My Festiva is running a little rough (black spark plug)

[CyberEric]

My 91 fuel injected has been running rough for a few months. Mostly dipping RPMs at idle and during coasting I bit of roughness as well. Never stalls, just a bit rough.

Pulled the spark plugs and the tip was completely gone on cyl # 3. It was basically disintegrated. I think I heard it happen too.

Put in all new plugs and new wires. No improvement. Checked the #3 plug after a week and it was already black. The others looked fine.

I am thinking too much fuel since it's black so soon... maybe a leaky injector? Is that common?

Any other ideas?

Thanks!

[skeeters_keeper]

Could be a burnt valve on that cylinder, can you do a compression test?

[CyberEric]

Could be a burnt valve on that cylinder, can you do a compression test?

Yeah, I have been meaning to do one. Thanks

[WmWatt]

You could also remove the valve cover and look around. A leaking exhaust valve will leave black depostis on the underside of the valve cover right above the valve. On mine it was carbon deposits keeping the valve from closing and I was able to use a chemical cleaner to fix it. However black depostis on the spark plug would be incomplete combustion from too much oil or gas in the combustion chamber? I guess the other thing could be timing. I find a cheap vacuum guage or a tachometer shows misses by vibrating.

PS this sort of question goes in the repair help forum where it's likely to get more respnses.

[CyberEric]

Thanks, I'll post there next time. And apologies for the double post, been having problems with this forum on my browser.