I have a Jeep Cherokee in the shop that won't pass smog because the nox is too high. They have fixed an intake leak and replaced the cat. They tell me that the cylinder pressures are on the high side of spec at 175 psi and as a result the nox is too high to pass. I have already looked into finding a thicker head gasket to lower compression but there doesn't appear to be one available. Can I lower the nox by richening the mixture with an AFPR?

 

Has the EGR system been inspected, valve and solenoid? Egr valve is what deals with Nox. Cat. converter helps also, but you have already addressed that. Also, if compression readings are high, then there could be some carbon buildup. You could try to decarbon the chambers. I work at a Chevy dealership, so we use GM Top Engine Cleaner. Works good.

 

Thanks for the tip on top engine cleaner, I'll look into that. These cars have no EGR system.

 

Yeah, I wasn't sure if they had EGR on it or not. I know GM trucks and others eliminated EGR valves and just changed the camshaft profiles to deal with the EGR. Then yeah, try that top engine cleaner. That stuff has amazed me more than once. Damn good injector cleaner when performing an injector flush.

 

I sent my wife to pick up some top engine cleaner from the local Chevy dealer. They told her they no longer sell it, instead they want the car brought in so they can do the cleaning procedure themselves. I guess I'll run over to Napa and get some seafoam and try that instead.

 

That is B.S. That dealership just wants you to bring them the car like you said. I just used some today mixed with gas to flush fuel injectors. Got it over the parts counter. GM tells you to use this in a bulletin to flush injectors, so they do still make it. Those people at that dealer don't know what they are talking about or just want more of your money.