Jim Grant's Tech Tips
Cherokee, Poor Idle
Q: I have an ‘88 Jeep Cherokee with a 4.0l
engine. At times, especially when warm, the engine starts hard and idles
rough. Sometimes it will start right up no matter what the conditions
are. Now when warm, the engine idles too high, approx. 1300 RPMs and feels
as though one of the cylinders is misfiring. However, above idle and at
cruising speed the engine does not hesitate or skip a beat. I'm suspecting
a vacuum leak but cannot find one. Please help!
would suggest checking the Exhaust Gas Recirculation (EGR) valve. The
EGR valve introduces (feeds) exhaust gases back into the engine at the
intake manifold. The reason for this is to reduce combustion temperatures.
This, in turn, reduces exhaust emissions. The EGR valve is suppose to
be closed at idle and opens at varying amounts while the vehicle’s being
driven. A sticking or defective EGR valve can really mess-up the engine
idle, causing erratic idle speeds and misfire or an engine skip that would
lead one to believe there is some sort of vacuum leak. If inspection of
the EGR valve finds that it is not sealing properly or is opening in an
untimely manner a replacement will be needed.