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98 k1500 engine misfire

Cornfield creations

Cornfield creations

1/2 ton status
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This thing is going to be the death of me. Little history update-
98 k1500 5.7. Bought used with 181,000 miles. Had random misfire when I bought it fuel pressure is good-
REPLACED:
Plugs, wires, cap, rotor, Dizzy gear. After all that it ran a little better- enough to drive it.
Later on replaced:#3-#4 injectors, upper plenum gaskets, cleaned upper plenum, cleaned all other injectors/tubes/ poppets. Slightly better than before, but misfire on 3 and 4 still there. Primarily #4.

Since then- replaced intake gaskets. Same thing



Drove it 5,000 miles, now it is worse again- only on #3, and #4. Misfire data stored 40,000 misfires on #4 alone :eek1:

I thought possibly the dizzy is just worse out- ignition module and upper bearing may be giving a false reading.

So I got a brand new GM dizzy. Put it in--- SAME THING!! So I took out the #4 spark plug and this is what I got once it finally broke loose.

IMG_7773.JPG


The one on the left is the #3, the one on the right is the #4 plug. The others all look fine. #3 isn't too bad, but #4 was horrible. There is gunk all the way inside the plug itself and it smells like it is cooked.

Well I changed them and it runs better but the misfire is still there and I am out of ideas. Any one else?:dunno::dunno: Sorry it's soo long!!
 
you are at the point of something being wrong with the engine itself. could be blown piston rings, could be leaking valve guides, worst case, hole in the piston.

Id do a compression check, im fairly certain youre gonna find something wonky with the measurement from cylinder #4

next would be a leakdown test of all 8 cylinders.

when those tests show proof that there is something wrong with that cylinder, its time to pull the head and see whats really going on in there.
 
Gotcha- it only misses during idle too, when driving it runs smooth and really strong- and barely uses any oil after 3,000 miles. Which is why I never checked compression because it ran awesome other than the misfire at idle.
 
seen this kind of thing 1 time on a tbi 350 driverside. head gasket blew out between 3 and 5 cylinders. caused funny running and slight skip.

took plugs out and air in 1 hole and out the other.

are you talkin true #3 and #4 ?

when you cleaned the spider /injectors what style old with all black to the ends in intake or the newer style with true little injectors down on the ends?

the first design had lots of problems.
 
sweetk30 said:
seen this kind of thing 1 time on a tbi 350 driverside. head gasket blew out between 3 and 5 cylinders. caused funny running and slight skip.

took plugs out and air in 1 hole and out the other.

are you talkin true #3 and #4 ?

when you cleaned the spider /injectors what style old with all black to the ends in intake or the newer style with true little injectors down on the ends?

the first design had lots of problems.
Click to expand...

Yeah, I can watch them on the misfire data as it is happening. Thats why I was leaning towards the dizzy because the cylinders follow each other on the firing order. Plus when I changed the stuff before it took away a lot of the mis.

Again today I drove it home and the thing ran like a screamin banshee.

Yes the injectors are the old style with the tubes and poppets. The MPFI upgrade was another $350 to update.

I had the bed off when I first got it, someone put in a pump and left the "O" ring gasket off????? Weird, but everytime I would fill up the gas would run out. Pulled it out and sure enough, no ring. The fuel pump is pretty new, plus if it was that I would be having issues on every cylinder, and more so at WOT. This thing will run hard and never blip all the way to 6000 RPMs.
 
Just drove it again. Plugged in the scanner prior to starting. At startup on a cold engine it will misfire constantly on the #4. Once you start driving it will quit, only at idle will it miss. Once the engine is hot, it will miss probably around 3/4 less than when it is cold, but is still noticeable at idle.
 
Could the fuel pressure regulator be a possibility? I read somewhere else a miss at idle but not farther in the RPM range could be the regulator being bad.
 
My older brother was a fleet mechanic,and more than one of the GMC's they had in the fleet had similar issues like yours with misfire codes--he replaced the injectors on the cylinders the codes said were misfiring and that solved the problem,for a day or two--then it would throw another misfire code for a different cylinder,finally it started showing "random misfire" codes...

When he went to a GMC dealer to buy more new injectors,the parts guy said "hey,I have news for you--many of the fuel injector rails have had RUST problems,and they lok perfect on the outside,but are flaky rust inside,and the silty rust clogs the screens built into the rails where each injector plugs in"...he said GM is aware of the problem but didn't issue any recalls, but they have fixed many disgruntled customers trucks under a "silent recall"...so my brother bought all 8 injectors and a new fuel rail (they had several in stock!)--and when he replaced both on the truck,it ran great and the codes never returned...he decided to saw up the old fuel rail and sure enough,inside of it was a ton of silty rust that looked like brick dust...the insides were all flaky and peeling,while the exterior looked perfect...

This might be your problem,maybe not-just thought I'd relate his experience here in case it is!..its one of those things you'd never find yourself most likely--dealers HAVE to fix things "right",so they leave no stone unturned in their search for the cause..
 
Yeah this has the CSFI, not the multi-port as the LS engines have. They do have an updated "multi-port spider" that I knew before GM extended the warranty silently but the dealership has to go through some procedures to get it to go through. Plus I think that warranty has expired on all with the CSFI issue.

I finally did a compression test (should have done it to begin with). ALL tested DRY- 175lbs, BUT #4 and #6. The #4(problem cylinder) has 125lbs, wet it has 140-145.


Shouldn't that be normal for a wet test? a 10-15% increase in pressure? So I am leaning towards valves. Or am I off on this?
 
been a while, but if memory serves, if a cylinder fails a compression test, i normally lean towards rings. if it passes a compression test and fails a leak down test, i lean towards the valves.

id wait for some more experienced engine builders to speak up on that though.
 
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