How to Set up 1984 C4 Crossfire TB's & TPS
03-23-2014, 02:28 PM
#1
Instructor
Thread Starter
How to Set up 1984 C4 Crossfire TB's & TPS
It turns out that the Crossfire set up procedure that I have lived in fear of for so long is actually fairly easy!
I have had the car for 4 years. It probably should have been used for parts but not knowing any better I spent a lot of time and money on it. When I first got the car the engine vacuum system was horribly mis-piped and the car idled badly, very fuel rich. I re-piped all the vacuum according to the diagram stuck to the radiator cover and that helped a lot. The car was still rich at idle and the idle speed would stay up at 1500 sometimes but come down if you goosed the throttle the right way (most of the time). But the car ran and there was always something worse to fix.
I would like to replace the engine but keep it Crossfire but you can't buy the Renegade manifold right now and my Crossfire throttle bodies have shaft play in them but should I have them enlarged or not? etc etc, but rather than answer all those weighty questions I figured I'd set it up anyway to try and improve things. It turns out that someone else had set it up in the past........ Badly.
After doing the setup the car does run better, the idle is way better - it reliably idles at 1000 rpm then goes down to 7 or 800. I have only driven it a few miles so far but the off idle stumble, while still detectable, is almost gone and the car does not smell as rich. The throttle bodies (TBI's) still have to be rebuilt of course but it's much better.
During the setup you do the following:
1. Plug the air passages to the IAC (Idle Air Control) valves which normally control idle speed on each throttle body. This prevents them from affecting the idle while you make adjustments.
2. Adjust the mechanical idle stop screw on the rear TB to get a reading of 6" of water column vacuum at the rear throttle body's special adjustment port.
3. Adjust the mechanical idle offset screw on the front TB to get a 6” reading on its special adjustment port, thus getting both throttle plates synchronized.
4. Since at this point the car is at a mechanical idle of 1200 RPM or so you then adjust the mechanical idle on the rear TB (this is the only TB idle stop that is used, it stops the front TB also via the linkage) down to 475 rpm
5. Check that the front TB idle stop screw is not stopping the TB at all, that there is a .005 of an inch gap between the screw & throttle.
6. Adjust the TPS (Throttle Position Sensor) to 0.525vdc +- 0.075 vdc at idle.
I plugged into the ALDL diagnostic link inside the car to get RPM and TPS position. I used the OBD1 diagnostic cable from OBD Diagnostics in Redondo Beach, CA. I bought it a few years ago – I think I heard about it on here. It comes with several different software programs. I used WinALDL which is written as freeware by a guy in Europe. Here’s a link to his website http://www.joby.se/ I picked his software because he actually has a C4 – his website is worth looking at. When you set up the cable you need to jumper in the 10k resistor. If you don’t it will transmit some data but not all so it looks like it’s working but it’s not. His software did not have a choice for my exact ECM but I picked the 1982 L82.This is the first time I really used it and it works good so I’m going to send him some money. However seeing the laptop screen outdoors in bright light is not easy. I did everything according to the Chevy FSM.
1. I started off by unplugging both ISC motors just so they wouldn’t be sent to extremes during the procedure. I then started the engine.
2. I plugged up the ISC air passages on each TB, the idle went right down as I plugged each one – once they are plugged only the throttle plate position affects idle, just like on a carburetor. The air passage entry is on the top driver’s side front of each TB. The FSM shows a special shaped plug tool. I used Storm King ¾” foam rubber insulation from Home Depot. I shaved the sticky part off with a razor and slid the flat side down into the hole, against the forward part where the air hole is, then packed some more in, about 3” in all.
3. I checked that the front TB throttle stop screw was well out of the way (it should never touch the stop) and saw that the rear throttle stop screw was not touching the stop and stopping the throttle as it should. I took the plug off of the rear throttle body’s special adjustment port and connected my Manometer. A Manometer measures pressure or vacuum in inches of water column. Inches of water pressure are much lower than Mercury pressure, the way engine vacuum is usually measured, since Mercury is heavier than water. I used a Dwyer brand “Magnehelic” pressure gauge. These gauges are quite common in the heating and cooling industry as air pressure in ducts is measured in inches of water. They have both high and low pressure ports. You connect to the low port to measure vacuum. I used a 10” range model. I just looked on E-bay and they go for $30 or so. It’s a very nice, accurate and well made instrument. You can adjust the zero easily via a front screw. It is position sensitive somewhat. At this point the engine was idling at 650 RPM or so and the Manometer read close to 0”. As the protective factory plugs on my car were long gone I adjusted the rear TB mechanical idle screw with a small Torx bit, turning it with an ignition wrench, as there was no room for anything longer. I opened the throttle plate up some with the screw and got the vacuum to 6”. The idle was now at about 900 RPM.
4. I moved the Manometer to the front TB’s port and plugged the rear’s port. The front vacuum was close to 0. I opened the front throttle plate some and got to 6” of vacuum by adjusting the idle offset screw on the front TB. The screw is on the linkage. The factory lock which has to be removed was long gone. I put some Locktite on the screw. I then checked the front one again and then the rear again to be sure the adjustment was good. At this point the idle was at about 1200.
5. I then adjusted the rear TB mechanical idle stop screw to get the idle down to 475 and checked that the front TB stop screw was not touching and that there was .005 inches of clearance between the front TB screw and the stop (I wonder if the .005 is to let the stop not influence the mechanical idle normally but to stop the plate if you quickly release the throttle). I think that the 6” of vacuum setting gets the idle so high just to get enough vacuum on the port for a decent reading with some meat in it and maybe because that position is important for off idle performance without stumbling. I think that finally setting the mechanical idle (minimum idle) down to 475 leaves enough headroom between there and the real idle that the ISC motors can control the idle well.
6. The TPS (throttle position sensor) voltage is supposed to be 0.525vdc +- 0.075 vdc at idle per the FSM. You adjust it with the ignition on but the engine off. At full throttle it is supposed to be 5 vdc but I have seen on here that often they don’t get all the way to 5 but that they must get over 4 vdc or the ECM won’t see it as WOT (wide open throttle) so you won’t get full power. The FSM has no adjustment procedure for full throttle, only idle and there is no full throttle voltage check procedure that I saw although in the description of operation it says 5. Mine was originally at about 0.47 vdc at idle and 3.90 vdc at full throttle (from inside the car with the gas pedal). I took the 2 screws out one at a time and put some Locktite on them and then adjusted the TPS to 0.580 at idle, which gave me 4.23 at full throttle from inside the car. I know that most of what I have read says 0.5 vdc at idle but being as how the FSM gave it a range of 0.45 to 0.6 vdc I cheated as high as I could to help get the full throttle voltage over 4 vdc. Thinking about it now though, the TPS had slots which offered front/rear and up/down adjustment I think that there is a possibility of getting closer to 5vdc at full throttle by moving the switch up higher and then adjusting the idle position voltage by sliding it front/rear. I may revisit that adjustment. Or does the ECM remember the highest and lowest readings it sees over time so that it doesn't matter?
7. I shut the engine down, re-connected the ISC motors, then drove over 30 mph to reset the ISC motors per the FSM.
The results were good, the car runs much better. The rear TB especially has a lot of play in the shaft and the front does not open quite as fully as the rear, possibly due to the rear’s play, but the procedure was well worth doing and I understand it now. It’s really just a couple of hours work at the most.
I have had the car for 4 years. It probably should have been used for parts but not knowing any better I spent a lot of time and money on it. When I first got the car the engine vacuum system was horribly mis-piped and the car idled badly, very fuel rich. I re-piped all the vacuum according to the diagram stuck to the radiator cover and that helped a lot. The car was still rich at idle and the idle speed would stay up at 1500 sometimes but come down if you goosed the throttle the right way (most of the time). But the car ran and there was always something worse to fix.
I would like to replace the engine but keep it Crossfire but you can't buy the Renegade manifold right now and my Crossfire throttle bodies have shaft play in them but should I have them enlarged or not? etc etc, but rather than answer all those weighty questions I figured I'd set it up anyway to try and improve things. It turns out that someone else had set it up in the past........ Badly.
After doing the setup the car does run better, the idle is way better - it reliably idles at 1000 rpm then goes down to 7 or 800. I have only driven it a few miles so far but the off idle stumble, while still detectable, is almost gone and the car does not smell as rich. The throttle bodies (TBI's) still have to be rebuilt of course but it's much better.
During the setup you do the following:
1. Plug the air passages to the IAC (Idle Air Control) valves which normally control idle speed on each throttle body. This prevents them from affecting the idle while you make adjustments.
2. Adjust the mechanical idle stop screw on the rear TB to get a reading of 6" of water column vacuum at the rear throttle body's special adjustment port.
3. Adjust the mechanical idle offset screw on the front TB to get a 6” reading on its special adjustment port, thus getting both throttle plates synchronized.
4. Since at this point the car is at a mechanical idle of 1200 RPM or so you then adjust the mechanical idle on the rear TB (this is the only TB idle stop that is used, it stops the front TB also via the linkage) down to 475 rpm
5. Check that the front TB idle stop screw is not stopping the TB at all, that there is a .005 of an inch gap between the screw & throttle.
6. Adjust the TPS (Throttle Position Sensor) to 0.525vdc +- 0.075 vdc at idle.
I plugged into the ALDL diagnostic link inside the car to get RPM and TPS position. I used the OBD1 diagnostic cable from OBD Diagnostics in Redondo Beach, CA. I bought it a few years ago – I think I heard about it on here. It comes with several different software programs. I used WinALDL which is written as freeware by a guy in Europe. Here’s a link to his website http://www.joby.se/ I picked his software because he actually has a C4 – his website is worth looking at. When you set up the cable you need to jumper in the 10k resistor. If you don’t it will transmit some data but not all so it looks like it’s working but it’s not. His software did not have a choice for my exact ECM but I picked the 1982 L82.This is the first time I really used it and it works good so I’m going to send him some money. However seeing the laptop screen outdoors in bright light is not easy. I did everything according to the Chevy FSM.
1. I started off by unplugging both ISC motors just so they wouldn’t be sent to extremes during the procedure. I then started the engine.
2. I plugged up the ISC air passages on each TB, the idle went right down as I plugged each one – once they are plugged only the throttle plate position affects idle, just like on a carburetor. The air passage entry is on the top driver’s side front of each TB. The FSM shows a special shaped plug tool. I used Storm King ¾” foam rubber insulation from Home Depot. I shaved the sticky part off with a razor and slid the flat side down into the hole, against the forward part where the air hole is, then packed some more in, about 3” in all.
3. I checked that the front TB throttle stop screw was well out of the way (it should never touch the stop) and saw that the rear throttle stop screw was not touching the stop and stopping the throttle as it should. I took the plug off of the rear throttle body’s special adjustment port and connected my Manometer. A Manometer measures pressure or vacuum in inches of water column. Inches of water pressure are much lower than Mercury pressure, the way engine vacuum is usually measured, since Mercury is heavier than water. I used a Dwyer brand “Magnehelic” pressure gauge. These gauges are quite common in the heating and cooling industry as air pressure in ducts is measured in inches of water. They have both high and low pressure ports. You connect to the low port to measure vacuum. I used a 10” range model. I just looked on E-bay and they go for $30 or so. It’s a very nice, accurate and well made instrument. You can adjust the zero easily via a front screw. It is position sensitive somewhat. At this point the engine was idling at 650 RPM or so and the Manometer read close to 0”. As the protective factory plugs on my car were long gone I adjusted the rear TB mechanical idle screw with a small Torx bit, turning it with an ignition wrench, as there was no room for anything longer. I opened the throttle plate up some with the screw and got the vacuum to 6”. The idle was now at about 900 RPM.
4. I moved the Manometer to the front TB’s port and plugged the rear’s port. The front vacuum was close to 0. I opened the front throttle plate some and got to 6” of vacuum by adjusting the idle offset screw on the front TB. The screw is on the linkage. The factory lock which has to be removed was long gone. I put some Locktite on the screw. I then checked the front one again and then the rear again to be sure the adjustment was good. At this point the idle was at about 1200.
5. I then adjusted the rear TB mechanical idle stop screw to get the idle down to 475 and checked that the front TB stop screw was not touching and that there was .005 inches of clearance between the front TB screw and the stop (I wonder if the .005 is to let the stop not influence the mechanical idle normally but to stop the plate if you quickly release the throttle). I think that the 6” of vacuum setting gets the idle so high just to get enough vacuum on the port for a decent reading with some meat in it and maybe because that position is important for off idle performance without stumbling. I think that finally setting the mechanical idle (minimum idle) down to 475 leaves enough headroom between there and the real idle that the ISC motors can control the idle well.
6. The TPS (throttle position sensor) voltage is supposed to be 0.525vdc +- 0.075 vdc at idle per the FSM. You adjust it with the ignition on but the engine off. At full throttle it is supposed to be 5 vdc but I have seen on here that often they don’t get all the way to 5 but that they must get over 4 vdc or the ECM won’t see it as WOT (wide open throttle) so you won’t get full power. The FSM has no adjustment procedure for full throttle, only idle and there is no full throttle voltage check procedure that I saw although in the description of operation it says 5. Mine was originally at about 0.47 vdc at idle and 3.90 vdc at full throttle (from inside the car with the gas pedal). I took the 2 screws out one at a time and put some Locktite on them and then adjusted the TPS to 0.580 at idle, which gave me 4.23 at full throttle from inside the car. I know that most of what I have read says 0.5 vdc at idle but being as how the FSM gave it a range of 0.45 to 0.6 vdc I cheated as high as I could to help get the full throttle voltage over 4 vdc. Thinking about it now though, the TPS had slots which offered front/rear and up/down adjustment I think that there is a possibility of getting closer to 5vdc at full throttle by moving the switch up higher and then adjusting the idle position voltage by sliding it front/rear. I may revisit that adjustment. Or does the ECM remember the highest and lowest readings it sees over time so that it doesn't matter?
7. I shut the engine down, re-connected the ISC motors, then drove over 30 mph to reset the ISC motors per the FSM.
The results were good, the car runs much better. The rear TB especially has a lot of play in the shaft and the front does not open quite as fully as the rear, possibly due to the rear’s play, but the procedure was well worth doing and I understand it now. It’s really just a couple of hours work at the most.
Last edited by ProjectC4; 03-23-2014 at 08:13 PM.
03-23-2014, 08:40 PM
#2
It turns out that the Crossfire set up procedure that I have lived in fear of for so long is actually fairly easy!
I have had the car for 4 years. It probably should have been used for parts but not knowing any better I spent a lot of time and money on it. When I first got the car the engine vacuum system was horribly mis-piped and the car idled badly, very fuel rich. I re-piped all the vacuum according to the diagram stuck to the radiator cover and that helped a lot. The car was still rich at idle and the idle speed would stay up at 1500 sometimes but come down if you goosed the throttle the right way (most of the time). But the car ran and there was always something worse to fix.
I would like to replace the engine but keep it Crossfire but you can't buy the Renegade manifold right now and my Crossfire throttle bodies have shaft play in them but should I have them enlarged or not? etc etc, but rather than answer all those weighty questions I figured I'd set it up anyway to try and improve things. It turns out that someone else had set it up in the past........ Badly.
After doing the setup the car does run better, the idle is way better - it reliably idles at 1000 rpm then goes down to 7 or 800. I have only driven it a few miles so far but the off idle stumble, while still detectable, is almost gone and the car does not smell as rich. The throttle bodies (TBI's) still have to be rebuilt of course but it's much better.
During the setup you do the following:
1. Plug the air passages to the IAC (Idle Air Control) valves which normally control idle speed on each throttle body. This prevents them from affecting the idle while you make adjustments.
2. Adjust the mechanical idle stop screw on the rear TB to get a reading of 6" of water column vacuum at the rear throttle body's special adjustment port.
3. Adjust the mechanical idle offset screw on the front TB to get a 6” reading on its special adjustment port, thus getting both throttle plates synchronized.
4. Since at this point the car is at a mechanical idle of 1200 RPM or so you then adjust the mechanical idle on the rear TB (this is the only TB idle stop that is used, it stops the front TB also via the linkage) down to 475 rpm
5. Check that the front TB idle stop screw is not stopping the TB at all, that there is a .005 of an inch gap between the screw & throttle.
6. Adjust the TPS (Throttle Position Sensor) to 0.525vdc +- 0.075 vdc at idle.
I plugged into the ALDL diagnostic link inside the car to get RPM and TPS position. I used the OBD1 diagnostic cable from OBD Diagnostics in Redondo Beach, CA. I bought it a few years ago – I think I heard about it on here. It comes with several different software programs. I used WinALDL which is written as freeware by a guy in Europe. Here’s a link to his website http://www.joby.se/ I picked his software because he actually has a C4 – his website is worth looking at. When you set up the cable you need to jumper in the 10k resistor. If you don’t it will transmit some data but not all so it looks like it’s working but it’s not. His software did not have a choice for my exact ECM but I picked the 1982 L82.This is the first time I really used it and it works good so I’m going to send him some money. However seeing the laptop screen outdoors in bright light is not easy. I did everything according to the Chevy FSM.
1. I started off by unplugging both ISC motors just so they wouldn’t be sent to extremes during the procedure. I then started the engine.
2. I plugged up the ISC air passages on each TB, the idle went right down as I plugged each one – once they are plugged only the throttle plate position affects idle, just like on a carburetor. The air passage entry is on the top driver’s side front of each TB. The FSM shows a special shaped plug tool. I used Storm King ¾” foam rubber insulation from Home Depot. I shaved the sticky part off with a razor and slid the flat side down into the hole, against the forward part where the air hole is, then packed some more in, about 3” in all.
3. I checked that the front TB throttle stop screw was well out of the way (it should never touch the stop) and saw that the rear throttle stop screw was not touching the stop and stopping the throttle as it should. I took the plug off of the rear throttle body’s special adjustment port and connected my Manometer. A Manometer measures pressure or vacuum in inches of water column. Inches of water pressure are much lower than Mercury pressure, the way engine vacuum is usually measured, since Mercury is heavier than water. I used a Dwyer brand “Magnehelic” pressure gauge. These gauges are quite common in the heating and cooling industry as air pressure in ducts is measured in inches of water. They have both high and low pressure ports. You connect to the low port to measure vacuum. I used a 10” range model. I just looked on E-bay and they go for $30 or so. It’s a very nice, accurate and well made instrument. You can adjust the zero easily via a front screw. It is position sensitive somewhat. At this point the engine was idling at 650 RPM or so and the Manometer read close to 0”. As the protective factory plugs on my car were long gone I adjusted the rear TB mechanical idle screw with a small Torx bit, turning it with an ignition wrench, as there was no room for anything longer. I opened the throttle plate up some with the screw and got the vacuum to 6”. The idle was now at about 900 RPM.
4. I moved the Manometer to the front TB’s port and plugged the rear’s port. The front vacuum was close to 0. I opened the front throttle plate some and got to 6” of vacuum by adjusting the idle offset screw on the front TB. The screw is on the linkage. The factory lock which has to be removed was long gone. I put some Locktite on the screw. I then checked the front one again and then the rear again to be sure the adjustment was good. At this point the idle was at about 1200.
5. I then adjusted the rear TB mechanical idle stop screw to get the idle down to 475 and checked that the front TB stop screw was not touching and that there was .005 inches of clearance between the front TB screw and the stop (I wonder if the .005 is to let the stop not influence the mechanical idle normally but to stop the plate if you quickly release the throttle). I think that the 6” of vacuum setting gets the idle so high just to get enough vacuum on the port for a decent reading with some meat in it and maybe because that position is important for off idle performance without stumbling. I think that finally setting the mechanical idle (minimum idle) down to 475 leaves enough headroom between there and the real idle that the ISC motors can control the idle well.
6. The TPS (throttle position sensor) voltage is supposed to be 0.525vdc +- 0.075 vdc at idle per the FSM. You adjust it with the ignition on but the engine off. At full throttle it is supposed to be 5 vdc but I have seen on here that often they don’t get all the way to 5 but that they must get over 4 vdc or the ECM won’t see it as WOT (wide open throttle) so you won’t get full power. The FSM has no adjustment procedure for full throttle, only idle and there is no full throttle voltage check procedure that I saw although in the description of operation it says 5. Mine was originally at about 0.47 vdc at idle and 3.90 vdc at full throttle (from inside the car with the gas pedal). I took the 2 screws out one at a time and put some Locktite on them and then adjusted the TPS to 0.580 at idle, which gave me 4.23 at full throttle from inside the car. I know that most of what I have read says 0.5 vdc at idle but being as how the FSM gave it a range of 0.45 to 0.6 vdc I cheated as high as I could to help get the full throttle voltage over 4 vdc. Thinking about it now though, the TPS had slots which offered front/rear and up/down adjustment I think that there is a possibility of getting closer to 5vdc at full throttle by moving the switch up higher and then adjusting the idle position voltage by sliding it front/rear. I may revisit that adjustment. Or does the ECM remember the highest and lowest readings it sees over time so that it doesn't matter?
7. I shut the engine down, re-connected the ISC motors, then drove over 30 mph to reset the ISC motors per the FSM.
The results were good, the car runs much better. The rear TB especially has a lot of play in the shaft and the front does not open quite as fully as the rear, possibly due to the rear’s play, but the procedure was well worth doing and I understand it now. It’s really just a couple of hours work at the most.
I have had the car for 4 years. It probably should have been used for parts but not knowing any better I spent a lot of time and money on it. When I first got the car the engine vacuum system was horribly mis-piped and the car idled badly, very fuel rich. I re-piped all the vacuum according to the diagram stuck to the radiator cover and that helped a lot. The car was still rich at idle and the idle speed would stay up at 1500 sometimes but come down if you goosed the throttle the right way (most of the time). But the car ran and there was always something worse to fix.
I would like to replace the engine but keep it Crossfire but you can't buy the Renegade manifold right now and my Crossfire throttle bodies have shaft play in them but should I have them enlarged or not? etc etc, but rather than answer all those weighty questions I figured I'd set it up anyway to try and improve things. It turns out that someone else had set it up in the past........ Badly.
After doing the setup the car does run better, the idle is way better - it reliably idles at 1000 rpm then goes down to 7 or 800. I have only driven it a few miles so far but the off idle stumble, while still detectable, is almost gone and the car does not smell as rich. The throttle bodies (TBI's) still have to be rebuilt of course but it's much better.
During the setup you do the following:
1. Plug the air passages to the IAC (Idle Air Control) valves which normally control idle speed on each throttle body. This prevents them from affecting the idle while you make adjustments.
2. Adjust the mechanical idle stop screw on the rear TB to get a reading of 6" of water column vacuum at the rear throttle body's special adjustment port.
3. Adjust the mechanical idle offset screw on the front TB to get a 6” reading on its special adjustment port, thus getting both throttle plates synchronized.
4. Since at this point the car is at a mechanical idle of 1200 RPM or so you then adjust the mechanical idle on the rear TB (this is the only TB idle stop that is used, it stops the front TB also via the linkage) down to 475 rpm
5. Check that the front TB idle stop screw is not stopping the TB at all, that there is a .005 of an inch gap between the screw & throttle.
6. Adjust the TPS (Throttle Position Sensor) to 0.525vdc +- 0.075 vdc at idle.
I plugged into the ALDL diagnostic link inside the car to get RPM and TPS position. I used the OBD1 diagnostic cable from OBD Diagnostics in Redondo Beach, CA. I bought it a few years ago – I think I heard about it on here. It comes with several different software programs. I used WinALDL which is written as freeware by a guy in Europe. Here’s a link to his website http://www.joby.se/ I picked his software because he actually has a C4 – his website is worth looking at. When you set up the cable you need to jumper in the 10k resistor. If you don’t it will transmit some data but not all so it looks like it’s working but it’s not. His software did not have a choice for my exact ECM but I picked the 1982 L82.This is the first time I really used it and it works good so I’m going to send him some money. However seeing the laptop screen outdoors in bright light is not easy. I did everything according to the Chevy FSM.
1. I started off by unplugging both ISC motors just so they wouldn’t be sent to extremes during the procedure. I then started the engine.
2. I plugged up the ISC air passages on each TB, the idle went right down as I plugged each one – once they are plugged only the throttle plate position affects idle, just like on a carburetor. The air passage entry is on the top driver’s side front of each TB. The FSM shows a special shaped plug tool. I used Storm King ¾” foam rubber insulation from Home Depot. I shaved the sticky part off with a razor and slid the flat side down into the hole, against the forward part where the air hole is, then packed some more in, about 3” in all.
3. I checked that the front TB throttle stop screw was well out of the way (it should never touch the stop) and saw that the rear throttle stop screw was not touching the stop and stopping the throttle as it should. I took the plug off of the rear throttle body’s special adjustment port and connected my Manometer. A Manometer measures pressure or vacuum in inches of water column. Inches of water pressure are much lower than Mercury pressure, the way engine vacuum is usually measured, since Mercury is heavier than water. I used a Dwyer brand “Magnehelic” pressure gauge. These gauges are quite common in the heating and cooling industry as air pressure in ducts is measured in inches of water. They have both high and low pressure ports. You connect to the low port to measure vacuum. I used a 10” range model. I just looked on E-bay and they go for $30 or so. It’s a very nice, accurate and well made instrument. You can adjust the zero easily via a front screw. It is position sensitive somewhat. At this point the engine was idling at 650 RPM or so and the Manometer read close to 0”. As the protective factory plugs on my car were long gone I adjusted the rear TB mechanical idle screw with a small Torx bit, turning it with an ignition wrench, as there was no room for anything longer. I opened the throttle plate up some with the screw and got the vacuum to 6”. The idle was now at about 900 RPM.
4. I moved the Manometer to the front TB’s port and plugged the rear’s port. The front vacuum was close to 0. I opened the front throttle plate some and got to 6” of vacuum by adjusting the idle offset screw on the front TB. The screw is on the linkage. The factory lock which has to be removed was long gone. I put some Locktite on the screw. I then checked the front one again and then the rear again to be sure the adjustment was good. At this point the idle was at about 1200.
5. I then adjusted the rear TB mechanical idle stop screw to get the idle down to 475 and checked that the front TB stop screw was not touching and that there was .005 inches of clearance between the front TB screw and the stop (I wonder if the .005 is to let the stop not influence the mechanical idle normally but to stop the plate if you quickly release the throttle). I think that the 6” of vacuum setting gets the idle so high just to get enough vacuum on the port for a decent reading with some meat in it and maybe because that position is important for off idle performance without stumbling. I think that finally setting the mechanical idle (minimum idle) down to 475 leaves enough headroom between there and the real idle that the ISC motors can control the idle well.
6. The TPS (throttle position sensor) voltage is supposed to be 0.525vdc +- 0.075 vdc at idle per the FSM. You adjust it with the ignition on but the engine off. At full throttle it is supposed to be 5 vdc but I have seen on here that often they don’t get all the way to 5 but that they must get over 4 vdc or the ECM won’t see it as WOT (wide open throttle) so you won’t get full power. The FSM has no adjustment procedure for full throttle, only idle and there is no full throttle voltage check procedure that I saw although in the description of operation it says 5. Mine was originally at about 0.47 vdc at idle and 3.90 vdc at full throttle (from inside the car with the gas pedal). I took the 2 screws out one at a time and put some Locktite on them and then adjusted the TPS to 0.580 at idle, which gave me 4.23 at full throttle from inside the car. I know that most of what I have read says 0.5 vdc at idle but being as how the FSM gave it a range of 0.45 to 0.6 vdc I cheated as high as I could to help get the full throttle voltage over 4 vdc. Thinking about it now though, the TPS had slots which offered front/rear and up/down adjustment I think that there is a possibility of getting closer to 5vdc at full throttle by moving the switch up higher and then adjusting the idle position voltage by sliding it front/rear. I may revisit that adjustment. Or does the ECM remember the highest and lowest readings it sees over time so that it doesn't matter?
7. I shut the engine down, re-connected the ISC motors, then drove over 30 mph to reset the ISC motors per the FSM.
The results were good, the car runs much better. The rear TB especially has a lot of play in the shaft and the front does not open quite as fully as the rear, possibly due to the rear’s play, but the procedure was well worth doing and I understand it now. It’s really just a couple of hours work at the most.
