2008 Chevy 2500 Running Rough

Greetings all. I have an interesting situation on my hands at work at the moment. I am working on a 2008 Chevy 2500 HD with a 6.0L engine that runs rough, but only at a cold start. I am not the first tech at my shop to have my hands in this truck, so here is what background I know about it. The first time it came in was for a knock sensor code. The tech replaced the knock sensor but was still having the same code. He then diagnosed it as needing a PCM and replaced that as well with the same code still returning. A wire was found to be at fault, and a new wire was run between the knock sensor and the PCM. This took seems to have taken care of that issue as the truck has not returned for the same issue. The truck later returned for a running rough condition and was looked at by a separate tech from the first time. He replaced the intake gaskets along with the throttle body and had the top end cleaned with BG's top end cleaner. This seemed to have mostly taken care of the problem. Now the truck is back, again, and it falls on me. My boss had me replace all 8 injectors as he felt that was where the issue was. Same result. The truck runs extremely rough on cold start and at times almost wants to stall out. I have looked at all the live data and can't seem to find anything that looks out of place for a cold start engine. I did smoke test the intake and cannot locate any leaks. This time, as well as last time from what I have been told, there are no DTCs to work from only symptoms. I have been on and off this truck for a little over a week now and cannot make heads or tails of it. Any pointers on where to go or what to look at would be appreciated. If there are any specific data points that might be useful, please ask for them and I will do my best to get them in as timely a manner as possible.

Hey mdeese89! You can check a few different things during your next cold start:

- Prior to startup, check the Engine Coolant Temperature and compare it to the Intake Air Temperature. They should be within 10°F of each other.

- Does the engine run better once Closed Loop is reached? What are the short and long term trims on both banks once in Closed Loop?

- Current and history misfire counts. Make sure the 'Misfire Cycles' PID is counting while you're watching the miss counts. If Misfire Cycles isn't counting, the PCM isn't watching for misfires correctly.

Tyler, thank you for replying. I was able to look at the things you suggested looking at this morning. Figure I will answer them in the order you mentioned them.

- Prior to startup, check the Engine Coolant Temperature and compare it to the Intake Air Temperature. They should be within 10°F of each other.

• The key on engine off ECT reading was 1°F and the IAT was 19°F - mind you it was probably somewhere in the mid to upper 50's this morning. Interesting thing that I noticed was that both with the engine off as well as running the PIDs would bounce around. With the engine not running the ECT would bounce up to about 75°F and the IAT would bounce to 66°F (not sure if that last bit helps at all).

- Does the engine run better once Closed Loop is reached? What are the short and long term trims on both banks once in Closed Loop?

• The short answer would be yes; it does run better when in closed loop. The odd part is, my scanner has a PID for cold start yes/no and even though the truck had not been started since sometime Friday and sat all weekend it did not consider this morning a cold start. Also, the computer was marking that the truck was already in closed loop after only running for about 4 min., but it did not seem to smooth out until the temp gauge was marking that it was at operating temp.
• The short-term fuel trim for both banks were sitting between -10% and -14% when I noticed the computer was reading in closed loop, but once the truck got to operating temp, the short-term trims started reading more positive (0% to -3%). The long-term trims stayed mostly steady at 8% on bank 1 and 5% on bank 2.

- Current and history misfire counts. Make sure the 'Misfire Cycles' PID is counting while you're watching the miss counts. If Misfire Cycles isn't counting, the PCM isn't watching for misfires correctly.

• There are no current or history misfires showing up at this time. As for the misfire cycle PID, I am unsure how it works as I've never had to use that PID for anything before, but it was counting, though it was very erratic until the truck was at operating temp. By that I mean it would count up, have a long pause and not count any higher, then it would start counting up again. As the engine warmed up the counting became steadier. Don't know if that means anything or not.

Don't know if this information helps or complicates things at all. This truck has had all of us baffled. We are just attempting to get the truck back to the owner running the way it is supposed to in a timely manner. It seems every time the customer brings it in it is at our shop for several weeks while we try and sort out what is going on. If there is anything else that can help, do not hesitate to ask and I will get it for you as quickly as I can.

• The key on engine off ECT reading was 1°F and the IAT was 19°F - mind you it was probably somewhere in the mid to upper 50's this morning. Interesting thing that I noticed was that both with the engine off as well as running the PIDs would bounce around. With the engine not running the ECT would bounce up to about 75°F and the IAT would bounce to 66°F (not sure if that last bit helps at all).

You might be onto something here. The ECT should definitely be higher than that if the ambient temperature was in the upper 50's.

An ECT that's skewed low would cause more fueling during cranking, which could lead to the rough running AND the negative fuel trims you saw.

I'd say give it another check tomorrow morning after sitting again overnight. If the ECT still isn't reading rationally, it's time for some pinpoint checks at the sensor.

Probably a long shot, but has anyone been working on the exhaust manifolds? The ECT is right in front of the #1 exhaust runner, and it's easy to damage if you're trying to get a broken manifold bolt out of the head. Ask me how I know! 

Checked again this morning and the ECT was still reading off. The ECT started at 78  and according to weather channel the outside temp was 62 this morning. While I was still key on engine off it started bouncing between -5 and back to 78. The IAT was bouncing between 21 and 69. Asked the boss about the ECT sensor and apparently we the last tech working on it replaced it (about a month ago). Not going to rule it out as a defect part yet. Just wanted to give feedback as I had a chance this morning. 

mdeese89 wrote:

Checked again this morning and the ECT was still reading off. The ECT started at 78  and according to weather channel the outside temp was 62 this morning. While I was still key on engine off it started bouncing between -5 and back to 78. The IAT was bouncing between 21 and 69. Asked the boss about the ECT sensor and apparently we the last tech working on it replaced it (about a month ago). Not going to rule it out as a defect part yet. Just wanted to give feedback as I had a chance this morning. 

Oh really? Isn't that interesting.  Did you boss happen to mention why the ECT was replaced?

I appreciate the feedback! I think you're ready for some pinpoint testing at the ECT. Backprobed at the ECT connector, key on engine off, during the signal bouncing would be best.

charm.li/Chevrolet/2008/Silverado%202500...omputer%29/Diagrams/

Does either/both wires show the same bouncing? Or are they steady?

So, no one remembers the exact reason why it was changed and it was not documented either. Based on the suggestion of my boss, I ran dedicated wires directly from the sensor to the ECM. The sensor was reading -5 still and was bouncing still. We decided to replace the ECM under warranty. It is supposed to be programmed this morning. I will update after the programming. 

mdeese89 wrote:

We decided to replace the ECM under warranty. It is supposed to be programmed this morning. I will update after the programming. 

That seems like quite a leap, IMO. But hey, I'm wrong all the time! Let us know how the new ECM works out.

Seems like the new ECM did the trick. No more bouncing numbers and started and ran very well from a cold start. Had a very slight miss for about a minute or so, but that smoothed out and the misfire counter didn’t count anything. Boss didn’t seem worried about it as he told me to make sure it was put back together and ready for pick up.