1986 XJ Cherokee Erratic Idle Problem

[Mikerainh2o]

Bought my son a nice 86 cherokee. It has an idle problem that is driving me crazy though. I've checked for vacuum leaks and found none. I've checked all the sensors according to what I've found on this site and they check good, except the tps, I havent checked it... I ran out of time the last time I got to work on it due to rain... So it is suspect. The problem is, when you start it, hot or cold, it will surge and sometimes die for about 45 secs to a min. I can see the IAC motor slowing decreasing the idle but then right before it dies, it will snap the throttle open. Somtime's it does die before it can open fast enough, but the IAC motor seems to be working. After that first min of surging, it runs great, won't die or surge again, until you shut it off for a few mins and restart. It will start right up everytime but go through its surge for a min and then run fine. You can keep it running by feathering it. Engine hot or cold it will do this. Could this be the tps?  Its the 2.5l with the manual trans. 

[Moses Ludel]

Mikerainh2o...Given the model year and age, I would do a cylinder leak down test to rule out cylinder sealing issues/low compression.  Also check the manifold vacuum at idle.

First off, I would not replace any parts before testing their function.  (Spare the expense!)  Any items I mention can be tested before replacing them.

I always begin with the basic fuel filter change on these models, though that may not make any difference.  Also confirm that the gas cap is sealing.  Make sure the EGR valve works properly and does not have a vacuum diaphragm leak.  This can create an erratic idle and will act like a vacuum leak.  Check vacuum hose routing if the EGR is not opening or closing properly.  The canister purge/EGR switch can be an issue on your model.  Test it.  Coolant thermal switches can also be an issue.

Check the sensor voltages (highlighted are most common trouble spots), beginning with the Coolant Temperature Sensor and MAT (manifold air/fuel temperature).  The TPS is worthy of a voltage reading, though it usually has a consistent problem that does not clear up.  The oxygen sensor should not be overlooked.  MAP is also important—even if vacuum hoses check okay.  Each of these sensors can be tested.

Glad the IAC motor seems to work properly.  That the engine runs well otherwise and does not misfire or cut out at speed is also a plus.  Fuel pressure is a concern but usually shows up as a miss or stumbling at all times.  If suspect, you can check fuel pressure at the TB test port...Check the emissions/fuel tank vapor system and canister.  Check for a vacuum leak within the front axle disconnect system.  This is an elaborate vacuum circuit with several possibilities for a leak. 

This should narrow the trouble down...Let us know what turns up.

Moses