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S10 Spark plug misfire mystery - any suggestions?

Hi there …

2000 Chevy S10 pickup

Had the check engine light come on, and when I checked on the code
it gave a 4th cylinder misfire. Autozone instructed me to simply replace my spark plug with a new oem plug.

I replaced the spark plug as instructed.
After about an hour of driving, the check engine light goes off and I think, great! Problem solved.
However, another hour of driving and the check engine light is back on again, without going away.

I headed back to autozone to get a readout, just to find that it’s the same code.
So I put another spark plug in.
Same result - check engine light goes off after an hour, only to come back on again permanently after another hour.

Now, normally I would think, this must be the spark plug line that needs replacement,
however I am sure there must be some clue in the fact that the check engine light turned off for an hour.

By the way, not sure if relevant, but I also found a lot of build up around the spark plug before I initially changed it.
In fact, there was so much that I had to scrape around the plug with an awl, and blow out the crud with a compressor,
just to get my socket into place without there being crud in the way.

Anyone have any clue what that check engine off/on clue might be?

Thanks so much for your time.



What engine?



It’s a 4.3

Just my 2 cents, but I’m a believer in always running a compression test. If you’re losing a cylinder the new plug may work fine at first but at some point the new plug may start misfiring due to lowered compression.
At least weed the possibility of mechanical faults out before continued wrestling and money spent.

A failing wire or coil can also cause plug issues.

What did the firing ends of the plugs look like when you removed them? Black and sooty? Super clean, like they had been steam cleaned?

The first one removed looked a bit black and sooty. The second one had only been installed for a few days when I removed it, but it did look a little grey on the tip of the center electrode and the metal on the side electrode looked a little like it had been flashed. Subtle though.

Could it possibly be a gap issue?

I would consider replacing the spark plug wires. has them for $12.00 to $82.00. If you have the original wires they are 16 years old and may well be faulty, and the act of disconnecting and reconnecting the old ones to replace the sparkplug may get better connections for a while.