damnyankee36

I know this is a shot in the dark for someone to troubleshoot this online, but here goes...

I have a standalone 2006 GTO LS2/T56 swap with a bone stock tune and factory engine components; no speed parts. After the engine is running at normal temps the engine runs rough for a second and stalls. The mixture smells very rich when this happens. It usually takes up to an hour or two to happen. It usually happens when the engine has been idling for a while or going down the road at a crawl. If I can get the RPM's up fast enough I can usually prolong the eventual stall, all the while while running rough and rich. The engine is not overheating BTW.

I can wait a few minutes and it usually starts and might run for a half hour or more or might quit again in a few minutes.

I feel that it might be heat related, like a sensor that eventually gets too hot and fails. Since it takes a while after the engine is at operating temps, I initially thought of the two components not mounted directly to the engine, the MAF and ECM. After a stall I shot them with a freeze spray, basically 134A, and restarted to see if that would help. Didn't seem to. I even tried to use my heat gun on the MAF to hasten the heat soak process; nothing. The problem is too random. On both units I never saw more than 140 degrees. But because it can usually start within a few minutes, I don't see how a sensor could cool off that fast.

A shop scanned the engine and said everything is running great. (Although I smell what I think is too rich of a mixture all the time.) He never had it stall on him to see what was happening at that instant.


Any ideas?

Injuneer

If you find a shop with a scanner that has "GM Enhance Parameters", it can read the misfire count by individual cylinder. Might help point in the correct direction.

Probably wouldn't hurt to borrow or buy an OBD-2 scanner and check for yourself. Even the cheap ones will read codes.

A faulty coolant temp sensor can make it run rich. Generally, it takes a total failure to set a code, while just a large error in the sensor readings can throw the A/F ratio into the rich cold start condition. But a large error may not set a code. The problem with the coolant temp sensor can be intermittent, particularly if its a problem with the wiring harness. I'm helping someone now with an earlier engine that has totally random reading of the coolant temp sensor, ranging from -40*F to the actual operating temp of 210*F. No rhyme or reason to the readings, and so far we have not been able to track down a problem with the wiring. The sensor (including several replacements) all meet spec.

Do you have that setup in an F-Body? Does it use a throttle cable, or the electronic throttle control? F-Bodys all use a throttle cable, while the Corvette and truck LSx engines use the electronic throttle actuator. Have no idea what the GTO uses.

damnyankee36

Thanks Fred. You're not the first person to mention the ECT sensor. Reading your explanation, it makes sense now. If it were to suddenly read cold it would richen the mixture when the engine doesn't need it.

I think I will try it!

It has DBW, BTW.

damnyankee36

In case it's the wiring, would the ECM get a "cold" signal if there's an open?

Injuneer

An open circuit is an infinte resistance. Infinite resitance will cause the PCM temperature to bottom out at the lowest value it can read. But in that case the PCM would set a code for a faulty circuit/sensor.

I don't know the specifics of the actual low temp that would set the code. That would be found in the factory manual for the year the PCM is from.

damnyankee36

Well, it's a no-go on the temp sensor. Died again.

Someone noticed how I had the fuel line clamped to the back of the head. He suggested vapor lock. I suppose vapors could be making to the injectors causing it to die. After it sat for several minutes, I could put my hand on the braided hose. It was definitely hotter at the clamp area. It was even cooler in the area over the header collector, (about 8" above). Earlier, I also shot the hose with an IR thermometer, it read no more than 170-180 degrees. I don't know what the temp needs to be for it to percolate.

I moved the line away from the head and right up against the firewall, about 2-3".