2005 5.3 Tahoe running rough/stalling only when hot

[1919]

Hi guys, I hope yall can help with this. '05 Tahoe 5.3, non flex, runs rough/knocks and stalls after it warms up completely. Runs fine when cold and while in the process of warming up. Usually you can smell gas and funky smell from exhaust while it's running rough. 200k miles. I bought it used, but to my knowledge it's all original. Has run fine for the few years we've had it prior to this. The first thing I checked was fuel pressure. 60 psi running. In spec. Never fluctuates. It's a returnless setup. Originally only had a knock sensor code and downstream o2 heater circuit code. Replaced knock sensors and harness, and cam sensor. Replaced plugs which were covered with burned oil due to valve seals. Replaced crank sensor. Bought 2 new coils and plug wires and moved positions with them until all coils had been checked. Cleared codes and got a 0300 random misfire code once it started acting up again. Unplugged the MAF sensor and there was no change. To me the fact that it only acts up when it's thoroughly hot suggests an electronic component, and I believe I've ruled out everything but the ECU, but wanted to get some outside input and suggestions before messing with that. Any input is appreciated. Thanks.

 

[1919]

Also the rough running does not correspond to open or closed loop. If I kill the vehicle for a couple minutes and restart, it begins in open loop but is still running rough.

 

[Fless]

If it hasn't been done, do a CASE relearn with a capable scanner after having changed the crank sensor. Quick relearn to rule it out for your misfire code.

Also, what do all the fuel trims look like when it's running good, and then when it's running rough?

How does the intake air temp compare when it's acting up?

What's the MAF reading in g/s at idle?

 

[justirv]

Also the rough running does not correspond to open or closed loop. If I kill the vehicle for a couple minutes and restart, it begins in open loop but is still

Also the rough running does not correspond to open or closed loop. If I kill the vehicle for a couple minutes and restart, it begins in open loop but is still running rough.

You can also check for leaky intake manifold gaskets. They shrink with age, and add unmetered air, causing a lean condition. Use you favorite spray in the area and observe any change in RPM's.

 

[1919]

If it hasn't been done, do a CASE relearn with a capable scanner after having changed the crank sensor. Quick relearn to rule it out for your misfire code.

Also, what do all the fuel trims look like when it's running good, and then when it's running rough?

How does the intake air temp compare when it's acting up?

What's the MAF reading in g/s at idle?

I don't have the capability to do the relearn. The sensor I bought said the relearn wasn't necessary. In any case, changing it made no discernable difference in the way it runs.

I also am unable to see fuel trims. All I have is a cheap code reader.

IAT and MAF appeared normal, but it's been a few weeks since I was at that point. I'll double check tomorrow and post actual values. Unplugging the MAF entirely also made no discernable difference.

All replacement parts were Delco also, no cheap junk.

 

[1919]

You can also check for leaky intake manifold gaskets. They shrink with age, and add unmetered air, causing a lean condition. Use you favorite spray in the area and observe any change in RPM's.

Intake gaskets were changed when I changed the cam and knock sensors. If a vacuum leak in the intake area was a factor, would it cause problems before the engine warms up also, or only when hot?

 

[rockola1971]

Intake gaskets were changed when I changed the cam and knock sensors. If a vacuum leak in the intake area was a factor, would it cause problems before the engine warms up also, or only when hot?

Unmetered air really shows its ugly head when in closed loop. Spray some starting fluid or the like around the intake manifold flats at the heads and see if RPM raises or sputters. Have any current UPSTREAM O2 sensor codes? Active FLASHING check engine light? All plug wires should be new or newer at 200k miles now. Saw that you did say you put in new plugs. The relearn for crank/cam sensor is mandatory.

 

[1919]

Unmetered air really shows its ugly head when in closed loop. Spray some starting fluid or the like around the intake manifold flats at the heads and see if RPM raises or sputters. Have any current UPSTREAM O2 sensor codes? Active FLASHING check engine light? All plug wires should be new or newer at 200k miles now. Saw that you did say you put in new plugs. The relearn for crank/cam sensor is mandatory.

I'll spray it today and see what happens.

Haven't had any upstream codes at all. O2 sensors were changed preemptively a few years ago when I bought the vehicle. In my head I ruled out O2 sensors because it acts up in both open and closed loop, but I don't know if that's the right way to look at it.

No check engine light, but I suspect it may have been tampered with if it's not coming on during all this.

Thanks to everyone for the input thus far.

 

[1919]

Someone I talked to also suggested injectors. Could an injector be sticking once it warms up?

 

[strutaeng]

Someone I talked to also suggested injectors. Could an injector be sticking once it warms up?

Umm, yeah that's possible. Injectors hardly ever fail, however.

You need a scanner capable of reading misfire counts on each cylinder. Then swap plugs, wires and coils and see if the misfire moves along.

If you have a misfire, your O2 waveform is going to be of of wack. Focus on finding and fixing the misfire first would be my advice.