I recently bought a 2007 H2 Lux with 92,000 miles on it and a periodic CEL that multiple mechanics couldn't fix. I've seen numerous posts where people have had their CEL start blinking then go off or sometimes blink and then stay on and the code comes up P0300 misc. misfire on various cylinders. I spent a lot of money trying to cure it. Hopefully this will help others before they spend a lot. In my case I found that if I traveled through rolling hills on cruise control set at 65 the CEL would come on going up hill in a geared down situation and when I would crest the hill just before it would shift back up or just after and as I would then start to decelerate going down the hill it would set it off. The engine never ran poorly or seemed to miss or anything.
Here is what my mechanic did to fix this. (Not necessarily in order and not all in one visit.) Two tanks of quality premium fuel with injector cleaner. Made sure computer was current on updates. New spark plugs. New spark plug wires. New coils. Check vacuum for leaks. Check fuel pressure. New intake manifold gasket. New knock censors, New exhaust gaskets. Scrubbed fuel system twice. Full compression test. Checked valve lifters. Gave up.
I researched everywhere I could. Found it was a common problem, but never found a cure. Then I found buried in an old forum post, that a very successful cure was to do a Crankshaft Relearn. Did that. Boom Fixed.
If you aren't familiar, all that is, is connecting a quality OBD to your rig, click on crankshaft relearn, starting the engine, give the gas a quick goose and release up to over 3,000 rpm. Done. Really. That's it. The mechanic even did it for free. I have driven it about 300 miles since with no CEL.
I don't know if this will cure all the cases, but it did mine. I thought it would be worthwhile to post a new thread here. Good luck!

 

Thanks for taking the time to share. That's great info.

 

temp fix, then boom, replace engine.

 

Hummerz, I can't tell if you are being sarcastic or if this is in fact what I can expect. Thanks.

 

everything you did is sound advice i do not foresee any boom in the near future. at least not from what you stated was done.

 

Post updates.

 

hummerz, i'm interested in your theory as to why a crank relearn could cause a failure, please explain

Neal

 

Over time the the plastic on the sensor that can deform some and cause the sensor to slightly change position and the crank bearings as well can cause it to be off some as they wear can cause a slight variation on the position of the sensor in relation to the reluctor wheel. On the older engines it can even cause the sensor to contact the wheel. GM even makes a shim kit to reposition it slightly. I think that was mostly on the older 4.3, 5.0, 5.7 engines though. Symptoms can be anything from terrible fuel economy to unable to start. Either way, its generally not a problem but I've seen a crank variation relearn smooth a good number of engines up over the years and it hurts nothing to try. Also costs nothing to try if you have a Tech2 or a diagnostic tool that can do a relearn. I actually need to chim the one on an older Blazer we have with the 4.3. Classic symptom on the older engines is they will start rough and at low rpm's and then settle down after about 10 seconds or so which is what ours has started doing.

 

MixManSC nailed it.
Crank relearn merely resets the calibration of the crank shaft sensor to the crank trigger wheel due to slight variances in the clearance between the sensor and the trigger wheel which affects the signal strength the PCM sees. Usually only needed if/when crankshaft is swapped out or a new CKS, (Crank position sensor) is installed or a new PCM is installed, typically throws a P1336 code for Crank Relearn, (I use HP-Tuners for this on LSx conversions from time to time). For what it's worth, your engine will not blow up from "crank relearn" being performed.

 

I do understand MixManSC's explanation and believe there is a lot more to it than the crank sensor and the reluctor wheel. GM says that any changes to the rotational components can cause the P0300 code to be triggered. The ECU monitors the acceleration and deceleration of the crank and based on these events can sense a change in the learned acceleration and deceleration of the crank, if these changes continue for a certain number of rotations the code can be set. These rotational components include the balancer and torque converter along with the entire rotating assembly along with changes in the ignition events, there a multitude of things that can cause large enough changes to trigger the P0300 code. I don't disagree with hummerez that it could be possible that there is a component failing with the ability of causing a failure but I don't think that performing a crank variation relearn would resolve the P0300 in this case. The op commented that his mechanic updated the PCU, just this alone can cause the need for the crank variation relearn. The op commented that he did not list the repairs that the mechanic had performed in order so its possible he actually found the issues but due to the absence of performing the crank variation relearn after the PCU update caused the issue to continue.

The crank variation relearn is easy enough and usually cheap enough to get done that I think its a viable shot to fix this issue. If the true cause of the P0300 is serious enough this will not resolve it and trouble shooting will continue for the true cause. I do feel that this is a easy procedure to perform and should be tried before getting to deep into replacing parts if you cannot truly identify the real cause.

Neal