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Why Do You Suppose My Spark Plugs Look Like This?

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 makes weird rattley noises for a second or two after you turn it on (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?

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