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.
Friday, January 12th, 2007 AT 3:38 AM