92 LT1 OBD1
I have been having an issue where my engine idle becomes unstable after the engine comes up to running temperature. It will rev up a couple of hundred RPM’s then sputter to a halt. I am not getting any trouble codes. Beyond this, the car runs great.
I initially thought I had a fuel issue as it sounds like it runs out of gas. But I have eliminated the fuel system as I am getting the proper fuel pressure and regulator is working properly. The fuel pump is tested with constant voltage continues to run a good 5 seconds after halt. And I have good fuel. I tested the resistance on all my injectors and are within spec.
I did a swap out of the ICM with proper thermal grease, no change. I am getting high res and low res from the opti all the way through the episode and all the way to halt as tested with an oscilloscope. I get #1 spark plug fire all the way to halt as tested with a timing light. I can also see that timing is maintained which eliminates a knock sensor issue. MAP Sensor, TPS, Coolant Temp Sensor, Idle Air Control Valve, Ignition Control Module, Knock Sensors all within specs. I replaced the O2 sensors and did not make a difference.
I did a smoke test on the vacuum system and test was positive. I went through EGR system, replaced the valve and is operating properly. I tested the evaporative system and it is working properly.

After all these tests, I tested the reference voltage and output voltage of the throttle position sensor while the engine was warming up. The reference voltage from the ECM was maintaining 5.02vdc at idle and as I reved the engine. As I was reading .65vdc output voltage at idle, suddenly, the reference voltage spiked to 5.07vdc while the output voltage jumped to somewhere around 1.1vdc. When this happened, idle became unstable, reved slightly and ground to a halt.

My thoughts on all this, maybe I have a voltage regulating issue within the ECM where as it is not maintaining the 5 volt reference voltage.
I have to tell you that I recently had an issue while replacing my alternator, where the tang had broken off the alternator connector, and the alternator was sending my system 17vdc for a few miles before I caught it. I replaced the connector and the regulating voltage returned to a normal 12.5-13vdc. I am thinking I may have cooked something in the ECM during all that.

My plan is to test the output voltage of the alternator to see if it spikes at the episode. If it does not spike, tear into the ECM to see if I can see anything that is possibly burnt before I send it out for repair.

To the experts out there, am I full of crap on this? I mean, the reference voltage spike is the only thing that I can find happening that doesn't seem to be the result, but a cause of the engine dying. Shouldn't the ECM be maintaining the 5 volt reference voltage?

 

Look at your throttle position switch. Then smack your ecm while running and see if it changes. And clean your iac and throttle body look for loose bolts.

 

Over 16v is not good for ecm. 92-3 ECMs are hard to find, to get one rebuilt you have to have one to send in. ECM is only used in 92-93 vette and 93 lt1 z28 or firebird. I would check grounds and make sure there is TPS connector terminals aren't spread. If both are ok if I would try borrow a ECM. If that is not possible I would replace TPS before sending ECM out for repair.

 

That minor fluctuation in 5V reference should not make a huge difference. You've done your home work on this one.

Try running it with the TPS unplugged. Does it do this at idle?

Does the car have the stock Multec I injectors? Did you Ohm test them hot or cold?

You have a oscilloscope great. Do you happen to have a amperage clamp to do current ramp testing on the injectors?

Does the car restart right away or do you have to let it cool down?

This only occurs at idle right? Runs fine down the road at temp?

The TPS could be at fault with that jump in voltage with no other changes.

 

My first thought is dirty/noisy TPS.

 

Stock injectors measured cold.
The engine will start right back up after episode.
I do not have an amperage clamp.
I will try unplugging the TPS. Good call there.
A .07dcv spike on the input seems to initially rev the engine to approx 1500rpm.

I tested the charge terminal of the alternator. No spike during the episode.
I pulled the ECM. Opened the memcap port. Smells slightly like burnt resistor.

I would not think a spike on the input of the TPS with a resultant spike in the output would be the fault of the TPS. But i could see at idle, if the ecm is seeing an opening of the throttle plates, would signal the injectors to spray in compensation. But since the throttle plates are actually shut might flood the engine to halt.

I guess throwing a new TPS would be a lot cheaper than sending the ECM out for repair.

Anyway. Thanks for your help. You have given me a few more things to check.

 

I got an idea.
I could remove the screws on the TPS and spin it up at idle (counter clockwise). It would ramp up the voltage output while leaving the throttle plates closed to simulate and see if it kills the engine. Being electrically connected would prevent any codes from being set for not having the TPS connected.

 

Well, that certainly didn't work. Spinning the potentiometer in the TPS didn't kill the engine. Time for a rethink.

 

I have installed a different ECM and no change. I think I have eliminated the Idle Air Control unit as when the unit is disconnected from the system, the engine still dies. I disconnected the TPS completely, engine still dies. I probed one of the fuel injectors and have noticed at idle, I see .25vdc on my analog meter, but I get a 8-10V spike when the engine dies. I wonder if the ECM is sensing a WOT situation and is max pulsing my injectors, flooding the engine. Please correct me if I'm wrong, but I think the 92 is a Speed Density fuel injection system. In that the ECM senses the engine speed from the Ignition Control Module. I did a swap out for another Ignition Control Module a while back and seem to have the same issue. I think my next step would be to probe the Ignition Control Module at the ECM and see what the voltage does when the car dies. If anyone has any input would be appreciated.

 

Do you have access to an oscilloscope? The spike you are seeing doesn't sound right, but you will get a lot more detail on what is happening at the injectors with an oscilloscope. Did you use the same MEMCAL with the other ECU? You could be losing ground to the ECU as it grounds one end of the injectors, I think, I don't have the FSM in front of me right now.

 

Yes, I have an oscilloscope. Right now, i have the test ECM and different Memcal.

 

I would hook up the scope across one of the injector banks and see what is happening when the car stalls. When you were measuring the voltage was it across and injector bank and referenced to ground or the other side of the injector bank? So, is the test ECM a different MEMCAL number than what is in your original ECM?

 

Have you done a WOT run to see if it can supply full demand for a bit? I would pull out the FPR hose and seal the nipple to prevent air leaks and simulate a WOT condition where it has to constantly put full pressure and do a WOT run to see if it can keep up under load.

Worthless at best unless you do have a burnt coil, misleading at worst. All you know is the coil is good and not if the injector is clogged partially. I have never done an Ohm test. I send them out for cleaning and testing to KNOW as opposed to guessing.

Have you done it under the load of a WOT run? It should have some knock but not excessive. Not sure what to tell you for a stock motor since I don't have it stock.

How did you check the CTS? Did you scan it and hit it with an infra red thermometer? How did you eliminate the IAC as being out of adjustment? Did you see how many idle counts once everything is at operating temperature and everything but the motor is off?