I have a '98 GMC Sierra with a 305 ,auto, with 122K+ miles on it. I have owned it since new and this truck has been very good to me . Recently (3 weeks)the service engine soon light came on . I had a friend who used to wrench put a code analyzer on the port under the dash and he came up with a 306 which is a #6 cylinder misfire code. Late last year I changed the cap,rotor and plug wires .This past week after this problem came up but before I had the code ,I had changed plugs (went from AC platinums to NGK platinums)and #6 was definitely gooked up with carbon. I hoped it would solve the rough idling and the code. It didn't . Just yesterday I put nearly a whole can of spray carb cleaner through the throttle body with was very blacked up with carbon also. At the same time I put a bottle of Gumout fuel system cleaner in the tank to run through . I gave my buddy the old plug , he read it and saw that there is a sign of calcium deposting on the side electrod meaning a coolant leak possibly which I am also losing but a very small amount. We ruled out any EGR because it is up front and seems to be working fine , same for the O2 sensor. Has anybody else had this problem and if so , what did it end up being? I'm hoping if I put 2 or 3 more fuel loads with cleaner it may come out of this by itself. I have also thought I may have a bad #6 plug wire . I may swap it with #4 or #8 just to see if the code moves. My bud is also bringing home a compression tester to rule out a valve or rings. I need to get another 2 yrs out of this truck . I'm almost prepared to tear it down this summer and make it a fresh motor - I have a 14 yr old daughter that will need something in a couple years. Thanks in advance.

 

You know #6 misses,just not why...Look at the cap...did ya put on a good cap?..And good wires?..cheap indonesia crap gives ya these headaches.
Another strong possibility is the injector,or as you mentioned,a valve

 

cap, rotor and wires are all from Napa and were installed just last fall. My buddy works for a GM truck dealership and suggested I put Techron in the fuel to clean the system . He also says by the model year, I should have port fuel injection - to me it looks like an electric carburetor which appears to look just like a regular carb except it has one big butterfly . I might go out there now and swap #6 wire for the one at #8 to rule out the wire - that is if the code wanders over to 8 . Then I'll know its the wire.

 

Argon,
I was just over at another automotive forum , with another GMC Sierra folder- seems a guy had the same similar thing happen with his -he had #4 and #6 misfire and it ended up being a bad intake gasket on that bank ,does it sound like it could be my problem also?

 

Thats always p[ossible,but not real common...Be sure to look for a cracked plug..

 

My money is still on an injector though...a tuff diagnosis on that vortec..

This needs to be narrowed down...It must be determined if its spark,fuel or other...

I know ya don't wanna hear this..but it needs to be in a shop if a few simple things don't check out..(like a plug...a wire melted to the exhaust..)

You can't "see the injector,..its under the upper intake...it could get involved.

Another possibility is a fuel pressure regulator leaking under trhat intake...dripping gas down the 6 hole...see if #6 is gas-fouled.

 

Argon,
My buddy that works as a light truck mechanic came over tonight - put his scanner on it ,still a 306 code. He also went through a more detailed scan and saw that (basically)it found a problem with the fuel delivery system ,or as you suspected the injector. It(#6) was running on 21%.He says that 98% of misfires that come in to their shop end up as injectors plugged or "Coked" due to such bad gas we're paying the high prices for , some service vans come in once a month to have their injectors cleaned. So ,he says I will still be needing to change the intake gaskets due to coolant loss and as long as I have it apart I should get the injector and change that too. So - back to Napa I will go as soon as the cash is in hand and buy the upper manifold gasket and the injector . I'm also refraining from driving the truck any more than necessary .He says as long as I'm careful with it and just back and forth to work(17 easy mileseach way) I should be ok until we get this changed . He gave me the service procedure for the job . We'll have it done in a Saturday morning asap. I will report back here with actual end results but til then - thanks for your help.

 

Are you changing the lower gasket for the coolant loss?..

 

yep , will be doing -upper and lower intake gaskets and the injector.

 

cool..your buddy can set the timing with his scanner then.

These injectors are tuff to diagnose as you know...about all you can do is eliminate other things....especially on that 5.7,where you can't unplug itwhile its running