Jeep Comanche:

Truck cranks, but cuts off when you let off ignition key

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# 1  5/28/2011 8:22 PM

gaj265123
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Registered: 5/28/2011
Posts: 3

Truck cranks, but cuts off when you let off ignition key

I have a 1989 Jeep Comanche pickup.  Last week, it cut off while being driven.  We thought it was low on gas.  Put gas in it.  It cranks.  But as soon as you let go of the ignition key, it cuts back off.  It will only stay cranked if you hold the ignition key forward.
Thanks for your help.

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# 2  5/29/2011 12:21 AM

TechHelp2
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Registered: 11/24/2009
Posts: 1299

Re: Truck cranks, but cuts off when you let off ignition key

Are you talking about the starting of the engine or the engagement of the starter?

Does the vehicle have a 4 cylinder or a six cylinder?

If you have a 6 cylinder then the problem could be the crank sensor. There was a service bulletin on this; it describes a hard start condition related to the crank position sensor.

More detailed information can help me confirm this and also help me diagnose the problem better.

When you get back to me I will be glad to help you from there.


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# 3  5/29/2011 2:04 AM

gaj265123
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Registered: 5/28/2011
Posts: 3

Re: Truck cranks, but cuts off when you let off ignition key

It is a six cylinder.  When you turn the key, it turns over.  When you stop turning the key, it tries to fire.  But it won't stay cranked.

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# 4  5/29/2011 12:50 PM

TechHelp2
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Registered: 11/24/2009
Posts: 1299

Re: Truck cranks, but cuts off when you let off ignition key

Check to see if you have spark at the spark plugs when you crank the engine, if you have spark when it died out it may be a fuel problem.

If you have someone who has a scanner; check the computer system to see if any trouble codes have been stored in the memory of the computer. If any are stored repair them first.

On the scanner ( or if you have a tachometer on the dash or a meter to check engine RPM's), if you lose the RPM signal or don't have any signal when cranking the engine then it would be and indication the crank position might be at fault.

A dirty throttle plate inside the throttle body could be caked up with carbon. Clean up the throttle plate with a recommended throttle plate cleaner and a small brush. The idle air control motor could also be stuck in the closed position causing the engine not to idle. Clean the idle control motor or replace if cleaning doesn't help the problem.

Check the fuel pressure. You should have a two second prim when the key is turned on. That means when you turn the key on you should be able to hear the fuel pump run for two seconds and then turn off.

The MPI fuel system employs a vacuum assisted pressure regulator. Fuel pressure should be approximately 55-69 kPa (8-10 psi) lower with the vacuum line attached to the regulator than with the vacuum line disconnected. System fuel should be 214 kPa (31 psi) with the vacuum line connected to the regulator and 269 kpi (39 psi) with the vacuum line disconnected.

Try these few suggestions and get back to me with the results.


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# 5  5/29/2011 3:28 PM

gaj265123
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Registered: 5/28/2011
Posts: 3

Re: Truck cranks, but cuts off when you let off ignition key

When you turn the key, the engine cranks.  It shuts right back off.  If you hold the key forward, it keeps running and you can rev it up and everything.  It runs just like its supposed to.  As soon as you let the key go, it cuts off.

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# 6  5/30/2011 2:37 AM

TechHelp2
Administrator
Registered: 11/24/2009
Posts: 1299

Re: Truck cranks, but cuts off when you let off ignition key

Check the ignition switch, not the key the ignition switch inside the steering column. It could be out of adjustment or it may be malfunctioning in the run position. If it winds up to bad it will probably need to be replaced.

Check the fuel pump ballast resistor, it is placed in the circuit between the furl pump relay and fuel pump. It is used to reduce fuel pump noise. Voltage to the fuel pump is supplied through the ballast resistor except during start up and throttle acceleration, that's when the engine runs, at start up and acceleration, in the run position it won't run. It may be cutting of the fuel pump circuit. The ballast resistor is mounted on the fire wall of the engine compartment. The fuel pump voltage is supplied also through the starter relay, and oxygen sensor relay respectively.

That's why I told you to check the fuel pressure in the previous post, it may be cutting out.

Let me know you make out.


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