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Amedee
Amedee, Jeep Technician
Category: Jeep
Satisfied Customers: 21980
Experience:  ASE Master Technician advanced level specialist. Wisconsin certified emissions state inspector
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Jeep Cherokee Laredo: My 1991 Jeep Cherokee will crank but

Customer Question

My 1991 Jeep Cherokee will crank but not start. I have spark, fuel, air.
Any ideas?
Submitted: 2 years ago.
Category: Jeep
Expert:  Amedee replied 2 years ago.
Hello!

Did you still need help?

Does the engine run on starting fluid?

Have you checked engine compression?
Customer: replied 2 years ago.
no it doesn't.
Expert:  Amedee replied 2 years ago.
Ok.. start by checking the TPS and MAP sensor for 5 volts with the key on. This will ensure that the computer is awake. If not, then all of the powers and grounds going into the PCM need to be checked with the key on. If the TPS and MAP sensor have 5 volts with the key on, then see if the ignition coil has power while cranking.

Let me know what you find!
Customer: replied 2 years ago.
I have 5 volts at both.
Expert:  Amedee replied 2 years ago.
You said you have spark when cranking correct? If so, try to disconnect power to the fuel pump or disconnect the injectors and see if the engine runs on starting fluid. If should. If not, the ignition or valve timing is off.
Customer: replied 2 years ago.
When this scenario started yes, But I changed the CPS, and didn;t check for spark, but it still wouldn't start. So last night I installed a new coil, cap and rotor. When it still didn;t start, I checked spark, and I don;t think it's there. I had my 5yr old cranking it and he couldn't keep the key turned for very long.

I need to start from the ground up and go from there. Do you agree?

Lets pretend that I haven't looked into anything. Where would you start?
Expert:  Amedee replied 2 years ago.
YES.. I would agree! You need to start by checking the basics.

We will have to pinpoint what the engine is missing.


Spark


Fuel


Compression



All three of these are tested when the engine is being cranked over. There are two way of checking for fuel. Fuel pressure and fuel trigger. Both are very important and must be present for the car to start. Fuel pressure is self explanatory. You just hook up the gage and see what the reading is. Fuel trigger is check with a noid light. The injector gets unplugged and the noid light gets plugged in to the injector clip. With the engine cranking over, this light must blink. This is the signal to the injectors from the computer. You can have all the fuel pressure in the world, but it wont matter if the injector is not pulsing gas into the cylinder like it should! All three tests play a very important part in diagnosing a no start condition. Be extremely careful when doing these tests.



A simpler test would be to see if it runs on starting fluid. If it does, then you know you have spark and compression and you simply have a fuel delivery problem
Customer: replied 2 years ago.
I went out and verified fuel.
I took the noid light back, but I have good pressure at the rail, 33psi. and a strong smell of fuel in the exhaust and it will NOT start on starting fluid.
Expert:  Amedee replied 2 years ago.
Hello!

What about spark?
Customer: replied 2 years ago.
so Far, no spark.
Expert:  Amedee replied 2 years ago.
Ok.. it sounds to me like you either have a bad ignition coil or a wiring issue preventing the ignition coil from firing spark. First thing to do is check to see if the coil has power with the key in the on position. You can use a test light or a voltmeter. Next step would be to see if its getting a ground signal trigger. This is done with a test light. But only you would hook it up backwards. Hook the alligator clip of the test light and hook it to battery positive, and take the tip of the test light and touch it the signal wire of the coil. The test light should blink when cranking over the engine if its getting a signal. If you use a conventional test light, it will flicker very very dim (so look closely) if the coil is getting a signal to fire the spark.

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Amedee
Amedee
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ASE Master Technician advanced level specialist. Wisconsin certified emissions state inspector