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Misfire on #6 and #1

27K views 82 replies 18 participants last post by  herbsalign 
#1 ·
As you can read about here transmission/torque-converter-t7975-40.html#p84247, I've been having quite the problem with my truck, but only at 65 MPH and above.

The mechanics I've taken the truck to say that it's an internal issue, since the plug, coil, wire, and injector all check out. A bad misfire on #6 and #1 is what they say it is, even though you can't feel it at all. I've taken it to two reputable, trusted shops, so I know they're not fibbing on me.

2 questions: Are there any other things to test? And, if something is wrong inside, do I have a timebomb on my hands? It hasn't gotten any worse in the 3 months that it's had the issue, but ya never know.

If I've got a bad valve or two, it's really not worth it financially to rebuild the motor. I'd like to know what I'm dealing with here, so any opinions are welcome.
 
#3 ·
118,500. Not a single issue otherwise; just that one on the freeway. I'm going through a wide range of emotions right now, as you can imagine... my gut feeling is that it will keep running strong, but I'm no expert on engine internals.
 
#4 ·
I guess the guy to ask would be ASTE.. but man thats low miles for one to act up! I would think maybe you could do a compression test to narrow it down and maybe a pressure test to make sure it isn't a head gasket or something..im kinda just shooting in the dark here. But if it was a valve or two i mean you would think it wouldn't be too expensive to pull a head and replace one, ya know.
 
#5 ·
That's always an option. They've got the contact info for a good engine guy around here, so I'll check him out when I get the truck back tonight.
 
#7 ·
How did they test the coil and wire? If they just did an ohm check, that would not necessarily rule out that one of them has a weak area allowing spark to jump. If I were to bet, I'd say it's a plug wire. But, that is only because it is the normal "weak link" in the ignition system. The mileage is about right for the wires to go bad. If it were me, I'd start with replacing all the plugs and wires with AC Delco parts. If it continues, I would then replace the two coils. I'm not big on just throwing money at a problem, however, those are components that go bad enough of the time that I think it'll fix it up.
 
#10 ·
Yeah i was kinda thinking the same thing guys, but i figured maybe the mechanic shop thought of that. I have had this issue before on vehicles in the past with a coil sparking, but putting out a "weak" spark causing a misfire condition. But on that vehicle it did it at idle.
 
#11 ·
Crankcase position sensor (CPS) malfunction a possibility? I was reading some reliability reports on 2003 GM trucks and it said; An occasional problem on this vehicle is failure of the Crankshaft Position Sensor.
 
#15 ·
I think you can ohms test the CPS not a hundred % shure on that but you can ohms test the coil packs that will tell you right there if its those switching the coil packs only work when you have a bad missfire because of one dead, since you said you cant feel the miss i wouldnt worry about switchting them around the ohms test should tell if they are faulty.
 
#16 ·
They can pass an ohm test and still be faulty in a way that would cause a misfire. All an ohm test will tell you is if the wires in the coil have a high resistance (or open) or are shorted. They can pass the ohm test and still have a bad insulator on the "high" side causing the spark to jump to ground.
 
#17 ·
Thanx for all the help, boyz. They looked at it for 3 hours yesterday, and couldn't find anything. I guess there's thousands of misfires stored in the computer... I just find it weird that the motor still runs as smooth as honey. The mechanic swapped wires around, to see if he could get it to misfire on other cylinders, but to no avail. I might try changing out the wires for new ones, though. What's gonna provide the biggest bang for the buck? New coils? New plugs? New wires? Money is extraordinarily tight right now, so I want to start on what's most likely the problem.
 
#18 ·
This is gonna sound dumb, but if you disconnect the battery, doesn't that clear the memory of the computer? That being said, and if it does work, couldn't you do that and then bring it back to the shop and have them test it again; this way you would have fresh misfires, if in fact it were still having problems.
 
#19 ·
I don't think that really would help, since it's still misfiring. They know which cylinders, they know it's happening a lot, but they don't know why.
 
#22 ·
Well, I did more searching on Google. A lot of hits came up, and they all describe the same problem I have. Unfortunately, the only solution I found was a guy had weak springs in #1 and #6, and replacing those springs fixed the misfire.
 
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