Rough Idle problem - Jeep Cherokee

'84-'85 CHEROKEE 2.5L ENGINE DIAGNOSIS - I just spent 6 weeks pursuing a rough idle problem with my '84 Cherokee. (2.5L engine, 100k miles) This might be useful to someone else so here's the story.

1. Found the brake vacuum booster leaking - replaced. Still have the problem. (Symptom was lowering vacuum while holding brake pedal depressed.) 2. Checked the intake manifold bolts tight - they needed a bit of a tweak. (Actually, one stud was broken. Replaced, retorqued.) Idle still a problem. 3. Noticed that the usual "clack" when moving throttle off of idle was missing.

After pursuing the service manual diagnostics for the PCV valve actuation, I was stumped. I'd replaced the anti-diesel relay by rotating relays along the line of them at the RF fender. No symptoms changed. I had to re-derive the function of the computer and its interaction with the carb from the schematic.

After enough work, I found that one of the relays was not shifting back to close the right contacts when de-energized. It was shifting ok, but the contacts weren't making When I'd moved the relays, the symptom hadn't changed because the same faulty relay would either malfunction as computer-controlled output (Anti-diesel relay) , or when installed in another position (closed throttle, WOT logic) , would give faulty computer input which resulted in the same faulty output. In this case, the faulty relay was not taking the idle/part-throttle/WOT input to ground so the computer thought it was at part throttle. When at part throttle, the idle mixture solenoid duty cycle (dwell) wanders quite a lot resulting in varying rough idle. When the computer knows it is at idle, the PCV solenoid closes ("clack!") and the variation in the MC solenoid duty cycle is very small. (When restoring all carb settings, remember to set the sole-vac high idle first, then set normal idle speed.) There is a block of 3 relays on the RF fender behind the headlight. They are all open to atmosphere at their bases. The anti-diesel relay cycles every time you move the throttle off of idle. This can be a million cycles in 100,000 miles. I strongly recommend replacing all 4 relays (the block of 3 and the idle relay on the LF fender) every 50k or so. Cheap insurance. Or if you otherwise suspect an electrical problem, use an ohmmeter to check contact resistance for the relevant relays.

BTW- Blue relays are inscribed LE UK. Meaning, I assume, Lucas Electric. (some know what that means!)

Ron Miller Ft. Collins, Colo.

ron@fc.hp.com

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