Sounds like the crank sensor. One way to check is while cranking watch the tach. If there is no movement then most likely it needs a crank sensor. If the sensor is good you will see a slight bounce in the tach.
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If its a wrangler with the carberator/Ford computer, replace computer. If you cannot find replacement, if the distributer has mechanical and vacuum advance on it, add an MSD with magnetic trigger and it will run great.
Great question. On a 1991 vehicle, that's all what the computer was about-engine management functions, that and some emission requirements. Turns the pump on, runs the injector circuits, and figures spark timing. Only has so many sensors, and so many output controllers. If your spark is good as in a blue, snapping spark, and the injectors are pulsing, and nothing wrong with the pump circuit (check your fuel pressure to verify), then the computer has that much going for it. Even so, a diagnostic flow chart could lead you down a long list of tests or things to check before it arrives at the computer as the possible problem. And when all is said and done, they will often say "Substitute a known good computer and see if problem persists."
Jiggle the wire connectors going in to the computer under the hood -- first check battery cables/terminal connections at battery -- beleive it or not, connection could SEEM good because you get crank & lights, but if it's faulty, loose, engine won't start - computer requires good solid voltage flow.. next check a fusible link very close to the battery off of the positive cable. check fusible link carefully. this could cause your problem for sure.
You need to find out what is missing at the time that it is not starting. You are either not getting ignition (spark) or not getting fuel.
A real common problem for Jeep 4.0L engines is bad crankshaft position sensors and bad connections at the crankshaft position sensor connectors. When these mess up, you will have a "No Spark" condition. I would check the connector and make sure it is clean and tight before running out and spending money on buying a new crank sensor - especially if this Jeep has been operated in mud and/or water.
The only other suggestion I have is to check the fuel pressure when it is not starting. You could have a fuel pump or a fuel pump relay that works only when it wants to.
BAD CRANK SENSOR ON TOP OF ****** HARD TO SEE FOLLOW WIRES, OR FUEL PRESSURE LOW BAD PUMP IN TANK..RENT TOOL AT AUTO ZONE HOOK UP TO FUEL RAIL SHOULD HAVE AT LEAST 45 50 PSI OF PRESSURE IF GOOD THEN CRANK SENSOR.WHEN IT DOESNT START CHECK FOR SPARK IF NO SPARK GOOD CHANCE ITS BAD.ITS ABOUT 45.00 BUCKS FOR NEW ONE AND ABOUT 35MIN TO PUT ON.GOOD LUCK
I would look at the fuel delivery system including the fuel pump and fuel filter to make sure your getting the proper fuel pressure to the injectors.I would also check the power control module to see if that's operating properly.Some auto repair shops have machines to check the operation of the module.I hope this helps or maybe gives you an idea for a new starting point.
Sounds good ninemil, I would also check for any bent or disconnected fuel lines that would restrict the fuel flow as well. I do not know if this has a fuel pump assembly but you should be able to hear it actuate on when the key is in the "on" position. How did it act before you shut it off? Was it harder to start? Make sure you buy a high quality pump (Carter) many of those cheaper brands will fail trust me.