1990 Jeep Cherokee failed smog really badly

I just got a 1990 jeep Cherokee 2WD 4.0 6 cyl with 162,000 miles and the damn thing failed the smog test as a gross polluter and I am not sure where to start. I failed the HC and CO. With the 25 MPH test coming out as a gross polluter.
The smog guy said its running rich but if I lean it out my NO will climb and I barely passed on that part. It has a new O2 sensor, new distributer, new coil, valve cover was cleaned, plugs and wires are all good. It had a few vacume leaks but other then that I am not sure where else to start aside from replacing the injectors (smog guy made a reference to it). Any tips?
Do you
have the same problem?
Thursday, January 11th, 2007 AT 10:45 PM

1 Reply

I have run into 2 problems with the 4.0 motors that stand out. These may or may not solve your problem, but certainly worthwhile regardless.

The intake valves tend to carbon up, not allowing good combustion.
I would try what I call an air induction service.
I like to use castle hydroblast. You spray it into a vacuum line such as the brake booster line if nothing else is accessible. Full spray for 3 minutes, stall the engine with it or have someone kill it while your spraying. Let it set for a few hours then take for a short drive in low gear, revving the rpms to help break it up. First use Castle cleansrite to clean the throttle bore. Try to get the throttle plates scrubbed good with a brush, while your spraying. Use a rag to wipe the carbon out between brushings. Seafoam is another good product, but smokes a lot.

I have a professional fuel injection service where the fuel pump is disabled and high quality cleaner is run through the motor have been very beneficial.
Was this
Friday, January 12th, 2007 AT 3:38 AM

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