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88 Jeep Comanche 4.0L 4x4 Mileage Drops by Half

Background: 1 owner (me), 312k miles, motor replaced with long block @ 298k miles. Installed aftermarket Borla header with engine swap due to long history of unrepairable weld cracking on stock header (replaced 2x with stock)…the Borla header later replaced with Pace Setter header due to interference problems with front axle in extreme 4 wheeling conditions (NOTE: PACE SETTER HEADER HAS O2 SENSOR LOCATED ABOUT 3’ DOWN PIPE FROM STOCK/BORLA HEADER LOCATION). Mileage was 18-21 city/highway. At approximately 308k miles, about the same time that the Pace Setter header was installed, gas economy dropped to almost 1/2. Failed emissions (of course) with over double the allowable HC/CO, NO2 OK.

Troubleshoot: Injectors checked- electrically OK (new ones installed @ about 275k miles), Ignition checked with timing light (timing about 10 degrees BTDC @ idle) all cyclinders checked & appeared to be firing OK, replaced O2 Sensor, Knock Sensor, & ECU. Emissions tested after each replacement with virtually same failing results. Emissions personnel @ Emissions Waiver Station consulted during Waiver process…they were asked about the change in O2 Sensor location and said this would have no affect…this was the only substantive difference after the mileage problem started.

Your thoughts ???

It might be cheaper just to buy a 1974 Comanche, restore it, and never worry about oxygen sensors, catalytic converters, EGR flow rates, ECM loops, failed emissions tests, Voodoo, Magic, and greatly reduced fuel mileage…Just drive it…

Check the air in the tires. You would not believe how many vehicles out there have low tire pressure. I use 32psi in everything I drive and get great gas mileage and the tires wear perfectly. Low tire pressure can kill fuel mileage.