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91 Jeep Cherokee Laredo

ran smooth until suddenly stalled; would restart for only a few seconds. Visually diagnosed and installed fuel pump; then ran for 2 minutes & rough before stalling. Now we have a new crankshaft sensor; now runs for 5 seconds. Any idea before we rebuilt one part at a time? Diagnostic equipment not available…TKS

What made you replace the fuel pump and crankshaft sensor? Did you get a shotgun, aim it under the hood and replaced whatever the buckshot hit? I ask because those subsystems are not really related.

Seriously, you need to tell us a bit more.

If you cycle the key switch on-off-on-off three times and stop in the on position the Jeep’s computer will go to diagnostic mode and blink out a code if it has recognized any problems. All the codes are 2 digit and when 5-5 is blinked it is at the end of the list.

But you really must come up with some basic information or you will spend more for unneeded parts than you would spend at a good shop. No expensive equipment is needed to determine if the ignition is working or if there is pressure in the fuel rail.

The problem might be with the ignition control module.

The ignition module functions in two modes. These are the START and the RUN modes. When starting the engine the ignition module goes into the START mode. This allows full battery voltage to the ignition coil to ensure the cold engine starts. Once the engine starts and the ignition switch moves to the RUN position, the ignition module switches to the RUN mode and drops the voltage to ignition coil. This done so that when the charging system comes on line once the engine starts the higher voltage from the alternator doesn’t burn up the ignition coil. So it functions the same way that older ignition systems did when they had a ballast resistor or wire to the ignition coil.

So it could be that the ignition module functions in the START mode, but is failing in the RUN mode.

Tester

I don’t believe that the 91 Jeep has an ignition module. The engine has a distributor that is controlled by the computer.

If the OP has not opened the distributor and inspected the rotor and cap he might do so. The rotors often burn through and allow the spark to ground into the distributor shaft. Of course, that eventually toasts the pickup coil on the models that use them.

http://www.partsgeek.com/catalog/1991/jeep/cherokee/engine_electrical/ignition_control_unit.html

Tester

That Ford Motorcraft variety of module was used on carbureted Jeeps for more than a decade but never on fuel injected engines from Ford or Jeep.

That’s funny. Because here’s the coil/ignition module out of the wifes’ old 90 Jeep Cherokee.

Tester

Is that comparable to the Motorcraft style module in the earlier post, Tester? The 91 model Jeep parts listings that I find show no module. And that Motorcraft module never made the transition to fuel injection.

The first module was for the Jeep with the 2.5 liter. The second module is for a Jeep with a 4.0 liter.

Tester

I believe 1990 was the last year the Bosch fuel injection system was used on the Jeep 4.0L.
1991 and up had the Chrysler system, the coil driver is inside the PCM.
4 cylinder is different, would need to know which engine the OP has.