Cherokee 4.0L MPI Idle and Hot Start Problem

We have a problem with fast idle and no start when warm on an 89 Cherokee with 4.0L, the Renix multi-port injection, and an automatic.
A search of the web shows this is fairly common, but we seem to have addressed all of the likely possibilities.
What we've done: - replaced and adjusted per factory service manual the TPS - replaced the O2 sensor - replaced the IAC ("Idle Stepper Motor"). - cleaned the throttle body, including the IAC port. - tested the EGR components - tested the coolant temperature sensor per FSM - tested the MAP sensor per FSM - tested the manifold temperature sensor per the FSM - verified grounds - searched for manifold pressure leaks (none found, except as noted below)
Symptoms/history: - would idle very high (~1500 rpm) after warming up. Intermittently, it would return to normal operation after a period of time. - we replaced the TPS and adjusted per the FSM. Subsequently, idle speed was variable, but not fast. One day later, the vehicle would not restart after being shutdown hot. Subsequently, we performed the other tasks outlined above. - when the engine cools, it starts okay. As the engine warms up, the rpms gradually increase slightly. About the time antifreeze starts gurgling in the reservoir (I'm assuming this means the thermostat opens), the idle rpm increases to 1500 rpm or more (no tach) - with the engine idling fast, plugging the IAC port on the top of the throttle body has little if no effect on RPMs. Although the throttle plate is fully closed, pressure on the throttle plate (butterfly valve) to close it further causes the idle to drop significantly. There appear to be very small gaps along the edge of the plate. - I reved the throttle to see if this effected the behavior - it did. When the throttle closed, the rpms remained high. The IAC valve had opened, and was remaining open. We shut off the engine, then attempted restart. The engine would not start until I plugged the IAC port. Once the engine started, the port closed, and idle was normal. We shut the engine down, then removed the IAC, cleaned the valve seat, and replaced the IAC. The engine would not restart (still hot).
Any ideas? I think the CPS is the only remaining sensor, but it is not clear how that would cause such behavior.
Best regards, Jon
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I vote ground issue.
The sensors for the computer ground on a bolt by the dipstick. I would be checking those.
Mike 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: Non members can still view! Jan/06 http://www.imagestation.com/album/pictures.html?id !15147590 (More Off Road album links at bottom of the view page)
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Per the FSM, the CPS is also the "Engine Speed Sensor". It feeds crank position and RPM data to the computer, and the computer commands the IAC to adjusts idle as required.
The CPS on these models has a long history of causing intermittant no start conditions. A quick fix I have used twice for the no start problem is to "clean" the CPS connector by unplugging and replugging it. The CPS will probably test ok with an ohmmeter even if the connector is the problem. I believe there is a service bulletin that suggests doing away with this troublesome connector by simply soldering the wires at that point.
As you did not mention doing anything yet with the CPS, I would do the cheap and easy thing first. Try cleaning the connector first and see if that helps.
Good luck, Joe '90XJ
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Just for "fun" make sure that something is not keeping the throttle "butterfly" from closing all the way. Linkage, throttle cable, carbon in the throttle body? I like to do the "free" stuff first, even if it sounds wrong!
Good Luck, Bruc
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