4 weeks ago or so I ran into a problem with my 1991 3.1 V6 Firebird. My fuel mixture was way too rich and it ended up being a combination of bad O2 sensor and vaccuum leak. I fixed both and put in new plugs.

Everything was fine but a couple weeks ago it slowly started running worse and worse. No error codes, but the spark plugs have carbon foul on them again. It is definately not oil foul. It's carbon foul.

This book I have suggests a hotter spark plug. What temperature spark plug should I get to see if that solves the problem, and if that isn't the problem, what else can I check to see why I'm getting carbon foul? Are there any common issues that lead to this?

 

Noone knows?

 

Check the ignition system in general first. You shouldn't need a different plug unless you have a decently mod'ed motor. Check the ignition system, then if everything there checks out, check the fuel delivery. Is your rail pressure too high? Do the injectors leak?

Something else to think about, now that you have corrected the rich condition, did you erase the computer memory? The old block learn info might be in there causing you to still be running to rich because of what it learned from the old O2 sensor. Just a thought.

 

If by erase the computer memory you mean leave the car battery unplugged for awhile, then yes.

 

What might have changed is that they gave you the wrong part # for the spark plugs... do you know if they gave you the same plugs that you had before?

A hotter plug will reach it's self cleaning temperature faster. You can experiment with it, but I agree 100% with 2_point8_boy- if you didn't have this problem before, it's not the plugs fault. Unless of course they gave you the wrong plug part #.

Actually do you know the part # that you got? The heat range should be part of the plug #. Example, my plugs are AC's, number R42TS... R=resistor, 4=14mm hole, 2=heat range, T=taper seat (no washers), S=I forget.