1993 40th anv won't idle NEED HELP
05-04-2009, 05:55 PM
#1
4th Gear
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1993 40th anv won't idle NEED HELP
I have a 1993 40th anv edition and need some advice and help.
before winter my corvette would stall every once in awhile after sitting on and idle. then it would not start for a couple hours past by. I figure time for a tune up. So during the winter took it to a garage good friend of mine great mechanic with all the tools and scanners. Did complete tune up. Plugs, wires, distributer, ect etc. Started car and it still does the same thing. Idle for five minutes then shuts off but know it at least restarts. So replaced TSP sensor, Idle sensor, ERG value. still same thing. It acts like it almost out of timing but the timing is controlled by the computer. So we are stump and not sure where to look now. And yes I made sure all my vacuum lines are good. and when we scan the car no codes come up. How ever i did notice everytime before it almost stalls because it idles normal then to a low idle seven or eight times before it quits it says on my dash SYS light flashing. Any help would be awesome. Thanks
before winter my corvette would stall every once in awhile after sitting on and idle. then it would not start for a couple hours past by. I figure time for a tune up. So during the winter took it to a garage good friend of mine great mechanic with all the tools and scanners. Did complete tune up. Plugs, wires, distributer, ect etc. Started car and it still does the same thing. Idle for five minutes then shuts off but know it at least restarts. So replaced TSP sensor, Idle sensor, ERG value. still same thing. It acts like it almost out of timing but the timing is controlled by the computer. So we are stump and not sure where to look now. And yes I made sure all my vacuum lines are good. and when we scan the car no codes come up. How ever i did notice everytime before it almost stalls because it idles normal then to a low idle seven or eight times before it quits it says on my dash SYS light flashing. Any help would be awesome. Thanks
05-05-2009, 07:43 AM
#2
Le Mans Master
It basically seems like the IAC (idle speed control motor) could be causing the problem. It is a stepper motor that controls air flow for setting idle speed. They do get sticky and can cause some of the problems you have. Its on the right side of motor, up by the MAF, held in with 2 screws. If stuck wide open it might cause a starting problem.
It is driven by the ECM and if you have a scanner it will show up as IAC counts. Some people like to clean them and reinstall. Some times it works and sometimes it don’t. You can try that for diagnostic proposes but if defective I just like to install a new one.
The only other secondary thought might be a bad ICM. I only say this because you say you could not get it started and it would stall. But the indications are weak to make a hard case for the ICM.
It is driven by the ECM and if you have a scanner it will show up as IAC counts. Some people like to clean them and reinstall. Some times it works and sometimes it don’t. You can try that for diagnostic proposes but if defective I just like to install a new one.
The only other secondary thought might be a bad ICM. I only say this because you say you could not get it started and it would stall. But the indications are weak to make a hard case for the ICM.
05-05-2009, 07:54 AM
#3
It basically seems like the IAC (idle speed control motor) could be causing the problem. It is a stepper motor that controls air flow for setting idle speed. They do get sticky and can cause some of the problems you have. Its on the right side of motor, up by the MAF, held in with 2 screws. If stuck wide open it might cause a starting problem.
It is driven by the ECM and if you have a scanner it will show up as IAC counts. Some people like to clean them and reinstall. Some times it works and sometimes it don’t. You can try that for diagnostic proposes but if defective I just like to install a new one.
The only other secondary thought might be a bad ICM. I only say this because you say you could not get it started and it would stall. But the indications are weak to make a hard case for the ICM.
It is driven by the ECM and if you have a scanner it will show up as IAC counts. Some people like to clean them and reinstall. Some times it works and sometimes it don’t. You can try that for diagnostic proposes but if defective I just like to install a new one.
The only other secondary thought might be a bad ICM. I only say this because you say you could not get it started and it would stall. But the indications are weak to make a hard case for the ICM.
Also, I'd check the fuel pressure as well...
05-05-2009, 04:53 PM
#4
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It basically seems like the IAC (idle speed control motor) could be causing the problem. It is a stepper motor that controls air flow for setting idle speed. They do get sticky and can cause some of the problems you have. Its on the right side of motor, up by the MAF, held in with 2 screws. If stuck wide open it might cause a starting problem.
It is driven by the ECM and if you have a scanner it will show up as IAC counts. Some people like to clean them and reinstall. Some times it works and sometimes it don’t. You can try that for diagnostic proposes but if defective I just like to install a new one.
The only other secondary thought might be a bad ICM. I only say this because you say you could not get it started and it would stall. But the indications are weak to make a hard case for the ICM.
It is driven by the ECM and if you have a scanner it will show up as IAC counts. Some people like to clean them and reinstall. Some times it works and sometimes it don’t. You can try that for diagnostic proposes but if defective I just like to install a new one.
The only other secondary thought might be a bad ICM. I only say this because you say you could not get it started and it would stall. But the indications are weak to make a hard case for the ICM.
05-05-2009, 09:13 PM
#6
Le Mans Master
Sounds like you already changed the opti. Problem does not really sound like an opti anyway except for a wondering idle.
When problems get strange on a 93 my thoughts go to the ECM. They can do some strange things and cause weird running problems, no codes many times.
I would monitor some items. Get some node lights on the injectors and see if you are loosing the injector pulses. Also if it totally dies, check the ECM output to the ICM. It’s the white wire to terminal “B” of the ICM. You can measure it with a regular DVM on the AC scale, should measure between 1 – 4 vac as the engine is cranked. You might want to try it when things are running normal so you know what indication you are looking for. It is really a low frequency square wave.
When problems get strange on a 93 my thoughts go to the ECM. They can do some strange things and cause weird running problems, no codes many times.
I would monitor some items. Get some node lights on the injectors and see if you are loosing the injector pulses. Also if it totally dies, check the ECM output to the ICM. It’s the white wire to terminal “B” of the ICM. You can measure it with a regular DVM on the AC scale, should measure between 1 – 4 vac as the engine is cranked. You might want to try it when things are running normal so you know what indication you are looking for. It is really a low frequency square wave.
05-05-2009, 09:44 PM
#7
Race Director
From Shoebox.com you might try this it cost nothing !
Reset the IAC Position
Depress accelerator slightly
Start engine, then release accelerator pedal, run engine for 5 seconds
Turn ignition "OFF" for ten seconds
Restart engine and check for proper idle operation
It is NOT recommended to to push or pull on the pintle of an IAC that has been in service. The force required can damage the threads on the worm drive. Also, do not soak the IAC in any liquid cleaner or solvent, as damage may result. When installing a new IAC, you may move the pintle to match the measurement of the old one. The force required to move a NEW valve will not cause damage to it. Use engine oil to lubricate the o-ring. Tighten attaching screws to 27 lb. in.
Just a simple reset to make your computer act right . worked on mine !
Normally if you haven't had your IAC out it should not need reset !
Reset the IAC Position
Depress accelerator slightly
Start engine, then release accelerator pedal, run engine for 5 seconds
Turn ignition "OFF" for ten seconds
Restart engine and check for proper idle operation
It is NOT recommended to to push or pull on the pintle of an IAC that has been in service. The force required can damage the threads on the worm drive. Also, do not soak the IAC in any liquid cleaner or solvent, as damage may result. When installing a new IAC, you may move the pintle to match the measurement of the old one. The force required to move a NEW valve will not cause damage to it. Use engine oil to lubricate the o-ring. Tighten attaching screws to 27 lb. in.
Just a simple reset to make your computer act right . worked on mine !
Normally if you haven't had your IAC out it should not need reset !
Last edited by larry00; 05-05-2009 at 09:46 PM.
06-19-2009, 10:48 AM
#9
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Fixed
After long had work and 8 month later I got my 40th FIXED (YAHOO) and it was not the ECM or the PROM. We decided to take apart the distributor it self not the cap. Upon opening it up and as you now they are sealed unit we found the whole inside was rusted solid Replaced with a new one and the car runs awesome. My Mechanic said he could not believe it was the ECM because it ran like it was out of tune. Even though the ecm will do that but he was not convinced until he check out everything first. So if you ever have the same problem you might want to start there first. I have emailed a corvette guru and he said that is a common problem because the distributor is located under the water pump. So thanks again for everyone’s help now off to wash and wax and clean it thru so I can get back to driving.
