I've got a stock 85 that has had an irregular idle all summer long. Sometimes idles high, sometimes normal, sometimes low. Always seems to be hunting. It has also recently begun to ping like heck under more agressive acceleration, suggesting lean condition.
I ran the WinALDL diagnostic recently. Below are results:
Got one error code - 32, or EGR. It checked out in terms of holding vacuum and bleeding down over normal timeframe, but I haven't tried pulling 10 inches of vac, then starting to verify it releases..
Flag data is as follows:
Only boxes checked are: MW2.. ALDL mode, MWAF1: Closed loop flag, BLM enable flag, MCUIO: No A/C requested, not used, no high gear, park/neutral, air divert solinoid on (but, only 1 box was checked, and there are two boxes total).
BLM: In the column marked "24" there are 4 boxes with values in them. In the intersect with 1200 RPM/MAF, the value is highlighted in green and reads 145.0. In the 800 RPM/MAF intersect the value reads 135.0. In the 1600 RMP/MAF intersect the value reads 127.7. In the 2000 RPM/MAF intersect the value reads 142.6.
In the column marked "48", only two values; in 1600 RPM/MAF intersect, value is 155.0, in RPM/MAF 2000 intersect, value reads 150.0.
In the column marked 72, only one value; 1600 RPM/MAF intersect value is 151.0, in column marked 96 again only one value, 2800 RPM/MAF intersect value reads 151.0.
I ran the same diagnostic yesterday with AIR pum tubes disconnected and manifold pipes capped (in case my AIR unit was feeding air in closed loop). Data is as follows:
One thing that jumped out at me was EGR - it was reading 0 at 72 mph and 2000 RPM - is that normal? It did periodically read "255 dc", so it does seem to work, albeit maybe intermittently? Solenoid??
Also, flag data kind of toggled between rich condition box checked and unchecked while driving, and remained checked after the test. Remember, for the time being, I have my AIR pump disabled.
Other flag I don't recall seeing before is "DRP Occurred". No idea what a DRP is. Last new flag is "Old High Gear".
BLM data follows (narrow latest - there is also a wide latest that has values running out to the column marked "144"; values there are reported in .xxx, not whole values if anyone is interested).
BLM Raw or converted in latest test is 137, down from 145 with AIR pump operating (yesterday's post). MAF is down at 7.73 g/s. Rich / Lean counter this pass was 250, up from yesterdays 227.
Column Marked "24" shows 4 reads as follows:
At the 800 RPM/MAF intersect, value is 139.7
At the 1200 RPM/MAF intersect, value is 133.7
At the 1600 RPM/MAF intersect, value is 137.8
At the 2000 RPM/MAF intersect, value is 141.6
Column marked "48" has one value:
At the 2000 RPM/MAF intersect, value is 149.7
Column marked "72" has two values:
At the 1600 RPM/MAF intersect, value is 153.0
At the 2000 RPM/MAF intersect, value is 150.3
Anyone familiar with WinALDL that can help me decipher this data? Thanks very much in advance!!
The BLM values are all quite lean. BLM > 128 means that fuel must be added in closed loop to compensate.
Also 7.73 gm/sec at 1050 rpm seems very low. I am used to seeing approx. 1 gm/sec/100 rpm.
Either its getting extra air from somewhere, or the MAF calibration needs to be adjusted (raise the output) to reflect the actual airflow.
I think it's got a leak somewhere. Darned if I can find it, though. One place it may be coming from is the intake manifold gasket. I have a leak on the back side that I cannot see, but that you can trace from underneath back up to the manifold area. I also see a slight leak on the front, but it's very minor. The one in back will leave a spot on the floor if the car sits in one place for a few weeks, though. Not a puddle mind you, but a damp stain.
I think that may be the next step, pulling the manifold and re-gasketing everything.
Been watching you chase this idle thing all over the place, I have an 86 that surges and don't want to idle smooth cold, it straightens out pretty good at operating temp or closed loop so it's not really bugging me too much but I would like to get it fixed so I'm hoping you find the cure and maybe that will help me. I've read the whole thread on T/P wow have you got a lot of patience. I'm thinking that small oil leak out of the back of the manifold isn't your culprit but needs to be fixed anyway and maybe in the course of pulling the manifold you'll discover something else, you've had everyhting else apart can probably do it in your sleep now. Good luck you can't give up now. Well I'm off to the garage with my can of carb cleaner to look for vac leaks, I know what your saying "been there done that"
Very unlikely any vac leaks are coming from the back or front of the manifold. That's one design (gaskets) I've always hated in GMs, oil leaks yeah, but vac leaks...I've never experienced from that area.
Hey man I dont know if this helps but I had a similar problem with my 85 And it drove me nuts, what I found was the EGR valve was not closed off all the way it was always adding exhaust gasses into the intake manifold and so it was lean and the idle would surge up and down between 450 to as much as 1200 rpm while in park and when it was in gear it would die sometimes. Try pulling the valve and get a vacuum source and open the valve and make sure it is clean inside where the seat is. This solved the problem I had.
That area across the rear of the intake manifold is prone to oil leaks but not vacuum leaks. Think about it. If it was, you would be blowing clouds of white smoke. Vacuum leaks come from the gaskets between the intake and the heads, I think you probably have a vacuum leak but that's not where it is.
I have had similar trouble to what you have. Mine surges from 500 to 1300 at idle and will not drop below 1200 when moving.
I found two things.
The evaporative emission solenoid is dead and I would love to know where to get one. I have checked everywhere and they are history.
The second thing I did was unplug the MAF and the idle returned to normal. Dead MAF.
The problem with the vapor solenoid is that it is open when dead. The purge valve on top of the canister opens with vacuum. Mine has a second purge valve on the tube coming from the gas tank and also opens with vacuum. With the engine running and the solenoid dead there is vacuum to the gas tank and sucks gas vapor into the manifold and on to the TB. This is a vacuum leak with extra smack.
To check the solenoid, turn the ignition on, ground ground the diagnostic terminal, unplug the solenoid and use a test light to see if there is current. If there is current the solenoid or the connector on it are bad order and can cause your idle problem. There are other steps in this vapor system to test in the shop manual.
I unplugged the vacuum from the bottom of the TB going to the vapor system and plugged the line from the TB for now.
Is there any alternative vapor system?
If anyone knows where to get one of these solenoids please let me know.
I forgot to add a couple important steps in the test.
First test the purge valve on the canister. Try to blow air into the lower hose on the purge valve on the top of the canister. You should not be able.
Then put 15 inches of vacuum to the top tube. This should hold for 20 seconds. If it holds then try to blow into the canister and you should be able to do it this time.
To test the solenoid, turn the ignition on, ground the diagnostic terminal, and put 10 inches vacuum to the TB side. This should hold. Now pull the hose off the solenoid going to the purge valve and check to see if the vacuum drops. It should not drop. If ok then unplug the solenoid and you should see the vacuum drop. Use a test light to check for current if the vacuum drops when you pull the hose from the canister side.
for lefoy...DRP occured=Distributor Reference Pulses, I can thank tequilaboy for that one. never saw it flag on my 87..and didn't see it mentioned in the users manual. Always knew about the pulses, but first time I've heard about them cropping up while scanning.
for lefoy...DRP occured=Distributor Reference Pulses, I can thank tequilaboy for that one. never saw it flag on my 87..and didn't see it mentioned in the users manual. Always knew about the pulses, but first time I've heard about them cropping up while scanning.
How to Adjust your Early C4 TPS and Idle Speed by Lars Grimsrud SVE Automotive Restoration Musclecar, Collector & Exotic Auto Repair & Restoration Broomfield, CO Rev. New 6-15-00 This tech paper will discuss the procedure for correct adjustment of the Minimum Idle Speed and for adjustment of the Throttle Position Switch (TPS) on the early C4 Corvette TPI systems. These steps apply specifically to the 1985 model year, and in general to other years. Later model years do not have adjustable TPS’s. General Idle speed and off-idle throttle response on the early TPI systems is determined by correct adjustment of the minimum idle speed screw combined with a correct setting of the TPS. I’ve seen many of these cars that have had their idle speed “corrected” by well-intentioned mechanics and owners by simply screwing the minimum idle speed screw in a few turns. This really messes up the settings, and will not make your car perform properly. Doing a correct setup of the TPS is one of the easiest ways to make your car feel and respond better. To maximize the benefit of this procedure, I recommend that you first remove your Throttle Body (TB), disassemble it (it’s incredible easy – there are a total of about 5 pieces in it…), clean the TB up really good with some spray carb cleaner, and put it back together. A nice clean TB will really put an edge on the performance improvement you will get by doing this procedure. The Service Manual has instructions for doing these operations, but the directions are scattered through several sections of the Manual. Here is the complete, step-by-step process for doing this (not including TB rebuild). All specs and steps are taken directly from the Manual (all 3 different sections), and this process is absolutely correct. Tools & Equipment You will need the following tools and equipment: 1. A set of Torx wrenches. You can buy a complete set in a nice, genuine plastic pouch at Sears. 2. A good digital voltmeter that will read voltages less than 1 volt. 3. A paper clip. 4. A small screwdriver. Procedure There are two electrical components on the TB that you will be working with: The TPS and the Idle Air Control Valve (IAC). Make sure that the connectors for these two components are easily accessible and that you can easily disconnect the IAC. You will also be playing with the diagnostic connector under the dash. Remove the cover (if it’s still in place). Bend your paper clip into a “U” shape. You will be playing with the two top right hand terminals (“A” and “B”) in the connector. · First step is to set the minimum idle speed. If nobody has messed with this on your car before, the set screw will be covered by a pressed-in plug. It’s located on the driver’s side of the TB. Remove this plug if it’s there. · With the IAC connected and the ignition “OFF,” stick the paper clip into the diagnostic connector from “A” to “B.” This grounds the diagnostic lead. · Turn the ignition to the “ON” position without starting the engine. Wait 30 seconds. · Now, with the ignition still in the “ON” position, disconnect the IAC connector at the IAC. · Remove the paper clip from the diagnostic connector. · Start the engine and allow it to reach normal operating temperature. The idle speed will probably be really low, and you may have to coax the engine a bit with the gas pedal to keep it running for a while. · If your car is an automatic, set the parking brake and put the transmission in “DRIVE.” If your car is a manual, leave it in neutral. · Adjust the idle speed screw to obtain 400 rpm in drive or 450 in neutral. · Shut off the engine and re-connect the IAC. That’s it for idle speed. Now on to the TPS. There are 3 wires stacked vertically on the TPS. You will need to be able to measure the voltage between the two top wires. You can either buy a special harness connector that breaks these wires out (from Mid America), or gently pierce the insulation of the wires with the pointy prongs on your volt meter. You can also stick a paper clip into each of the two top locations of the connector and clamp onto the paper clips to measure the voltage. Whatever is easiest for you. · Turn the ignition to the “ON” position without starting the engine. · Loosen the TPS Torx adjustment screws. · Set your volt meter to a low scale DC volt setting that will accurately read less than 1 volt. · Measure the voltage between the two top TPS wires. · Adjust the TPS by rotating its position until you get a reading of .54 volts. · Tighten the Torx screws and recheck the voltage. Re-adjust if necessary to make sure voltage is right at .54. · Turn the ignition “OFF.” You are now in perfect adjustment on idle speed and TPS output. Start the engine. It may take a few seconds for the car to “catch on” to its new settings.
Hope this helps.
Followed this procedure and my 85 idles at 700rpm....
Did adjusting the TPS and IAC according to the last post fix your idle surge? I am curious because my 85 is doing the same thing. According to the Haynes Automotive Repair Manual, "A broken or loose TPS can cause intermittent bursts of fuel from the injector and an unstable idle because the ECM thinks the throttle is moving." Sure sounds like exactly what is going on with my car. Please let me know if you have had any luck! I sure don't want to go through everything you have if you already know this works.
Corvette Store
Stock 85 w/irregular idle
never saw it flag on my 87..and didn't see it mentioned in the users manual. Always knew about the pulses, but first time I've heard about them cropping up while scanning.
