Join Date: Sep 1999
Location: San Diego, CA
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Re: Having 4.0 trouble
Does it run better at low vs. high rpm? You mention a slight whistling noise at higer rpm, does that correspond to the missing HP? It could be a failing fuel pump if it doesn't have much top end to the motor (or filter, but you changed that). Maybe a loose crank sensor or wire. Very well could be a failing cat too. When was that last changed?
Have you been able to use the check engine light codes to diagnose the problem? Good luck and post again once you've checked out some more of these things.
258 4.0 Head/MPI,NV4500,D300,4.10's,HS9500