Idle problems w/ low mileage '81
08-23-2005, 08:19 PM
#1
Racer
Thread Starter
Idle problems w/ low mileage '81
I have a friend that recently purchased a 35,000 mile 1981, automatic trans, loaded. It's in very fine original condition. I'm on here because he's having 2 different problems. The 1st is with the idle. When you first crank the car, the automatic choke engages as it's supposed to. As the choke dis-engages, the car will shut off several times. Once the car comes up to operating temperature, it idles at 700 rpm with no problems. If you drive the car around the block and turn it off for 10 min, it repeats the original problem by shutting off several times before idling correctly. We've checked for any vacuum leaks and there are none. All of the accessories are hooked up per GM spec. All of the cylinders are firing correctly and the (new) plugs & wires are fine. There is no missing as you accelerate. I cleaned the carb and changed the fuel filter. There's fresh gas in the tank. I know that this was the 1st and last year for 'computer command control' on a carbureted car (last year for a carb period), so there really aren't any adjustments on the carb to go to. After the car is up to temperature, even with the AC on (which is new and works fine) the car idles smoothly. Only after you shut her down for 10 to 15 minutes do the problems idling return. The other problem is that when you engage the cruise control, the car 'surges' slowing down and speeding up. I know that it will do this when you go up and down a hill, but this's happening while on a level road. I'm not convinced that it's not related to the problem with the idle. I want to say that with such low mileage, the car has spent a reasonable amount of time sitting & it's possible that there's some varnish residue in the carb that's causing this problem. I wonder if having the carb re-built would be the way to go? I would welcome any suggestions or a dirrection to steer me in that I'm not aware of!
One detail: when he got the car, the MAP sensor was disconnected and the the port on the base of the carb was capped. He had the MAP sensor replaced with a new GM part and hooked back up. But even if you unhook the MAP sensor and re-cap the port on the carb now, you still have the same idle problem. I wonder why the MAP sensor was being by passed? When it was not connected, the 'check engine' light would light at idle. As soon as you were underway, it would go off.
One detail: when he got the car, the MAP sensor was disconnected and the the port on the base of the carb was capped. He had the MAP sensor replaced with a new GM part and hooked back up. But even if you unhook the MAP sensor and re-cap the port on the carb now, you still have the same idle problem. I wonder why the MAP sensor was being by passed? When it was not connected, the 'check engine' light would light at idle. As soon as you were underway, it would go off.
Last edited by MarkV; 08-23-2005 at 08:22 PM.
08-23-2005, 08:51 PM
#2
Melting Slicks
Member Since: Jan 2005
Location: Madison Ms
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What you are describing is why many of us 81 owners have disconnect the computer, smog crap, and stuff. Not difficult to change. New distrib., new carb, welding up smog crap, and your off!! Then the fun begins, because you actually can tune your car!! 
08-24-2005, 03:35 AM
#4
Melting Slicks
Originally Posted by MsVetteMan
What you are describing is why many of us 81 owners have disconnect the computer, smog crap, and stuff. Not difficult to change. New distrib., new carb, welding up smog crap, and your off!! Then the fun begins, because you actually can tune your car!!
I went this route with my 81 and did a serious overhaul on the engine and I now have the power that I want. 
08-26-2005, 01:09 AM
#6
Racer
Thread Starter
Originally Posted by adam
Any codes being reported? That's the best place to start now...
I know that there's a port under the ash tray. Where would you go to have someone check the codes? (Other than a Chevy dealership!)
08-26-2005, 07:13 AM
#7
Racer
Member Since: Feb 1999
Location: Garner NC
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Checking codes
You can check the codes yourself. Just pull out that test term under the ashtray and ground the test terminal (connect terminals D & E with a wire) Then turn the ign to on (not start) and start counting how the check engine light flashes.
1st it'll start flashing code 12, that's flash one time slight pause then it'll flash 2 times. I think it does this 3 times then it'll start flashing other codes if there stored in the ECM. I think each code flashes 3 times then on to the next, when done it'll start flashing code 12 again.
If you don't get a 12 the ECM is probably fried, or something else is wrong.
Good luck!
1st it'll start flashing code 12, that's flash one time slight pause then it'll flash 2 times. I think it does this 3 times then it'll start flashing other codes if there stored in the ECM. I think each code flashes 3 times then on to the next, when done it'll start flashing code 12 again.
If you don't get a 12 the ECM is probably fried, or something else is wrong.
Good luck!
08-26-2005, 07:27 AM
#8
In the connector under the ashtray you have to short 2 of the outer pins together (with a wire/screwdriver/paperclip/etc). Ignore what the manuals (or the post archives here) say about which 2 as those connectors were fitted both ways round by the factory. Look at the plastic housing & you'll see that the outer 2 connections at one end of the connector have a much smaller (or no) divider between the pins. Those are the pins to short together. Then you turn on the ignition (or do you short the pins after the ignition has been switched on? I can't remember but it'll be in the archives here). Don't actually start the engine, just turn on the ignition & watch the check engine light. It should flash once, then pause & flash twice. This is telling you that code 12 is set (wich is expected as the engine isn't running & is a visual confirmation that the codes are being displayed). This should be repeated 3 times & then it will flash any other stored codes before repeating them all again, starting with 12. If no other codes are stored then it'll just keep blinking code 12. If there are other codes stored then post what they are & somebody will tell you what they are.
The idle problem could be due to the car standing and the fuel drying out. I've recently had to strip a carb that's stood for 2 years with unleaded fuel in it & the fuel dried into something like vaseline & had blocked up some jets, so there could be crud in the carb causing this. My '81 has occasionally cut out after starting from warm but I think that's because I've set the idle quite low. If I slightly increase the rpms for about 10secs after starting from warm then it'll idle OK. If I start it from cold & blip the throttle before it's fully warmed up (to get the choke off) then it will also sometimes cut out. This is just as you've described, but mine does it because I've dropped the idle speed down below what it should be. When it's warmed up I get a nice slow, regular idle & I like that
If I set it to the speed stated on the under-hood label then it doesn't cut out at all. There's a screw to manually adjust the idle speed on the carb. Again, I can't remember where it is as it's been a while since I touched it!! I think it's on the d.side. If you look at the throttle linkage you should see a screw (with the head facing to the front of the car) which adjusts where the linkage sits in the idle position. Turn this in slightly to bump up the idle speed a bit (remembering where the screw was set to in case you want to go back to original - or I'm feeding you a bunch of BS & you're looking at the wrong screw!). It shouldn't take much adjustment to stop the problem if it's due to the idle being to low (like I've done). Make sure that the engine is up to running temp & that the aircon is OFF when doing this. There should be a solenoid mounted to the front of the carb which pushes a "bolt" out onto the throttle arm when the A/C is on. This should bump up the idle speed about 100rpm to prevent the engine cutting out at idle when the A/C is on.
Something else to check is for wear in the throttle spindle. The way the Q-jet works puts a lot of wear on the throttle shaft where it goes through the soft metal of the carb body. With the return spring & operating cable both pulling on the d.side end of the shaft, wear can rapidly occur. This then lets the throttle shaft move about in the body, with more movement on the d.side. The primaries may not close properly (giving a slightly different idle speed each time you shut the throttle) & there'll be a slight air leak due to it. So grap the throttle spindle by the bracket on the end & see how much wear there is in the pivot through the carb body. If there's a lot of wear (mine was in a fore & aft direction) it can be cured by fitting some brass bushes in the carb. There's kits to do this yourself, or somebody can to it for you (assuming that there is any wear). It's an easy job, but a lot of really ****ly problems disappear once done.
So, I'd check for error codes, increase the rpms slightly, check for excessive wear in the throttle shaft (this isn't a problem with just the electronic carbs) & see what happens. I'd also check the float level & clean the filter before even thinking of stripping the carb down.
I wouldn't suspect the computer or settings yet as there could just be some dirt in the idle circuits, or a small vacuum leak.
Haven't a clue about the cruise control as mine has been removed, but it does sound suspiciously like a small vacuum leak. Are all the emissions systems still connected & working? Two small vacuum leaks I found on mine were real b'stards to find. One was the purge cannister which had a very small leak from one of the valves. the other was a solenoid thing for the EGR system. There was a 90 deg plastic vacuum connector that connected to the underside of it & somebody had pulled it off. Both those small vacuum leaks caused me lots of flashing check engine lights, although the car felt OK to drive. What I did was to follow the vacuum diagrams, work out what was what & then disconnect each system from the carb in turn, plugging the hole at the carb & seeing if there was an improvement. The only system I wouldn't advise doing that on is the vacuum to the brake booster!
Unplugged MAP sensor is an uknown. Bubba does some strange things to these cars
Good luck
The idle problem could be due to the car standing and the fuel drying out. I've recently had to strip a carb that's stood for 2 years with unleaded fuel in it & the fuel dried into something like vaseline & had blocked up some jets, so there could be crud in the carb causing this. My '81 has occasionally cut out after starting from warm but I think that's because I've set the idle quite low. If I slightly increase the rpms for about 10secs after starting from warm then it'll idle OK. If I start it from cold & blip the throttle before it's fully warmed up (to get the choke off) then it will also sometimes cut out. This is just as you've described, but mine does it because I've dropped the idle speed down below what it should be. When it's warmed up I get a nice slow, regular idle & I like that
If I set it to the speed stated on the under-hood label then it doesn't cut out at all. There's a screw to manually adjust the idle speed on the carb. Again, I can't remember where it is as it's been a while since I touched it!! I think it's on the d.side. If you look at the throttle linkage you should see a screw (with the head facing to the front of the car) which adjusts where the linkage sits in the idle position. Turn this in slightly to bump up the idle speed a bit (remembering where the screw was set to in case you want to go back to original - or I'm feeding you a bunch of BS & you're looking at the wrong screw!). It shouldn't take much adjustment to stop the problem if it's due to the idle being to low (like I've done). Make sure that the engine is up to running temp & that the aircon is OFF when doing this. There should be a solenoid mounted to the front of the carb which pushes a "bolt" out onto the throttle arm when the A/C is on. This should bump up the idle speed about 100rpm to prevent the engine cutting out at idle when the A/C is on.Something else to check is for wear in the throttle spindle. The way the Q-jet works puts a lot of wear on the throttle shaft where it goes through the soft metal of the carb body. With the return spring & operating cable both pulling on the d.side end of the shaft, wear can rapidly occur. This then lets the throttle shaft move about in the body, with more movement on the d.side. The primaries may not close properly (giving a slightly different idle speed each time you shut the throttle) & there'll be a slight air leak due to it. So grap the throttle spindle by the bracket on the end & see how much wear there is in the pivot through the carb body. If there's a lot of wear (mine was in a fore & aft direction) it can be cured by fitting some brass bushes in the carb. There's kits to do this yourself, or somebody can to it for you (assuming that there is any wear). It's an easy job, but a lot of really ****ly problems disappear once done.
So, I'd check for error codes, increase the rpms slightly, check for excessive wear in the throttle shaft (this isn't a problem with just the electronic carbs) & see what happens. I'd also check the float level & clean the filter before even thinking of stripping the carb down.
I wouldn't suspect the computer or settings yet as there could just be some dirt in the idle circuits, or a small vacuum leak.
Haven't a clue about the cruise control as mine has been removed, but it does sound suspiciously like a small vacuum leak. Are all the emissions systems still connected & working? Two small vacuum leaks I found on mine were real b'stards to find. One was the purge cannister which had a very small leak from one of the valves. the other was a solenoid thing for the EGR system. There was a 90 deg plastic vacuum connector that connected to the underside of it & somebody had pulled it off. Both those small vacuum leaks caused me lots of flashing check engine lights, although the car felt OK to drive. What I did was to follow the vacuum diagrams, work out what was what & then disconnect each system from the carb in turn, plugging the hole at the carb & seeing if there was an improvement. The only system I wouldn't advise doing that on is the vacuum to the brake booster!
Unplugged MAP sensor is an uknown. Bubba does some strange things to these cars
Good luck
08-26-2005, 07:31 AM
#9
Le Mans Master
I would suggest diagnostic and replacement of those vacuum systems (hoses, t-fittings, etc.) that are apparently leaking. The rubber hardens, cracks and will leak. Also, the heat sensitive sensors that are mounted in your intake manifold leak; I removed all of mine as even the new replacements leaked. My '80 now starts well and idles without difficulty.
08-27-2005, 01:20 PM
#10
Racer
Thread Starter
Originally Posted by UKPaul
In the connector under the ashtray you have to short 2 of the outer pins together (with a wire/screwdriver/paperclip/etc). Ignore what the manuals (or the post archives here) say about which 2 as those connectors were fitted both ways round by the factory. Look at the plastic housing & you'll see that the outer 2 connections at one end of the connector have a much smaller (or no) divider between the pins. Those are the pins to short together. Then you turn on the ignition (or do you short the pins after the ignition has been switched on? I can't remember but it'll be in the archives here). Don't actually start the engine, just turn on the ignition & watch the check engine light. It should flash once, then pause & flash twice. This is telling you that code 12 is set (wich is expected as the engine isn't running & is a visual confirmation that the codes are being displayed). This should be repeated 3 times & then it will flash any other stored codes before repeating them all again, starting with 12. If no other codes are stored then it'll just keep blinking code 12. If there are other codes stored then post what they are & somebody will tell you what they are.
The idle problem could be due to the car standing and the fuel drying out. I've recently had to strip a carb that's stood for 2 years with unleaded fuel in it & the fuel dried into something like vaseline & had blocked up some jets, so there could be crud in the carb causing this. My '81 has occasionally cut out after starting from warm but I think that's because I've set the idle quite low. If I slightly increase the rpms for about 10secs after starting from warm then it'll idle OK. If I start it from cold & blip the throttle before it's fully warmed up (to get the choke off) then it will also sometimes cut out. This is just as you've described, but mine does it because I've dropped the idle speed down below what it should be. When it's warmed up I get a nice slow, regular idle & I like that If I set it to the speed stated on the under-hood label then it doesn't cut out at all. There's a screw to manually adjust the idle speed on the carb. Again, I can't remember where it is as it's been a while since I touched it!! I think it's on the d.side. If you look at the throttle linkage you should see a screw (with the head facing to the front of the car) which adjusts where the linkage sits in the idle position. Turn this in slightly to bump up the idle speed a bit (remembering where the screw was set to in case you want to go back to original - or I'm feeding you a bunch of BS & you're looking at the wrong screw!). It shouldn't take much adjustment to stop the problem if it's due to the idle being to low (like I've done). Make sure that the engine is up to running temp & that the aircon is OFF when doing this. There should be a solenoid mounted to the front of the carb which pushes a "bolt" out onto the throttle arm when the A/C is on. This should bump up the idle speed about 100rpm to prevent the engine cutting out at idle when the A/C is on.
Something else to check is for wear in the throttle spindle. The way the Q-jet works puts a lot of wear on the throttle shaft where it goes through the soft metal of the carb body. With the return spring & operating cable both pulling on the d.side end of the shaft, wear can rapidly occur. This then lets the throttle shaft move about in the body, with more movement on the d.side. The primaries may not close properly (giving a slightly different idle speed each time you shut the throttle) & there'll be a slight air leak due to it. So grap the throttle spindle by the bracket on the end & see how much wear there is in the pivot through the carb body. If there's a lot of wear (mine was in a fore & aft direction) it can be cured by fitting some brass bushes in the carb. There's kits to do this yourself, or somebody can to it for you (assuming that there is any wear). It's an easy job, but a lot of really ****ly problems disappear once done.
So, I'd check for error codes, increase the rpms slightly, check for excessive wear in the throttle shaft (this isn't a problem with just the electronic carbs) & see what happens. I'd also check the float level & clean the filter before even thinking of stripping the carb down.
I wouldn't suspect the computer or settings yet as there could just be some dirt in the idle circuits, or a small vacuum leak.
Haven't a clue about the cruise control as mine has been removed, but it does sound suspiciously like a small vacuum leak. Are all the emissions systems still connected & working? Two small vacuum leaks I found on mine were real b'stards to find. One was the purge cannister which had a very small leak from one of the valves. the other was a solenoid thing for the EGR system. There was a 90 deg plastic vacuum connector that connected to the underside of it & somebody had pulled it off. Both those small vacuum leaks caused me lots of flashing check engine lights, although the car felt OK to drive. What I did was to follow the vacuum diagrams, work out what was what & then disconnect each system from the carb in turn, plugging the hole at the carb & seeing if there was an improvement. The only system I wouldn't advise doing that on is the vacuum to the brake booster!
Unplugged MAP sensor is an uknown. Bubba does some strange things to these cars
Good luck
The idle problem could be due to the car standing and the fuel drying out. I've recently had to strip a carb that's stood for 2 years with unleaded fuel in it & the fuel dried into something like vaseline & had blocked up some jets, so there could be crud in the carb causing this. My '81 has occasionally cut out after starting from warm but I think that's because I've set the idle quite low. If I slightly increase the rpms for about 10secs after starting from warm then it'll idle OK. If I start it from cold & blip the throttle before it's fully warmed up (to get the choke off) then it will also sometimes cut out. This is just as you've described, but mine does it because I've dropped the idle speed down below what it should be. When it's warmed up I get a nice slow, regular idle & I like that
If I set it to the speed stated on the under-hood label then it doesn't cut out at all. There's a screw to manually adjust the idle speed on the carb. Again, I can't remember where it is as it's been a while since I touched it!! I think it's on the d.side. If you look at the throttle linkage you should see a screw (with the head facing to the front of the car) which adjusts where the linkage sits in the idle position. Turn this in slightly to bump up the idle speed a bit (remembering where the screw was set to in case you want to go back to original - or I'm feeding you a bunch of BS & you're looking at the wrong screw!). It shouldn't take much adjustment to stop the problem if it's due to the idle being to low (like I've done). Make sure that the engine is up to running temp & that the aircon is OFF when doing this. There should be a solenoid mounted to the front of the carb which pushes a "bolt" out onto the throttle arm when the A/C is on. This should bump up the idle speed about 100rpm to prevent the engine cutting out at idle when the A/C is on.Something else to check is for wear in the throttle spindle. The way the Q-jet works puts a lot of wear on the throttle shaft where it goes through the soft metal of the carb body. With the return spring & operating cable both pulling on the d.side end of the shaft, wear can rapidly occur. This then lets the throttle shaft move about in the body, with more movement on the d.side. The primaries may not close properly (giving a slightly different idle speed each time you shut the throttle) & there'll be a slight air leak due to it. So grap the throttle spindle by the bracket on the end & see how much wear there is in the pivot through the carb body. If there's a lot of wear (mine was in a fore & aft direction) it can be cured by fitting some brass bushes in the carb. There's kits to do this yourself, or somebody can to it for you (assuming that there is any wear). It's an easy job, but a lot of really ****ly problems disappear once done.
So, I'd check for error codes, increase the rpms slightly, check for excessive wear in the throttle shaft (this isn't a problem with just the electronic carbs) & see what happens. I'd also check the float level & clean the filter before even thinking of stripping the carb down.
I wouldn't suspect the computer or settings yet as there could just be some dirt in the idle circuits, or a small vacuum leak.
Haven't a clue about the cruise control as mine has been removed, but it does sound suspiciously like a small vacuum leak. Are all the emissions systems still connected & working? Two small vacuum leaks I found on mine were real b'stards to find. One was the purge cannister which had a very small leak from one of the valves. the other was a solenoid thing for the EGR system. There was a 90 deg plastic vacuum connector that connected to the underside of it & somebody had pulled it off. Both those small vacuum leaks caused me lots of flashing check engine lights, although the car felt OK to drive. What I did was to follow the vacuum diagrams, work out what was what & then disconnect each system from the carb in turn, plugging the hole at the carb & seeing if there was an improvement. The only system I wouldn't advise doing that on is the vacuum to the brake booster!
Unplugged MAP sensor is an uknown. Bubba does some strange things to these cars
Good luck
Yes, the car is 'all there' and completely original. This's why he'd like to keep it that way, as you rarely see these C3's so complete and NCRS original today. This is what I needed & I'll (we'll) check out all of these little details. As I've said, once the car reaches operating temp, it runs like a dream; it's only at start up are there problems. I already had tweaked the throttle linkage adjustment screw and bumped up the idle by 500 rpm, but that wasn't the problem. I still suspect a problem in the idle circuit of the carb and believe that a re-build would be in order, especially taking into consideration how long this car sat with it only having 35,000 on the clock. The previous owner of the car rarely drove it & I'm sure that fuel stabilizer wasn't used! I'd bet $$ that there's some varnish buildup somewhere in the carb. Also a small vaccum leak is possible & that needs to be checked as well, although I did the 'starting fliud' check (spraying around the carb & vacuum lines) to see if the RPMs increased which would indicate a vacuum leak. There was no change trying this method. The car is very driveable but it's annoying to have it shut off 3 or 4 times before it will hold a stable idle!
