Can't Get Enough
Join Date: Sep 1999
Location: Cotopaxi, Colorado
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Re: Help, 95 running Rich, or lean?!
A catalyst will "lite off" with the presence of high hydrocarbons. High HC's usually come from a misfiring clyinder. I would verify the mechanical condition of the engine first. Do a compression check. If it is good, do a quick check on the valve train using a vacuum guage. Mostly checking that there is no rapid flutter to the guage. Then change your complete secondary ignition, that means, cap, rotor, wires, and plugs. That should fix your problem.
Ignition timing will also effect HC's, but since you have a 4.0l, and the timing is controlled by the p.c.m. and can't change too much because the crank angle sensor is fixed, you can safely rule that out.
Most catalyst overheat and "melt down" from unburned fuels (HC's) and not rich mixtures (CO%) as most people think.