p0300 code. NEED URGENT HELP!!!

[spivey94]

the engine seems to run ok in park and neutral and when at higher rpms when in drive, but when its at idle in gear it misfires really bad. any ideas???

1998 GMC Yukon 5.7l

 

[SunlitComet]

leaking sparks, cloggy injectors or vacuum leaks. what is rpm in park and in gear idle?

---------- Post added at 06:10 PM ---------- Previous post was at 06:09 PM ----------

 

[spivey94]

In park its like 700 then in gear it goes to around 500 or so

 

[BattelWagun]

700 seems a little high.(I'm not sure though if its out of the acceptable range though) I would check for leaking spark/clogged injectors too.

 

[SunlitComet]

and a vacuum leak for sure with those numbers. Also keep in mind if it was that low in gear the PCM should have boosted it with IAC(Idle Air Control) valve so it may be be stuck or plugged. Test it and find out. It may be that the IAC valve is the cause of your problems. It also boosts the rpm when A/C compressor is engaged.

 

[spivey94]

Ill definitely see if that's it. Could it be the egr valve? I've read up about it and a few sights have said that.

 

[BattelWagun]

It's a possibility that it could be the EGR, but that won't cause misfires as much as it would roughen up the idle and create issues under low throttle situations. My advice is to park the truck in a dark place (like a garage), get a spray bottle and fill it with water, the pop your hood and spray down all of your ignition system. Don't soak it, just mist the water onto it, if there is any spark leak you will see it (like little blue lightning bolts pretty much). if nothing shows up, post back here and we can go to the next step.

also, are all your ignition components GM parts?

 

[SunlitComet]

Or MSD?

 

[spivey94]

No mad quite yet wifey wont let me. I just put the msd off road ignition in my wrangler with the Mallory distributor. So that was my monthly allowance lol.

 

[BattelWagun]

Or MSD?

There goes sunlit flaunting his new ignition haha