Engine misfire

[suburban_lee]

1999 Suburban K1500, 5.7L

I've got an intermittent misfire problem that collectively, my regular mechanic and I cannot seem to diagnose. The misfire is a recurring misfire on the #4 cylinder (P0304). I experience the misfire at highway speeds, typically pulling a hill, and especially when carrying a decent size load in the truck. Sounds like a spark issue, but here's what we've done so far:

Replaced plugs, wires, rotor with AC Delco parts
Replaced distributor cap with Borg-Warner cap
Replaced the #4 fuel injector with AC Delco injector
Replaced PCV valve with AC Delco part
Ran a carbon cleaner through the system
Performed a compression test on the cylinder (normal results)
Replaced intake manifold gasket.

The inteake manifold gasket showed a crack at the #4 cylinder, so we thought we found it with this one, but the misfire returned again this past weekend. Removal of the intake manifold and the EGR valve showed no carbon buildup. The truck is driven daily, with most trips topping 20 miles of expressway driving. Any help would be appreciated.

Thanks!

[777stickman]

My Sub is a '98 7.4 and had a similar problem. When the intake gaskets were replaced we found a couple of the injector wires were chaffed under the upper plenum. Repaired that and the missing is gone.
Hope this helps........Steve

[suburban_lee]

Thanks for the reply, Steve.

I'd love to avoid taking the top plenum back off, but it sounds like it may be worth a shot. Was your misfire fairly regular, or very intermittent? In my case, I can go months with no problem, and when I least expect it, it happens.

Is the 7.4 the same as the 5.7 where injection is concerned - fuel meter body with all eight injectors installed and plastic hoses to each cylinder intake port? My reason for asking is that I don't remember much wiring when I changed the injector. If I remember correctly, there was a harness going to the fuel meter body. Is that replaceable?

[landyacht1957]

I am having the same problem. And have replaced all the same components.
I wonder if anyone knows who to fix this?

Have you found anything?

[suburban_lee]

Originally Posted by landyacht1957

I am having the same problem. And have replaced all the same components.
I wonder if anyone knows who to fix this?

Have you found anything?

Actually, yes. In my case, it turned out to be a faulting cam sensor. Nothing in the diagnostic codes pointed to this, and we changed it on a hunch, but it definitely fixed the problem. With as inexpensive as this part is, I would definitely recommend changing it out.

[landyacht1957]

Originally Posted by suburban_lee

Actually, yes. In my case, it turned out to be a faulting cam sensor. Nothing in the diagnostic codes pointed to this, and we changed it on a hunch, but it definitely fixed the problem. With as inexpensive as this part is, I would definitely recommend changing it out.

That make alot of sense.

I am going to buy one and change it.
Thanks, I hope this works. Your problem and mine are the same except that mine shows #5 misfire instead of your #4.

And your right, most of the time everything is fine, but then some days it just runs like crap. Today was one of those days.

[rsmith8411]

Thanks for posting the solution. I had a P0304 misfire code also on my 99 Suburban, and I started down the same path of changing plug wires, plugs, etc. That gets expensive. What's weird about my problem was that it always happened on the same stretch of interstate (and nowhere else but this same place!). It was on a long uphill climb up a mountain where the vehicle was at a high speed (about 70 mph). Every time I drove this section of interstate I would get a code. I even went 10 months oncewithout driving this stretch of interstate (and no codes), but just like clockwork, it set again when I went back up that mountain. Weird. After reading this post, I replaced the camshaft position sensor and it is fixed! I just went up Monteagle mountain without incident for the first time in four trips. This is why I love Automotive Forums. Thanks fellows and keep up the great work!

[DrDeath81]

I've had similar misfire patterns for quite awhile now. I took it to the dealer and proposed the crankshaft / camshaft position sensor could be the problem. After a Level 2 diagnostic the dealer stated that there were no codes of any misfires, nothing wrong with the position sensor. His "Tech" feels that it is blocked catalytic converters and they'd be happy to replace the cat assy (2 cats and pipe assy) and all four oxygen sensors since they have never managed to get the old ones out without damage. The quote was $1,600.00, but no guarantee that it would solve the problem.

I told them to do nothing. My Subdivision still misses. Now it looks as if the trial and error repair method is all that is left.