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need help jeep wont start

bartlett91

NAXJA Forum User
Location
York, Pa
Ok so I have a 1990 jeep cherokee, and I cant get it to stay running. It will start and shut right back off, only run for a second or 2. as long as i hold the key on it will stay running but as soon as i let off it dies. So far ive replaced the crank sonsor, ignition switch down on column, ignition tumbler, fuel filter, idk what it would be im lost
 
Ok so I have a 1990 jeep cherokee, and I cant get it to stay running. It will start and shut right back off, only run for a second or 2. as long as i hold the key on it will stay running but as soon as i let off it dies. So far ive replaced the crank sonsor, ignition switch down on column, ignition tumbler, fuel filter, idk what it would be im lost

Did this start all of a sudden? Drive it everyday?

What's the fuel pressure? My first guess in the dark would be that the fuel pump stops running when you let off the key.
 
Yeah i drove it all week, came home, and went back out and it wouldnt start, i dont have a fuel pressure gauge but I kno fuel is getting up to the engine tho
 
I would check the MAP sensor, it's the one mounted to the firewall on the top of the engine compartment. It has a trapezoidal metal bracket, and usually a green plug. Check that and make sure the wiring harness hasn't rattled itself loose.

While you're under the hood, look around and see if maybe one of the fuel injectors got unplugged (seen it happen).

If it's not the map, I'm thinking possibly the solenoid... who's proper name I can't remember at this second, but it sticks into the throttle body on top of the engine and is right next to the throttle position sensor. Since your jeep starts, this part may not be working to keep your jeep running.

You might try starting the jeep and then putting your foot into the throttle to see if you can keep it going. Now that I think about it, it's probably worth checking your idle air sensor, and possibly your throttle position sensor (mounted right next to the solenoid).

From what I can tell, I think this problem is sensor related, not a deeper mechanical issue.
 
Jump the wires at the ballast resistor and give it another try.
 
Jump the wires at the ballast resistor and give it another try.

Prime suspect is the ballast resistor.

Then the Ignition switch and wiring
 
Yup, by design the ballast resistor is "out of circuit" when the key is in the full start position so fuel will be supplied. But as soon as you let off the key, the ballast resistor is "in circuit" and if it is bad, you get no fuel.

It's a very common failure on your vintage. Try jumpering it with a piece of wire; that will tell the story very quickly. Let us know what you find.
 
Renix fuel pump:

1. Key ON, the fuel pump is energized for a couple of seconds, then power is cut.

2. Key START, power to fuel pump from the starter.

3. Key ON, engine RUNNING, power to fuel pump from the fuel pump relay.

4. Key ON, engine running at WOT, power to fuel pump from the O2 heater relay.

Your problem is likely either the ignition switch--the electrical switch, not the lock cylinder, is loose or out of adjustment or bad OR when the key is in the ON position after starting there is no power from the fuel pump relay to the fuel pump.

Good luck.
 
4. Key ON, engine running at WOT, power to fuel pump from the O2 heater relay.

Really? What insane reason did they use for an excuse to do that? How did they even wire that to happen?
 
Really? What insane reason did they use for an excuse to do that? How did they even wire that to happen?

If you look at the schematic it actually makes sense, the oxygen heater relay bypasses the ballast resistor and you get a little extra fuel at WOT.

The head scratchier is the relay shuts off the heater when it energizes or so it appears. Must be the French part of the design. :)
 
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If you look at the schematic it actually makes sense, the oxygen heater relay bypasses the ballast resistor and you get a little extra fuel at WOT.

The head scratchier is the relay shuts off the heater when it energizes or so it appears. Must be the French part of the design. :)

The heater is only really needed at idle. And at WOT the ECU goes immediately to open loop, rich mode and ignores the O2 sensor per the FSM anyway.

I concur on the fuel pump peak flow at full power, bypassing the ballast resistor. It is a more complex system than we thought!!!
 
I would check the starter relay, its the silver box next to the rest of the relays on the passenger side engine bay inner fender. I have had issues with that damn silver box LOL!

I had a hard time finding one local, I had to go to the dealer.
 
I would check the starter relay, its the silver box next to the rest of the relays on the passenger side engine bay inner fender. I have had issues with that damn silver box LOL!

I had a hard time finding one local, I had to go to the dealer.

Last time I bought one Autozone had them, but they were not in the computer, they had to let me dig in an old style hard copy catalog to find the part. They had it on the shelf.
 
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