P0300 Trouble Code, Multiple Cylinder Misfire

[nickmcc]

I have a 99 Chevy Tahoe, 2 door, 5.7 liter, Throttle body injection with 96,000 miles. I have owned this vehicle for about a year and a half and sometimes notice a miss, this is usually at around higher speeds and was very minor but it bothered me. The SES light did come on awhile ago and I had it scanned (with a Snap-on, and another brand) and got the infamous P0300 trouble code. This was the first time that the code showed up and went away after replacing the cap and rotor(plugs and fuel filter recently done also). I still noticed a miss sometimes but the light did not come on. My tranny went out, I had it rebuilt, and about a week later started to notice a more noticeable miss, SES light came on, had it scanned, got P0300 code. So I tested the coil, replaced it, truck ran great for next two days. Third day started missing again, light came back on. Miss is now more present than ever before, it is most noticable at 1200-1500 rpm, and somewhat noticeable at others except idle. I have tried different brands of fuel from different stations with no change.
My questions are: What should I check next, if it is the EGR valve, wouldnt that trouble code be stored in the comp., could this be due to a bad fuel pump or fuel pressure, (pump is noisey, (loud), always has been). This problem was very intermittent now it is not, if that is any help, (so I dont think that it is a burnt valve), also wouldnt that point away from a vacuum leak? Any help is greatly appreciated. Thank You.

[foureyedbuzzard]

Yuck.

Do you have the freeze frame data from the stored MIL code. If the MIL(SES) has not been reset, you should be able to retrieve this data. This will give you a snapshot of what was happening in the engine at the moment the MIL was stored. Look for any sensor readings that seem way off. Also look to see what parts of the OBD II status systems are currently "incomplete". Note that it takes several driving/warm up cycles to get a complete status on certain systems such a cat coverter etc. Many times an EGR won't set or store a code(even a pending one) but the EGR system will come up incomplete. OBD II isn't perfect, there are known anamolies with certain makes and models that don't store codes as designed.

Generally a bad/dirty EGR will give a rough idle as it is bypassing exhaust gas at idle, but a sticky one could potentially give you problems at cruise, especially if it isn't modulating properly.

If you want to try something reasonably quick and cheap, a cleaning of the throttle body and EGR couldn't hurt.

[curtis73]

I kept getting random misfire codes. Mine ended up being MAF sensor. I always wondered why it didn't throw the MAF input code, but its a mystery of OBD2.

It does sound like a nasty EGR, but if it was that bad it should throw a code. At any rate, with that many miles the EGR valve should be replaced. Mine lasted about 36,000 on my 96 GM. Not one of their stronger reliability parts.

Otherwise, I'd refer to foureyedbuzzard for his excellent post.

[grapony]

Originally Posted by curtis73

I kept getting random misfire codes. Mine ended up being MAF sensor. I always wondered why it didn't throw the MAF input code, but its a mystery of OBD2.

It does sound like a nasty EGR, but if it was that bad it should throw a code. At any rate, with that many miles the EGR valve should be replaced. Mine lasted about 36,000 on my 96 GM. Not one of their stronger reliability parts.

Otherwise, I'd refer to foureyedbuzzard for his excellent post.

It could be low fuel pressure. You need to test the fuel pressue while driving and see if it drops out of spec.

[nickmcc]

I have replaced most of the items controlling spark including plugs, wires, coil, cap, rotor, with no change. This morningit was pretty cold 24 degrees and the truck ran great, once engine was warm, I shut it off, came back out and it didnt want to start right away, then started and ran like crap. What does this point to? Also thanks alot to everyone that has submitted so far I really appreciate it.