P0300 Trouble Code, Multiple Cylinder Misfire

[nickmcc]

I have a 99 Chevy Tahoe, 2 door, 5.7 liter 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 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. 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, (mine is noisey(loud), always has been). This problem is also very intermittent if that is any help, (so I dont think that it is a burnt valve), also wouldnt that point away from a vacuum leak? Another theory I am pondering is that it might be water in the fuel? Any help is greatly appreciated. Thank You.

[abaird]

I don't think it is the egr valve cause I have a SES light that goes on for a few days and then off. Had it scanned and it is a bad egr valve however I haven't had it fixed yet and haven't noticed any difference in the way it runs. Maybe that can narrow down your problem a little. Hopefully someone else can provide more info.

[J-Ri]

Check the connections for the coil. Sounds like the coil isn't charging fast enough to supply spark at higher RPMs. Could also be the ignition module.

[nickmcc]

Coil is new, connections good, truck ran rough last night and this morning, ran ok this afternoon. stumped.

[planedude]

Try some hi test fuel. Also did u put delco plugs in it?

[skipr]

are you using a generic code reader, A good mastertech scanner will be more specfiic and give u cylinder # ex: P0305, P0307...etc. and you can watch the misfire counters at diff RPM's. Gives you a better look to narrow down the intermitent.

[Fireplug]

FYI
I read somewhere on the net about the distributor having problems and setting the P0300 code. The shaft wares out and then starts a slight wobble and the rotor misses the plug wire and then the engine miss starts. Not saying that is your problem but it might be.

[Thunderbolt]

It could be the infamous poppet valve sticking in the injectors. The scanner I use will show a PO300 random misfire when it is a minor misfire or it will show a PO301 - 308 if the misfire is more severe and specific. I would try running a couple bottles of injector cleaner through it. You will want to use stuff with Techron in it as GM says it is the only type that is effective. The dealer sells it for $20.00, But the same stuff at wal-mart is only $6.00. I use Chevron with Techron and it has cleared mine up. Worn distributor bushings can cause it too, But don't think that is as common. I have 200k on mine and the bushings are still fine although every one is different.

[nickmcc]

I have throttle body injection on this truck, not Multi-Port like some. I have put Heet in the gas and tried the Techron cleaner, but it doesnt seem to have helped so far. The miss is constant now and noticeable between 1000-1200 rpm, I am still stumped, appreciate all the help so far.
Thanks

[Fireplug]

Can you get it to miss while in park and the throttle just cracked open or do you have to be driving??

[nickmcc]

I have to be driving.

[nickmcc]

Correction, today it seemed even worse, and this is the first time that I have been able to notice while NOT driving.

[Fireplug]

Well the next thing you need to do is ti find out if its a fuel system problem or spark problem, Lets deal with spark first.
1. Check all connections on dist coil ign wires looking for anything loose or a crudy gunk build up on anything.
2. Get a small plastic spray mist bottle and fill it with water. With the truck running at idle spray the entire ign system with the water mist and look for any arcing. Look close at everything because its easy to miss a small arc in the daylight. Look over the ing wires real close for any white spots because that is a good clue of lite arcing of the ign wires. If you have a helper have them block the wheels and put on the e-brake, start the truck and put it into gear and with 1 foot on the brake pedal and the other foot raise the rpm to about 2,000 and remist and look for any arcing.
3.If nothing found with the rpm still at 2,000 and if the miss is there remove 1 spark plug wire at a time looking to see if the engine rpm changes if you come across 1 that has no change then that is the cylinder we need to look at

[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.

[planedude]

get a fuel pressure guage and put on it, I just had a 98 doing the same thing and it was the fuel pressure reg.