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

Re: Cherokee 4.0L MPI Idle and Hot Start Problem
 

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/pi...?id=2115147590
(More Off Road album links at bottom of the view page)

jondmclin@yahoo.com wrote:
> 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
>

Re: Cherokee 4.0L MPI Idle and Hot Start Problem
 

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/pi...?id=2115147590
(More Off Road album links at bottom of the view page)

jondmclin@yahoo.com wrote:
> 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
>

Re: Cherokee 4.0L MPI Idle and Hot Start Problem
 

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/pi...?id=2115147590
(More Off Road album links at bottom of the view page)

jondmclin@yahoo.com wrote:
> 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
>

Re: Cherokee 4.0L MPI Idle and Hot Start Problem
 

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

Re: Cherokee 4.0L MPI Idle and Hot Start Problem
 

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

Re: Cherokee 4.0L MPI Idle and Hot Start Problem
 

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

Re: Cherokee 4.0L MPI Idle and Hot Start Problem
 

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

Re: Cherokee 4.0L MPI Idle and Hot Start Problem
 

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

Re: Cherokee 4.0L MPI Idle and Hot Start Problem
 

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