2004 Sierra 1500 runs like crap once in a while

[Tomh730]

I’m pulling my hair out here, I have 2004 Sierra with 120k on it and been having issues with it missing on 1-2 cylinders from time to time. It’s been going on for approximately a year. It started out that when I would start the truck it would be missing on 1 cylinder and I get the code of po300. The truck would run like crap for minute up to 10 minutes any then clear up and run fine. The best part is it would do this sometimes 3x’s a week and then run just fine for up to 3 weeks. It didn’t seem to matter with the weather, could be raining and it runs fine. Once the truck was running it runs just fine. I’ve replaced the plugs, wires, and coil packs. Recently I’ve had the truck start to miss on 2 cylinders and then it’s hard to make it even idle. 1st time I tried to get it clear by trying to drive around the block and that didn’t help, died in the driveway. After it died 10x’s in the driveway it sat a few minutes, I tried it again and ran just fine. Two days later when I was going to work it did it again, I had to make it go to get to work, ran like crap the whole way, approx 9 miles, and when turned the corner to work it started to run fine!!!!!  I’ve had it at a local shop now for 3 weeks now, and he only had it act up 1 day. He confirmed it was missing on 2 cylinders. Spark plugs are soaked, spark tester shows spark, has compression, he checked the injectors and said they seemed to sound the same as the others. He did get a code for crank position sensor and replaced that, no help. The next morning he went to move the truck and it ran fine!!!  He’s not sure where to go from here, it’s hard to track it down when it doesn’t always do it. Any help would be great.  Could it be a bad ecm ?

[dtnel78]

Has the shop bothered putting it on a scope? A scope can catch intermittent issues better than not but only when they show themselves. Fuel pressure? What's fuel trims at? Have you checked the fuel pump pressure resting with key forward, running and driving?

If they hook it up to a scan tool and watch certain live data while driving if it acts up then maybe they'll see something.

Check the plug connectors for the coil packs, check them for corrosion, tightness, etc. The P0300 is random multiple misfire if I recollect properly. IS there any other codes such as a cylinder misfire that could be giving a 2nd , 3rd code that you forgot to mention? Main thing is compare scan tool data when it's running fine vs running bad.

When it had the code it should've stored live data about the conditions at the time it set that could've been retrieved. IF the code is still in there then the data could be accessible.

Did they relearn the crankshaft position sensor? I can't recall if it needs done on that year model but I do know it needs done with some stuff when you replace it.

Definitely post back what you eventually find that solves the problem as it could help the next person plus it would be good to see what the end result yet.

I wouldn't call a ecm yet on it.

Head over to Scanner Danner .com , register and post the question in their forums. Very knowledgeable people there and it may get answered fairly quickly.

Sent from my SM-N960U using Tapatalk

[Tomh730]

Hey dtnel78, thanks for the response. He hasn’t scoped it yet, he’s been driving and it hasn’t acted up again to scope it.  The plugs, wires, and coils are all new and didn’t make a difference from the old. It usually doesn’t have any other codes than po300. It had crank position sensor twice, but that’s when we were trying hard to keep it running, so I figured with as rough as it was running it threw that code?  After the new sensor didn’t work I think he put the old back in. As far as I know there hasn’t been any stored data after po300 code. Hopefully it acts up again soon, I have been driving a truck from work for 3 weeks now!!  Thanks again and I’ll let u know if we come up with something. 

[carkhz316]

Not that it's likely to be the cause of your driveability issues, but you need to perform a crank angle relearn after the CPS replacement, otherwise you will get a intermittent P0300, particularly at highway speeds.

[Tomh730]

The mechanic did do a crank relearn after. He also checked fuel pressures and were good. He went as far as to pull the fuel rail to make there wasn’t an injector leaking by, all looked good.he couldn’t get it to act up again to put a scope on it and I needed a truck back.  I’ve been driving for 2 weeks now and have had 3 episodes where it has a hell of a time idling but with feathering the throttle I can get it to barely go, and after approximately 6-8 miles it smooths out and runs fine!!  Exhaust reeks of fuel but straightens out. Can’t even make it to the shop with it still running crappy!!!  Tonight it did it after work and took longer than ever to clear up!! Didn’t show any engine codes this time either. I’m at wits end with this damn thing!! I just ordered a new ecm/Ecu for it. We’ll see what happens. I’ll keep you posted

[Dnt]

I have a guess, it is a long shot but a real easy check. Have the mechanic check for a bad Diode in the alternator. I have seen this happen where the spike can cause the electronic control system to keep rebooting causing mysterious intermittent issues.   

[Rick2500]

Check the wiring on your O2 sensors and mass airflow sensor as well. Mine was pinched and cause my engine to misfire every now and then too. 

[Tomh730]

Well we finally figured out what it wrong with the truck. I have 1 sometimes 2 valves that are sticking open!!!  2 yrs ago I replaced the heads on the truck because they were cracked, those great castech heads that gm did nothing about. At that time my local gm dealer said there were no replacement heads available anywhere in the USA. I bought what I thought were good aftermarket heads!!!  Guess not. After a year the truck started burning oil and now this. Just put all new brake lines on it and now the abs pump shorted out and fried the abs module!!  And now to top it off the transfer case just started to leak from the seam in the case, not input or output. I’m done with gm vehicles, I’ve had 2 crappy ones and that’s 2 too many.