'98 GMC Sierra with engine problems

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'98 GMC Sierra with engine problems - 3/11/2007 3:14:01 PM    Birddog   Posts: 24 Joined: 3/11/2007 Status: offline  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. Post #: 1

RE: '98 GMC Sierra with engine problems - 3/11/2007 6:47:10 PM    Argon Posts: 2181 Joined: 2/25/2007 Status: offline 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 _____________________________ Gm tech for over 20 years (in reply to Birddog) Post #: 2

RE: '98 GMC Sierra with engine problems - 3/11/2007 7:07:07 PM    Birddog   Posts: 24 Joined: 3/11/2007 Status: offline 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. (in reply to Argon) Post #: 3

RE: '98 GMC Sierra with engine problems - 3/11/2007 10:59:52 PM    Birddog   Posts: 24 Joined: 3/11/2007 Status: offline 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? (in reply to Birddog) Post #: 4

RE: '98 GMC Sierra with engine problems - 3/12/2007 10:35:50 AM    Argon Posts: 2181 Joined: 2/25/2007 Status: offline Thats always p[ossible,but not real common...Be sure to look for a cracked plug.. _____________________________ Gm tech for over 20 years (in reply to Birddog) Post #: 5

RE: '98 GMC Sierra with engine problems - 3/12/2007 10:59:26 AM    Argon Posts: 2181 Joined: 2/25/2007 Status: offline 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. _____________________________ Gm tech for over 20 years (in reply to Argon) Post #: 6

RE: '98 GMC Sierra with engine problems - 3/15/2007 10:02:07 PM    Birddog   Posts: 24 Joined: 3/11/2007 Status: offline 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 miles each 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. (in reply to Argon) Post #: 7

RE: '98 GMC Sierra with engine problems - 3/16/2007 8:23:52 PM    Argon Posts: 2181 Joined: 2/25/2007 Status: offline Are you changing the lower gasket for the coolant loss?.. (in reply to Birddog) Post #: 8

RE: '98 GMC Sierra with engine problems - 3/17/2007 8:38:29 AM    Birddog   Posts: 24 Joined: 3/11/2007 Status: offline yep , will be doing -upper and lower intake gaskets and the injector. (in reply to Argon) Post #: 9

RE: '98 GMC Sierra with engine problems - 3/17/2007 10:51:36 AM    Argon Posts: 2181 Joined: 2/25/2007 Status: offline 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 it while its running _____________________________ Gm tech for over 20 years (in reply to Birddog) Post #: 10

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