93 Cherokee starts then shuts off after 1-2 seconds

History:
A week ago I started my Jeep and drove around the block then shut if off to run inside the house fast, when I came out I tried to start my Jeep and it cranked and cranked without trying to start. Only after holding the throttle to the floor was it able to start, then it ran fine until 2 days ago. (I am not sure if this is relevant to the current problem or not)

Current problem:
I was driving home from college and my Jeep just shut off while I was driving down the road. No lack of power, no bogging, no nothing, just shut off and I coasted to a stop. When I tried to start it back up it started right away then shut off within 1-2 seconds. I tried again with the same result, starts fine then shuts off after 1-2 seconds. If I hold the throttle to the floor it will stay running until I let the RPMs drop below about 3000, then it bogs and dies.

What I have tried and replaced based on reading other threads and talking to mechanics:
- Unhooked the MAP sensor, it starts and shuts off within 1/2 a second. When I plug the MAP sensor back in it stays running for the 1-2 seconds.
- Replaced the fuel filter (no change)
- Replaced the fuel pump
- Replaced the gas tank and cleaned out the fuel lines (because it was rusty, did fuel pump at same time)
- Replaced the ballast resistor (no change)
- Replaced the Throttle Position Sensor (no change)
- Replaced the Crank Position Sensor (no change)
- Unhooked every visible sensor to see if there were any changes, nothing had an impact besides the MAP sensor
- Checked all fuses, all are good
- Checked the fuel pressure, everything is good based on the specs from my Chitron manual and what i've read on this site
- Replaced the fuel pump again to rule out a bad pump (no change)

Other notes:
- While its running for that 1-2 seconds it runs fine with good oil pressure
- After the 1-2 seconds and the Jeep shuts off the fuel pump does make a noise, the noise sounds exactly like when I first turn on the ignition and the pump primes
- not sure how to check the flow, but with a new fuel pump I assume its good?
- not sure how to check the ignition but there are no burned or melted wires and would the ignition go bad and shut the vehicle off while driving down the road?
- I do not have an alarm system

This fix is becoming very expensive and I would love to get it running soon. If anyone has any other ideas please post them.


Thanks for the help in advance.


P.S. I am not a mechanic, just replaced all those parts based on suggestions from mechanics.

I'd swap out the fuel pump relay with another similar relay in the PDC.
Then do a volt test on both ends of the ballast resistor, during cranking and during run. My guess is as soon as you release the key from start to run the power dies. Or possibly your new ballast resistor is junk.

I'd do an ohm test on your new ballast resistor, new doesn't mean good.

I'm thinking either the fuel pump relay, associated wiring or connectors.

Have you checked it for trouble codes? Read how to do it here and see the list of codes down in the thread a little farther......

http://www.2carpros.com/questions/jeep-cherokee-1993-jeep-cherokee-diagnostic-fault-codes

Ignition switch itself may still be in play here.

Fuel Pump relay FTW...

Just in case it's NOT the fuel pump relay - I had a bad wire at the ignition switch that caused a shutdown in the Run position. You could remove the plastic cover on the bottom half of the column and check the multi-wire connector block close to the key for signs of corroded wiring or cracked insulation. If that first wire either has poor/no contact or is exposed and grounds out on the column it can cause a shutdown with the key in Run but it will still start just fine.

Will it stay running if you keep on the gas to hold the revs up? If so you could also check the voltage at the battery when you start up during the couple seconds of running. If the battery is weak, it can start but then die right away. I had this happen a few days ago when the GF left the lights on. Jeep started but would not stay running at idle or coming to a stop until after a 20 mile run to give it a little charge. It's now back to normal (good normal, not XJ "needs some work" normal.)

YELLAHEEP said:

Have you checked it for trouble codes? Read how to do it here and see the list of codes down in the thread a little farther......

http://www.2carpros.com/questions/jeep-cherokee-1993-jeep-cherokee-diagnostic-fault-codes

Click to expand...

- I replaced the fuel pump relay (no change)

I also checked my error codes and I recieved 2 of them: (12, 14)

:arrowr:12 Direct battery input to PCM was disconnected within the last 50 Key-on cycles.

:arrowr:14 MAP sensor input above or below acceptable voltage.


14 means possible MAP sensor which I suspected from the beginning (but listened to the mechanics instead of going with my instinct), but what does 12 mean? Could 12 cause the 14?

12 is informational only. Does not indicate an actual problem.

birchlakeXJ said:

12 is informational only. Does not indicate an actual problem.

Click to expand...

Thank you for that info, so the 14 code could be triggerin the 12 code?

I replaced the MAP sensor and it still does the same thing and I still get codes 12 and 14.

So the MAP sensor was good what could cause it to throw that code 14 then?

Another thought:
By searching this forum and reading other peoples posts, I have been coming across a reoccuring theme in the suggestions, and that is possibly the ignition switch is bad?

What I have not found is, if you start the jeep and keep it running by revving the engine does that mean the switch is good since it will stay running at that point until you let off the revs?

Nick_XJ said:

Another thought:
By searching this forum and reading other peoples posts, I have been coming across a reoccuring theme in the suggestions, and that is possibly the ignition switch is bad?

What I have not found is, if you start the jeep and keep it running by revving the engine does that mean the switch is good since it will stay running at that point until you let off the revs?

Click to expand...

If you can keep it running by tipping in the throttle, the problem is not likely an ignition switch.

Concentrate on fuel delivery and everything involved with the fuel circuit.

Let's go back to how you tested fuel delivery. Assuming you are using a known good gauge? You should have 31 psi at idle. THEN (very important step), you must disconnect the vacuum line leading to the fuel pressure regulator. When you do that, the pressure should jump to approximately 39 psi. Also check the vacuum line leading to the regulator. It should be pulling good vacuum and not be wet with gas; if it is, you have a ruptured fuel pressure regulator diaphragm.

Gotta make sure that your fuel pressure is verified properlyl

Thank you all for your help and suggestions.

I ended up taking my Jeep to a mechanic and he determined that the problem was a faulty Crank Position Sensor. Turns out I got a defective one from Autozone.

The jeep is now running. Thanks again!


Oh and he said a faulty crank position sensor will feed incorrect voltage to the MAP sensor. You can determine this by checking the middle wire on the MAP sensor since that is the INPUT, if its not 5 volts then it will throw error code 14. The code 14 disconnecting power to the PCM to prevent damage is the cause for code 12.

Hey I also have a 93 cherokee and had a similar problem last week and it turned out to be the ballist rersistor on the drivers fender well. It is up toward the fire wall and easy way to test it is to just put a jumper wire in between the two wires and try and start it, if it runs then thats the problem and Autozone does carry it but I don't have the part number.

Wow! Ballist rersistor, I never would have thought of that!