93' Coupe - Operating Temp Hesitation/Stall Problem

[XCTrackGuy]

Sorry for the long post...


I have a 1993 4spd auto Corvette coupe (only 16k miles) that's been misbehaving since August 2010. Back in August it developed a stalling problem that seemed to occur at high engine temperature. It would stall, sometimes run a bit rough beforehand, but it was like someone turned the key and shut the car off usually. It wouldn't start easily afterwards, and if it did, it would die fast, until it cooled down. RPM didn't seem to matter while driving, it would just die. Occasionally I'd get a DTC36 (optispark high res code), but usually no error codes. In March or April I replaced the ICM and ignition coil and that seemed to fix the problem: no more consistent high temperature stall.


However, a few weeks ago I discovered there is still a problem. The 'vette would intermittently run rough at idle, like it was trying to stall, and when you would accelerate with anything more than a slight throttle opening, the car had trouble revving, would pop and sputter, etc. I also got a DTC36 again; so I replaced the optispark this past weekend with a dynaspark unit.


The dynaspark didn't fix the problem. It still idles roughly when it gets hot (200+ degrees), seems particularly lopey, etc... The car also sounds like it is intermittently misfiring. If you rev it gently, it revs fine, but if you step on it, the revs drop and the car tries to die (the more throttle you apply, the more aggressively it falls flat). However, if you rev it gently to say, 2000rpms, and step on the gas, it runs fine (this problem gets worse the hotter it gets). No error codes, except for an intermittent DTC22 (low voltage to the throttle position sensor). Naturally, if you unplug the TPS the car still hesitates under throttle as previously described.


While running the motor at night, I noticed that if you rev the engine, the spark plug wires occasionally emit blue flashes around the plug boots. I'm not sure if this is arcing or not, but I decided to replace the plug wires. I replaced the passenger side today (didn't seem to fix anything; both sides still emit some blue flashes as you rev it, more prominent when the motor is hot/and/or trying to stall) and I'm doing the drivers side as soon as I can get an extra set of hands to help me out (maybe this weekend)...


Any thoughts on what may be causing this problem?

​

 

[John Robinson]

Despite the low mileage you have problems with the wires and connectors on the right side of the engine. One thing that is consistent with any car is that with time not miles things go bad. Start with cleaning all the ground wire connections (can do a search for for there location) Your first problem sounds like a dirty connector or broken wire with the wire plug that goes into the opti on the top. The TPS problem is either a failing TPS or more likely a bad wire in the wire harness or plug. On my 93 the wires were pulled out of the plug pins and the ground wire was broken between the connector and the ground bundle in the wiring harness. From my experience and others who like me have fought what you describe it almost always turns to bad wires and connectors needing cleaned. Just for your knowledge the ecm reads the ground voltage to control fuel, spark, wither in open or closed loop etc.

Again I can not overstress the need to clean all the grounds. Remember this is a plastic body car that requires many ground wires and they all must be clean. Also a DTC code does not tell you the part is bad but rather that there is a problem with the signal going to the ecm and it will drive the ecm crazy trying to correct all the systems to bring the car into compliance with what it is programed to see. I would be surprised with the low miles on the car that the actual sensors are bad. They have simply been in hibernation waiting to be put to use.

 

[boomdriver]

Sorry for the long post...


I have a 1993 4spd auto Corvette coupe (only 16k miles) that's been misbehaving since August 2010. Back in August it developed a stalling problem that seemed to occur at high engine temperature. It would stall, sometimes run a bit rough beforehand, but it was like someone turned the key and shut the car off usually. It wouldn't start easily afterwards, and if it did, it would die fast, until it cooled down. RPM didn't seem to matter while driving, it would just die. Occasionally I'd get a DTC36 (optispark high res code), but usually no error codes. In March or April I replaced the ICM and ignition coil and that seemed to fix the problem: no more consistent high temperature stall.


However, a few weeks ago I discovered there is still a problem. The 'vette would intermittently run rough at idle, like it was trying to stall, and when you would accelerate with anything more than a slight throttle opening, the car had trouble revving, would pop and sputter, etc. I also got a DTC36 again; so I replaced the optispark this past weekend with a dynaspark unit.


The dynaspark didn't fix the problem. It still idles roughly when it gets hot (200+ degrees), seems particularly lopey, etc... The car also sounds like it is intermittently misfiring. If you rev it gently, it revs fine, but if you step on it, the revs drop and the car tries to die (the more throttle you apply, the more aggressively it falls flat). However, if you rev it gently to say, 2000rpms, and step on the gas, it runs fine (this problem gets worse the hotter it gets). No error codes, except for an intermittent DTC22 (low voltage to the throttle position sensor). Naturally, if you unplug the TPS the car still hesitates under throttle as previously described.


While running the motor at night, I noticed that if you rev the engine, the spark plug wires occasionally emit blue flashes around the plug boots. I'm not sure if this is arcing or not, but I decided to replace the plug wires. I replaced the passenger side today (didn't seem to fix anything; both sides still emit some blue flashes as you rev it, more prominent when the motor is hot/and/or trying to stall) and I'm doing the drivers side as soon as I can get an extra set of hands to help me out (maybe this weekend)...


Any thoughts on what may be causing this problem?

​

I agree with john...
track down the grounds and clean, then pull & inspect every electrical plug in the eng bay and look for a broken wire or corroded plug. If that does not help test the fuel pressure and finally test the EGR. If you have MAF or MAP for air mngt, you might want to look at those as well.

The answer is usually electrical. With this many systems that interface, its sometimes hard to ID a particular trouble maker..

The 'arcing' around the boots is normal. Thats the spark "aura" that can be seen thru the insulator. No particular problem...just the sign of a strong spark.

 

[XCTrackGuy]

I guess my electrical knowledge isn't up to snuff, but I don't understand why these issues would only occur once the car is at or above operating temperature. I would figure if a ground was bad, the issue would always occur, or occur intermittently, not consistently at temperature.

 

[John Robinson]

Let me try to make this as simple as I can. Before automatic chokes you had to use a hand choke to start the car and then when it had run a little bit you would feather the choke until the engine warmed up enough that it ran smoothly. Now with the new electronic systems the ECM (basically a small computer) is programmed to do certain things to make the engine start and run smoothly. The old hand choke needs are now taken care of in what is called the Open Loop program. In this program the ECM will over ride or ignore some signals from some of the sensors on the engine. Now when the engine gets above 140 degrees the ECM wants to go into closed loop (Choke fully off). Now the ecm will be looking for certain input voltages from the ground wires for each sensor. If it does not see what it wants it than will try to adjust fuel rate electrical timing and on and on. If the problem is bad enought that it can not correct it to it's satisfaction the ecm will go back into open loop (sometimes called limp home mode). That is why your car may run fine until it gets to temp and then go bad.

What some of us are saying is that before you through a lot of parts at the problem you need to check that the grounds and wiring are secure. Some of us have started the parts exchange routine until we learned better also. If you get a code it does not necessarily mean that sensor is bad it only means that the ecm is not receiving the proper signal from that sensor.

I hope this will help you understand why the problem only appears when the car is warm. In a nutshell you have two tunes on your car one is for start up and the other is for continuous running

 

[Robertwav1]

In March or April you mentioned replacing the ICM. It cleared things up for a while?That would be my first pick on a temperature issue and the car dying. Unfortunately the computer only knows that the car was shut off, can't determine if it's by the key or not. Heat conductive grease used on the ICM? May worth be checking it again. Also do a fast check on the battery.

 

[XCTrackGuy]

John,

By that logic, when the car is in open loop mode, it should run fine. Is there a way to keep the car in open loop to test this hypothesis?

 

[John Robinson]

John,

By that logic, when the car is in open loop mode, it should run fine. Is there a way to keep the car in open loop to test this hypothesis?

I am not aware of any way to tell the ecm what mode you want it to be in. Perhaps someone with more knowledge then me can expand on this. As I said I was keeping a very complex subject as simple as I could since I work on the KISS system. With any luck GMJunkie will chime in. He is probable one of most knowledgeable people on this forum when it comes to what makes these cars tick. I will say this I went through probably 6 to 8 months with my car running good then bad. because of wiring problems and when I got them sorted out and finally understood that this system is very ground sensitive and focused on that I had the car up to full performance in just a couple of days. All I did was replace some broken wires in the wiring harness from the water pump temp sensor and the tps and I did not have to replace any of the sensors. For some reason these wiring harness's are notorious for going bad.

In thinking about what you asked I believe you already have proof that there is a problem with the ecm getting all the correct signals when it is warmed up. In open loop the ecm I believe ignores the signal from the tps
Hope this sheds some light on the tps for you

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Throttle Position Sensor (TPS)
Copied from 1993 Service Manual
The Throttle Position sensor (TPS) is a potentiometer connected to the throttle shaft on the throttle body. It is a potentiometer with one end connected to 5 volts
from the ECM and the other to ECM ground. A third wire is connected to the ECM to measure the voltage from the TP sensor. As the throttle valve angle is changed (accelerator pedal moved), the voltage output of the TP sensor also changes. At a closed throttle position, the voltage output of the TP sensor is low (approximately .5 volt). As the throttle valve opens, the output voltage should be approximately 5 volts.

By monitoring the output voltage from the TP sensor, the ECM can determine fuel delivery based on throttle valve angle (driver demand). A broken or loose TP sensor can cause intermittent burst of fuel from the injectors and cause an unstable idle, because the ECM detects the throttle is moving.

If the TP sensor circuit is open, the ECM will set a DTC 22. If the TP sensor circuit is shorted a DTC 21 will be set. A problem in any of the TP sensor circuits will set either a DTC 21 or 22. Once a DTC is set, the ECM will use a default value for TP sensor, and some vehicle performance will return.

A personal note, when my TPS failed it DID NOT set a code other than to tell me the ECM was bad.

 

[boomdriver]

grounds are an issue because many of the sensors are arranged in a 'loop' that is simple resistance to or from the ECM. If a ground is poor in that circuit the sensor value changes and can get out of normal operating parameters. Like the temp sensor for example...its a variable resistance sensor. If that scale changes due to poor ground contact (many do ground thru a wire and not frame) so does the value of the data to the ECM. Ground path in reference signals can change the reference value as well.

The temp sensor is a good place to look for a problem BTW...

You can force it to stick in OL mode by D/C the MAP or MAF sensor or the O2 sensor(s). Without this fresh data stream its forced to operate on programming with only the input of the TPS. This should be easy to verify by the service lite remaining on.

My car is MAF and had issues some time ago and I ran it for quite a while with the MAF unplugged just so it would run. Operation was near normal but the economy was noticably reduced and emissions were increased. Running OL isn't going to harm it short term, but certainly not something I'd do for an extended period.