Fouling 2 plugs

mudpie · Mar 14, 2023

I've got a '96 C2500, 5.7, and it's misfiring. CEL is flashing, but the code is "Random Misfire". I pulled the plugs and #3 and #5 are fouled black. The rest look good. Gap is good. Did a compression test on that bank and I get 200, 195, 190, 195, so I'm ruling out head gasket. Plus, I'm not losing coolant. Cap and rotor are brand new, and I've verified the firing order probably 10 times. Plug wires are a couple years old, but I've moved them around and the problem stays with 3 and 5. A couple years ago I replaced the injectors with a used spider injection assembly off a running motor and I've had no issues. Fuel pressure regulator was replaced with new at the time, as well as the manifold gasket.

I'm going to tear into it today. What should I be looking for? I'm thinking the spider is leaking maybe? Any suggestions appreciated.

Nad_Yvalhosert · Mar 14, 2023

Sounds like oil fouling. Replace the valve seals.
I had an '84 Suburban with the 5.7. The #1 cyl would get coated in 200 miles. I kept a set of 8 in the truck, along with a ratchet and plug socket. Easiest thing for me was to replace the #1 plug every time it would start misfiring. Then by the 8th time I'd take the 8 fouled plugs into work, clean them, re gap them and start the process all over again. That rotted out shat box just wasn't worth a tear down.

Schurkey · Mar 14, 2023

mudpie said:
I've got a '96 C2500, 5.7, and it's misfiring. CEL is flashing, but the code is "Random Misfire".

Is your scan tool capable of showing individual cylinder misfire? Cheap ones lie about the true code--they claim P0300 "random" misfire when the real code(s) may be cylinder-specific.

If the MIL is flashing, it's serious. Flashing MIL indicates the catalyst is in danger of meltdown.

mudpie said:
I pulled the plugs and #3 and #5 are fouled black.

Fuel-fouled, or oil-fouled?

Any chance some of the plugs have a conductive coating on the porcelains that causing misfire without visible contamination, or under the black fouling?

mudpie said:
Did a compression test on that bank and I get 200, 195, 190, 195, so I'm ruling out head gasket. Plus, I'm not losing coolant.

Head gasket can fail, without affecting coolant.

Your compression numbers are substantially higher than I'd expect--is your gauge accurate?

At least they're even.

mudpie said:
Cap and rotor are brand new, and I've verified the firing order probably 10 times. Plug wires are a couple years old, but I've moved them around and the problem stays with 3 and 5.

The fouling is limited to 3 and 5, or you've confirmed the misfire is both 3 and 5, but none of the other cylinders?

mudpie said:
I'm going to tear into it today. What should I be looking for? I'm thinking the spider is leaking maybe? Any suggestions appreciated.

Connect a scan tool that show individual cylinder misfire. Let's make sure you don't have "hidden" misfires aside from the two that foul plugs.

Before you take it apart, I'd put a full set of fresh plugs in the thing; preferably plugs designed for a .040 gap instead of those designed for a wider gap. At least replace the two that foul--let's make sure those plugs aren't defective/conductive porcelains.

Aside from the plugs, it doesn't sound like this is an ignition issue. Still leaves oil consumption (valve stem seals, intake gasket, stuck rings/scored cylinders) and maybe fueling as potential problems.

mudpie · Mar 14, 2023

I swapped 3 and 5 with 1 and 7 and it's misfiring as soon as I start it up. There's no time period where it runs ok, waiting for the plug to become fouled. I thought maybe I got a defective distributor cap, but I pulled each wire off and checked with a plug and they're firing.

mudpie · Mar 14, 2023

I put a gauge on it to check the fuel pressure. When running, it's showing 50 psi, and when I shut it off it holds that pressure for at least a couple minutes. If I had an injector or a line that was leaking, that pressure would drop, right?

Schurkey · Mar 15, 2023

Fouling that quickly? Doesn't seem like a fuel issue to me, especially since the fuel pressure holds when the ignition is shut off.

Kinda thinking of a massive oil leak into those two cylinders. Perhaps from totally-wiped valve guides, but more-likely from a failed intake gasket.

BeXtreme · Mar 15, 2023

I'm going to say you are almost guaranteed to have an intake manifold gasket leaking into the lifter valley. A bad enough leak will cause it to run super lean on just those cylinders(misfire) and suck in a ton of oil(foul plugs).

Erik the Awful · Mar 15, 2023

I'd bet on an intake gasket leak.

mudpie · Mar 16, 2023

Schurkey said:
Is your scan tool capable of showing individual cylinder misfire?

I just got the guy at AutoZone to check it. I've been looking for a scan tool, but haven't got one yet.

Schurkey said:
Your compression numbers are substantially higher than I'd expect--is your gauge accurate?

I'm not positive it's accurate, but I've had it for years and used it on other cars, mostly my Volvos when I need to find out if I blew another head gasket, and the numbers I've gotten in those instances were in line with the Volvo specs.

Schurkey said:
The fouling is limited to 3 and 5, or you've confirmed the misfire is both 3 and 5

The fouling is limited to 3 and 5. I put a timing light on all the wires and it flashes, and I pulled the wires one at a time and checked for a spark and I have spark at each wire. I can only assume that those two aren't firing when they're in the cylinder, based on the fouling.

Schurkey said:
Before you take it apart...

Oops. Too late.

Schurkey said:
...but more-likely from a failed intake gasket.

BeXtreme said:
...almost guaranteed to have an intake manifold gasket leaking...

Erik the Awful said:
I'd bet on an intake gasket leak.

In hindsight, that makes sense. I had a cap and rotor that were corroded pretty badly. The truck was backfiring on startup, and I assumed it was a misfire caused by the cap and rotor. That's why I replaced them. I'm guessing maybe the backfiring wasn't kind to the intake gasket, or the intake gasket was my problem to begin with and the cap and rotor were coincidental. I don't know.

So it's apart now. I didn't see anything on the gasket that was an obvious "ah-ha" moment, but then again I have no idea what I'm looking for. I figured I'd see an obvious spot, like you'd see on a head gasket, but I didn't. It'll go back together tomorrow and I suppose I'll find out if that was the problem.

On the plus side, looks pretty good inside for 485k miles.

Schurkey · Mar 17, 2023

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Well, there y' go. Gasket thoroughly oil-soaked between lifter valley and port. Although, I'm surprised that it looks like both ports are getting oil, when the pair of port would be 1&3, or 5&7. Both pair of ports on that gasket should be wet if 3&5 are oil-fouling.

Probably want to throw a straight-edge across the intake manifold, and also the head surface. I bet one or both is warped.

Look in the cylinder head intake ports, see how many of them are wet with oil. Look in the intake manifold ports, see if there's oil in any of them, and if that oil can be traced back to the PCV hose.

Fouling 2 plugs

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