Famous P0300 (5) Misfire

[pbellone]

I have a 98 GMC K1500 4X4 with a 5.7l. Back in Feb I threw a SES light. It went away and the truck was running ok. I read the code and decided on a full tune up. The number 5 plug was crusted with crap and was black kinda on one side. All ACDelco plugs, wires, cap, rotor, both pre-cat O2 sensors. Ran a bottle of top end cleaner down it before the tuneup also. Clearded the codes and went for as ride. seemed ok but for awhile. Then it started up again with the same P0305. Pulled the #5 out and the same crap was building up again. This happened 3 more times then research indicated maybe the injector system was at fault. Ordered and installed the MFI replacement for the SCPI one I had. Still had a rough idle and #5 still with the same black coating on half the plug and bridging building up on the electrode. I also have an oil coating on the threads and taper when I pull the plug. Don't know if that is an indicator of anything. I am at the point of trying valve stem seal replacement. The compression appeared good and similar and I know the black pattern may be from the intake valve opening and pulling the mixture across the plug. My mechanic want to replace the intake manifold gaskets but I'm trying other paths first that I can do. Since I bought the truck I have used Mobil One and never stressed it out towing.

 

I read on thread here that was the same as mine but no resolution after reading it. http://www.gm-trucks.com/forums/index.php?...valve+stem+seal

 

Any GM Techs have a resolution or do I keep changing parts? Or trade it in for maybe a better truck?

[superjim]

The oil you are seeing on the EXTERIOR of the plug, is probably from a leaking valve cover. I don't think that would have anything to do with the miss and the discolored plug.

 

Your mechanic may be on the right track about replacing the intake gaskets.

It seems like the mixture is messed up for the #5 plug.

 

Have you checked all of the "INTAKE BOLTS" to make sure that they are all tight?

They do sometimes work loose.

[coostv]

The oil you are seeing is likely coming from the lifter galley. You most likely have an intake leak at that port. I had the exact problem on my Tahoe except the plug would turn black. Is your idle higher than normal?

 

One thing you can do is plug off the PCV and intake tube ports on the valve cover and then start it and check your dipstick tube for vacuum. Not always a sure fire test, but one to try. My Tahoe pulled vacuum all the time at the dipstick even with everything hooked up like it was intended.

[ayppen396454]

I went through something similar a while back. I also was in denial. Intake gaskets fixed it. Give the dipstick tube trick a shot and read the post below.

 

http://www.gm-trucks.com/forums/index.php?...=61853&st=0