JCMINIS

I have a 90 cherokee,4.0 that will start and run for 45 seconds or so.The fuel pump runs while it is being cranked,but once the engine is running I never hear the fuel pump run again and it dies. I just installed a new pump assy,filter and a new fuel tank, within the past 3k miles I did tune up,new injectors and new cps sensor.I tried yesterday swapping a known good computer in it,but it ran the same.
This jeep was/is a beater that my 17 year old son and I have made a father/son project out of but it is whooping us at the moment.

Any help would be great !

Thanks JC

P.S I searched a bit but dial up makes it tough !!!!

jd4x426

did you ever change the fuel filter if it has a bad clog it could this to happen

fantic238

It is difficult to hear a new fuel pump when the engine is on, unscrew the tank cap and put your ear near the filling hole to hear better.
Does it start normally or does it crank longer?

McRay

Might check fuel pressure, I would lean more to a electrical problem being it starts in start position. Check also if you have fire to plugs and coil ignition. might have a switch problem.

ltlgreenXJ

might want to check the fuel pump relay

JCMINIS

I did change the filter, and I have had my son start the engine while I was laying under the jeep with my ear to the tank, I do hear it run while he cranks it then the engine starts and runs fine for 30-45 seconds.I just never hear the fuel pump run again and the engine dies.I assume it starts and runs because the pump runs while cranking it,but when that fuel pressure burns off the engine dies.If the fuel pump kept running im certain it would run just fine,I just dont know what controls that.
The key switch sometimes takes a little jiggling to get from the off position,I was just really dreading changing it.Is there a way to short it to test ? All my lights and gauges work fine.

Thanks for the help !!!!!!

O and it cranks right up.

fantic238

If the pump stops turning it takes much less than 30-45 sec. to shut off the engine, i would say 5-10 sec.
You can jumper the relay between connector 30 and 87 with the relay out, with a thick piece of wire to run the pump continuously. Don't supply a positive to the relay in place you could feed the current back to the ECU.
The ECU receives signals from ignition on and CPS and energizes the fuel pump relay, if you just turn ignition on and don't crank the engine the pump turns just for 4 sec. with an internal timer of the ECU.

JCMINIS

You were correct on the 5-10 sec time,wishfull thinking I guess.I jumped the 30 and 87 like suggested and it runs,when you say CPS is that crank position sensor ? Located on driver side top bell housing ? Could I test the other sides of the relay to see where my problem is ? Also if I just turn the key on with no attempt to crank the fuel pump never runs...

Thanks so much for the help !!!!!!!

Jamie57

Had this problem myself.
Does it have the factory security system installed?
Believe it or not the system has to be reset after you have disconnected the battery. It will start but not stay running. Pulled out what little hair I have for 3 days over this.
the simple reset is to put your key in the drivers door and lock and unlock it.
My jeep started right up after that.
I didn't belive my ears so I tried it again. Disconnected the battery. reconnected it and it would start and not stay running. did the door thing and presto running jeep!
I found out this little trick after a lot of searching on another forum.
Try it it can't hurt and costs nothing.

btw the fuel pump only needs to run when it senses that the fuel line pressure is down. I never hear mine a lot when starting.

fantic238

CPS=crankshaft position sensor located on drivers side top bell housing (it works don't bother)
THe other sides of the relay (85 86) are one grounded and the other is the energizing (+) coming from the ECU, i have to look on the wiring diagram when go back home and tell u which is which.
Previous owner probably has wired the pump directly under ignition on or engine cranking bypassing a problem with the ecu or the relay.

fantic238

On renix systems there is no such sensor, it has a timer in the ecu.
You have chrysler injection, it's different.

JCMINIS

I tested 85 and 86 , I didnt note which but one has power and one dosent just like you stated.The one that dosent have power if I ground it she runs like a champ however I feel if I just ground it it will cause me problems,lol.(the no power side is black w/yellow tracer if that helps).

Thanks

fantic238

Oh sorry, i forgot to look at the diagram, send me a PM in 9 hours, so that i remember.
What i exactly don't remember is if the ECU supplies ground or +12 to the relay.
I'll find out.

fantic238

Ok, i've looked at the diagram, the black/yellow wire is the ground like i thought. U can either build a new wire and ground it or trace the old one till you find where it is interrupted/rusted. Let me know.

89cherokeelimited

If it runs then dies I bet it's the fuel pump. Mine did it and I finally changed my filter and turned my key to the on position and made sure there was on air in the rail and started up and has ran perfect ever since. I bought a fuel pump and ended up returning it becasue it was a $10.00 fix