MitchM

I have only 51,600 miles on a used 07 3500 express w vortec , it's garaged and driven every couple weeks on long 200 to 600 mile trips w weeks in between runs. ( it's used as an EMT local backup search and rescue van as it s' a roadtrek 190P conversion w generator, furnace, stove etc. ) . i use only top quality fuel from high volume gas stations and keep fuel tank topped up and ready to go. A random p0300 code started to show in 11/20 only after an hour of high speed driving, never at startup or around town at speeds under 50 mph. since 11/20 i've rx d trying to fix the p0300 with: replaced o2 left bank sensor ( after its OBD code showed and was found during a routine state inspection. at that time it also got a new fuel filter, a major tuneup, new ac delco points, plugs wires and a brand new gas cap . it was still MIL'ing and coding p300 with a steady yellow MIL , only after being run about an hour at 65+ mph. so i took it in to the best local shop with a more sophisticated oBD2 than i have. they checked fuel rail , reported pressures all good, and replaced the crankcase position sensor . that seemed to fix the problem for about a month --then MIL p0300 returned. Next Rx tried was: relearn CCkPos sensor, had a professional injector clean and cleaned up and dielectric'd the ECU ground. (thru this whole time the engine never has lagged, lugged, lost power, ran rough or failed to start. i notice no difference in miles per gallon.) I took it out this weekend and recorded what it was doing : at cold start, no p0300 at all . after running for 1/2 hour at city speeds under 40 mph, no no MIL , no p0300. after 1/2 hour i drive onto the freeway, accelerating to 65 mph / hr going up a long hill, got the MIL w p0300 (per my fixd cheap scan tool) . MIL starts flashing 3x then goes steady yellow as i am going up the steep hill w/ pressure on the gas pedal to maintain 65 mph. at hill top, i let off the gas pedal and start coasting steep downhill, the p0300 MIL goes out completely. On the level , i gently press gas pedal to maintain my 65 mph speed , MIL starts flashing then goes steady. so the p0300 MIL can be instigated by my pressing gas pedal while going uphill at speeds over 60 mph, and also if i'm trying to maintain speed.. MIL flashes while i'm pressing the gas pedal, goes off completely when i'm coasting down hills. i replicated this scenario a dozen times in a 150 mile trip yesterday.
QUESTIONS: is there a messed up fuel rail component, like the throttle body position sensor, that could alter fuel flow to the rail and be giving the wrong fuel mix into random cylin ders to cause the p0300 MIL ? what causes the MIL to go flashing then go steady then return to OFF when i stop pressing the gas pedal ? is there some other component in the gs pedal/ throttle linkage like the main ECU that could be causing such weird MIL behaviors when i press the gas pedal? i live in a rural area w 2 excellent chevy mechanics who use high quality oscilloscopes and computer-connected OBDII professional-level scanners. but the chevy dealers are an hour away and each has a lousy reputation. i've spent about a grand trying to get this problem fixed. All suggestions more than welcome, i'm a frustrated EMT with a nonfunctional 3500 express throwing scary MIL P0300s... I and my mechanics have now tried just about everything this board has ever posted for fixing p0300 s ... all theories and ideas welcome.

MitchM

07 express 3500 (w 4x4 quigley package.) w 6.0 vortec . 52,000 miles. BAD DRVING INCIDENT: while driving at about 20 mph while entering a commercial area, the van 's inexperienced driver who was wearing heavy construction boots on big feet got a big foot caught between the gas pedal and brake pedal, while trying to brake us to a stop. the screaming of the engine alerted me that he was pressing both pedals at the same time, while trying to brake us to a stop. by the time he listened to me screaming at him to pull his foot away and take his foot off both pedals , it took a good minute or 2. during that time we had jumped a curb traveled a good 700 feet with engine screaming and nearly hit a large expensive generator .the engine was screaming like i never heard an engine scream except on a race track. we dont hve a tach but it hd to be redlined. we stopped and cooled it and us down. after 1/2 hour of rest we went on to drive uneventfully for the rest of the 1100 mile trip. (the engine was well maintained before this incident , had just been tuned, had oil changed w proper lube oil etc. we weren't towing anything, thank the gods. )

within a month the engine was throwing a MIL P0300 'random cylinder misfire' and also crankcase position sensor code. engine is now throwing MIL P0300 and flashing when gas pedal is pressed to go up steep hills despite replacing CkCPosS, cleaning injectors, replacing points plugs w all AC delco , etc..
could this overrevving the engine to redline have stretched the timing chain ? or done some other engine-related damage? what should we ask the Chevy mechanic to be looking at ?
should we be looking at possible damage to the gas pedal or throttle linkage ?
the van is pretty much undriveable til we get some diagnosis and get it fixed. flashing MIL codes are serious.

oilcanhenry

Probably floated the valves. That's not at all good. The valves have to transfer heat to the motor which they can't do while floating. Probably looking at a rebuilt set of heads, possibly more. A valve float condition can even cause damage to the tops of the piston even in a non-interference motor. I'd get those heads off and get a look at everything.

Triaged

What year did they start putting AFM active fuel management AKA displacement on demand into the Chevy vans? Lifter problem?

mountainmanjoe

never

Triaged

Well I guess that rules out a lifter. I guess I just assumed they did at some point glad to know I'll never have that problem.

mountainmanjoe

I would start with the absolute basics. You didn't say anything about checking air filter, spark plugs, MAF sensor, cat.conv etc.

hanky

I would have asked for the Freeze Frame info , or the data stream info, but it probably is not available.

I am inclined to agree the MAF sensor may be malfunctioning.

MitchM

thanks for the suggestion -- we did do all the basic stuff: i've rx d trying to fix the p0300 with: replaced o2 left bank sensor ( after its OBD code showed and was found during a routine state inspection. at that time it also got a new fuel filter, a major tuneup, new ac delco points, plugs wires and a brand new gas cap . it was still MIL'ing and coding p300 with a steady yellow MIL...

i am suspecting a stretched timing chain at this point. it's really low mileage for that. there have never been any knocking sounds whatsoever, & no loss of power , or hard starting . we keep the oil topped up religiously and have changed oil and filter every 3000 miles.

mountainmanjoe

I didn't say anything about replacing.
I said CHECK. Do you have visual confirmation on each spark plug? EFI signal? Is the MAF reading correctly? Did you make sure there's no exhaust restriction? Did you check fuel trims?
If you don't do the troubleshooting you're going to run out of ammo for your parts cannon.