Why Do You Suppose My Spark Plugs Look Like This?

"Future Heap Owner" (aperiodic)
08/04/2019 at 15:05 • Filed to: trying my darndest to own a heap one way or the other, gmt400, Troubleshooting

I’m finally starting to turn my attention towards the ‘92 Chevy pickup I bought a year ago. It has lots of issues to sort out, but right now I’m focused on trying to figure out what’s wrong with the engine.

It runs a little rough, will stall out trying to drive the power steering pump at idle if you crank the steering wheel all the way over to one side while sitting still, and !!!error: Indecipherable SUB-paragraph formatting!!! (this noise only started happening in the winter/spring , going from “never” to “only on hot starts” to “on every start” over the course of a few months and a couple dozen starts).

So yesterday I got around to pulling out the spark plugs and taking a look, and they definitely don’t look good, but apart from that I don’t know what I’m looking it. Comparing to pictures at the end of my Haynes manual, it seems most like it’s running rich, but I’m not sure. I only pulled the ones from the driver’s side bank, starting at the front of the vehicle and working back towards the firewall. The first one looked alright, but the second one doesn’t, and they get worse from there:

Plug for second cylinder from the front on the driver’s side.

Plug for the third cylinder from the front on the driver’s side

Plug for the rear-most cylinder on the driver’s side. Worst of the bunch

Sorry for the bad focus on this one, but notice how one side of the ground electrode is much more fouled than the other

There’s what looks like rust on one of them, which is concerning to me; does that mean that water is getting inside the cylinder somehow? This truck does sit for weeks & months during the wet Portland winter. Other than that, it seems like there’s signs of it running rich on the second and third cylinders, but the fourth cylinder might indicate oil getting in there? It’s much darker and has more crud than the others.

What do you all think, Oppo? I’m trying to figure out what to do next, since I think if I just replace the plugs that’s not addressing the real problem.

The consensus when I posted about the noise seemed to be it was something to do with the injectors, so maybe they need to be adjusted or just replaced. Keep in mind that this is a throttle-body injection vehicle (the GM TBI they used from like ‘88 until ‘94 or ‘95), not direct injection. Are there any tests I could do to diagnose problems with the injectors? I’ll go look at my paperwork to try to figure out how old they might be. I know the previous owner was in the habit of replacing the spark plugs, wires, and distributor cap every 2 years/30k miles, which seems excessive to me, so I think perhaps this problem has been going on for a while and that was how they were working around it. A throttle body rebuild is on my list of possible things to do, since that’s supposed to help a lot with these GMT400s; should I just do that and replace the injectors while I’m at it, and see how it’s running after that?

DISCUSSION (10)

Discerning > Future Heap Owner
08/04/2019 at 15:49

Could be lots of stuff. If that’s oil on the black one, check your valve covers and valve cover gaskets . The brownish color could be the result of your engine running hot.

MM54 > Future Heap Owner
08/04/2019 at 15:50

The ones with rust on them are from sitting, probably with the valves open. I’d do a compression check, the bores/rings might not be too happy about the corrosion.

The one from the back end of the back is definitely getting oil in the cylinder, could be from valve guides (based on the oil on the threads) or from corrosion on the bores letting oil past.

I’d replace all the plugs, and while you have them out do a compression check. Based on that, either continue with ignition tuneup (cap/rotor/wires) and make sure it’s got fresh gas and other fluids or more on to more involved repair .

dogisbadob > Future Heap Owner
08/04/2019 at 16:08

leaking oil?

AMGtech - now with more recalls! > Future Heap Owner
08/04/2019 at 16:25

Check your PCV system. When malfunctioning they can send oil into the cylinders. Clean/replace as necessary, including hoses if it has any (can’t remember on yours specifically if it does or not.

Replace plugs and clean throttle body.

Report back.

NKato > Future Heap Owner
08/04/2019 at 17:05

Alongside The advice given so far, I’d also look at getting as much carbon buildup cleaned outas possible. After you do all the diagnosis and stuff, get throttle body rebuilt and so on, consider doing the Seafoam intake treatment at the end of it all.

Future Heap Owner > MM54
08/04/2019 at 19:10

I don’t drive this very much, so a tank of gas lasts me three or four months. Is that enough time for the gas to start posing issues? Should I be only partially filling the tank so I fuel up every other month (or more)?

The coolant I replaced 10 months ago, and the oil about 11. After how long should I replace oil if I don’t drive enough to do it based on mileage?

MM54 > Future Heap Owner
08/04/2019 at 19:22

The gas should be fine in that short of an interval; if you were going 6+ between fillups I’d start considering stabilizer with each tank.

For oil I go with annually if the mileage isn’t there. Moisture will accumulate (from condensation) when sitting, if nothing else. I’m sure there are a million opinions on this (like anything else oil-related) but I’d go with change it every year if you’re not doing enough mileage to ‘need’ it.

Future Heap Owner > AMGtech - now with more recalls!
08/04/2019 at 22:37

Just checked the PCV system and it seems fine: negative pressure at idle, valve seems fully functional and not dirty.

Gonna do plug replacement and compression test next weekend.

Tripper > Future Heap Owner
08/05/2019 at 08:37

My ranger was stuttering under heavy throttle when I first got it. T he plugs looked like that, actually worse. They were really old, so I replaced plugs, wires, and the fuel filter. It was much better but was still every so slightly “skippy” under heavy load at low rpm. I let it be for a while then checked the plugs again. They weren’t terrible but they were bad for as short as a time they had been in there. I cleaned them up, check the gaps, and decided to replace the coil pack...

Bingo.

MontegoMan562 is a Capri RS Owner > Future Heap Owner
08/05/2019 at 13:05

Kinja'd!!!

Charts like these are real helpful. There’s probably a ton of them on the googles .