Cranks but won't fire... Not CPS issue

Ok guys. I've got a good one here. I've got an '00 xj 4.0 aw4 with 231. Blah blah BUT it won't start all the time :twak: so here are the issues I'm getting... Cranks fine, when it does fire it runs 110% but once I shut it off it won't restart all the time. I get no bus sometimes and other times the cluster works fine. The CPS is a new Napa one that I have tested and its not grounded out. I have also tried 2 other CPS with same issues. I have tried a different ecu I got the same issues. What I've done to try and fix the issues... Clean all connectors under good, connectors under the jeep, cleaned ecu connectors, cleaned the grounds for ecu and motor and other I could find, pull fuse box under the hood and cleaned it all the wires and connectors looked flawless, clean ecu, cleaned connectors under the dash driver side, traced some wire from fuse box under hood and ecu, the other weird thing is when my key is 'on' my aux fan will come on making me think the jeep is in a safety mode or something so it won't overheat if it was running. I did blow a 15a fuse under the hood the other day which caused the fuel pump to not work. But idk if that was just randomly or what. I'm no dummy when it comes to turning wrenches but honestly right now I'm pretty stumped lol :sure: the asd relay under the hood will sometimes click and purge almost, the aux fan purges on the same rhythm as the relay. I'm assuming its a grounded out bus wire or something but I have not seem anything but nice looking wires. Wondering if anyone else had any ideas to help me look.

I know this sounds like a common CPS issues but unfortunately I don't think it is at all. The other reason I don't think the CPS is the issue is cause when the cluster doesn't do anything unplugging the CPS does NOT make the fuel/ volts move like it should.

So help me out guys! Any advice is much appreciated!

Hmm that's a tough one. Does your Jeep have the Sentry Key system? It almost sounds like something is preventing it from starting if the relays are clicking.

You might try a new ignition switch also, since the problem seems to be somewhat intermittent.

A giant WAG (wild arsed guess) but it may be a shorted engine temperature sensor. The sensor itself can short or it can short through coolant splashed on the sensor.

Been my experience, on earlier models, a shorted sensor can cause bus issues that may or may not display a trouble code. Which sensor is anybodies guess, sometimes.

Test the orange wire at the CPS for five volts. That same orange wire supplies voltages to multiple sensors.

Mine was 02 sensor harness. One of wires of harness was scrapped in half. I tried hard wiring it but truck still won't start.

jeepkid03 said:

Does your Jeep have the Sentry Key system?

Click to expand...

Not how it works. Sentry system will let it start, run for a few seconds then die.

Re: Re: Cranks but won't fire... Not CPS issue

jeepkid03 said:

Hmm that's a tough one. Does your Jeep have the Sentry Key system? It almost sounds like something is preventing it from starting if the relays are clicking.

You might try a new ignition switch also, since the problem seems to be somewhat intermittent.

Click to expand...

Talyn said:

Not how it works. Sentry system will let it start, run for a few seconds then die.

Click to expand...

actually after several attempts the vehicle won't start at all.. the sentry system kills the fuel pump... It will start once or twice but once it looses fuel pressure it won't start...

jeepkid03 said:

Hmm that's a tough one. Does your Jeep have the Sentry Key system? It almost sounds like something is preventing it from starting if the relays are clicking.

You might try a new ignition switch also, since the problem seems to be somewhat intermittent.

Click to expand...

No, its the normal chipless key thankfully. I was thinking ignition switch too might try that next but that doesnt explain the NO BUS

8Mud said:

A giant WAG (wild arsed guess) but it may be a shorted engine temperature sensor. The sensor itself can short or it can short through coolant splashed on the sensor.

Been my experience, on earlier models, a shorted sensor can cause bus issues that may or may not display a trouble code. Which sensor is anybodies guess, sometimes.

Click to expand...

i will check them for resitance

8Mud said:

Test the orange wire at the CPS for five volts. That same orange wire supplies voltages to multiple sensors.

Click to expand...

Will do! the CPS wasnt showing infinity resistance like i read it should somewhere, but i tried 3 of them and they all were the same.

JeepNovice said:

Mine was 02 sensor harness. One of wires of harness was scrapped in half. I tried hard wiring it but truck still won't start.

Click to expand...

will trace the wires further back

Talyn said:

Not how it works. Sentry system will let it start, run for a few seconds then die.

Click to expand...

correct, a non issue for me though.




Were getting a bunch of snow here and I dont have a garage so its just gotta sit there for a little longer :twak: Thing is really driving me nuts why it wont fire. Im going to go to junkyard and try and find another ECU with matching numbers. I have an ecu outta a 99 but different numbers. either way it doesnt fire with it, but i did notice with the 99 ecu the aux fan doesnt come on like with my 00 ecu? maybe mine is shorted or something? also going to pull a TCM from junkyard. maybe ill grab an ignition switch too while im there. Ill def have enough spare parts lol poor jeep is sitting outside half dash riped out and wires all over haha depressing looks neglected