2000 Silverado 2500 6.0 121k miles. It started with poor running at cold start. Let it idle for 5 minutes, could drive but would buck if given too much throttle. Checked it and code came up for bad Bank 2, #2 O2 sensor (can't member exact code). Replaced it, and ran a little better for a few uses (not a daily driver). Now when I cold start it, it chugs and bucks so bad sometimes I turn it off for fear of other damages. It will do this for 5 minutes, as it gets warmer it gets a little better. Takes over 10 monutes before I'm able to drive. When it gets to full temperature, it clears up and runs so smooth it's hard to tell it's running. After that it's great the whole day. I can stop for 2 hours and it still runs fine. If I check codes, while running rough, it's giving P0336. Once warmed up and running smooth, no codes and CEL goes off. Also, when chugging and bucking at idle before warm-up, there is a LOUD abnormal sucking sound, coming from the air cleaner (whole K&N intake kit), that goes away when engine warms-up and smooths out. I have never changed fuel pressure sensor or evap solenoid ( original round unfiltered one).
Just hooked it up and took it for a long drive. Definitely gets P0336 (crank position sensor) code while warming up. With thoroughly warm motor, runs smooth and if I clear codes, the CEL light goes off and doesn't throw any new codes. O2 sensors seem to be working fine no missing while idling or cruising. Did a WOT pull through2 gears, all good, but got 2 small pops from intake around 4500 but climbed good past that and fine in second gear.
The Tach never misses a beat the whole time

 

If the PCM is being told the engine is warm when it is not , could be part of the problem. A good scan tool will be able to show this in data stream info, worth checking out.

 

Nothing is misreading the engine temperature. When the gauge says it's hot, it's hot. My point was it runs fine when warmed up, my problem is only before it's thoroughly warmed up. The scope says the coolant temperature is very close to what the IP gauge. I was reading about a process that will make the ECM relearn the crank position sensor.. I was hoping someone might give me advice in that direction.

 

Sounds like a intake leak to me. They do that over time. Replace the intake manifold gasket. I swapped one out and paid attention to torque specs on install and it still leaked. So I went a bit past recommended specs and it worked. Just be very cautious if you do the same, might squish the gasket too much and ruin it
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