This past Friday (7/12/24) I took my xj to the mountains for some camping with my dad and brother on SoCal. It was about 95 out, not that hot for July in SoCal, but hot enough to make the xj hit 220-230 going up the dirt road. It drove great until it overheated. I lost throttle, and the motor began to die significantly so I turned it off and threw it in park. Once it cooled down, the damn thing wouldn't start again. Got it towed home, theres a crazy picture of getting it down the mountain with no power, and continued with our trip. Got back home Sunday morning, decided to tackle the jeeps starting problem. Just for fun I decided to try to start it, just cranked. My dad originally thought fuel pump, so I had someone crank it while I listened to it. I heard it engaged, and just to be safe I unplugged the hoses and sure enough I had fuel. Next I took the filter out to see if it was contaminated, it was pretty clean. I ruled out I was getting fuel to the motor, and then I did some reading here on CF. Turns out the crankshaft position sensor goes out quite a bit when overheating and messes with the spark. I wasn't thinking and decided to replace that without checking if I had spark, which I later checked after the CPS was installed, a pain I must say. I have spark, I have fuel. I forgot to plug a spark plug back in, tried cranking it and didn't start. Then I realized the plug was unplugged, plugged it in, and then it started for like 1.2 seconds and then died quickly. I had the battery on the trickle charge from last night (7/15/24) to today (7/16/24) so it should've been good. But my battery gauge shows it just above red when cranking. I know the MAP sensor is another faulty connection, but that allows spark which I have. I don't know what to check or replace anymore besides my alternator? Any help is appreciated, and sorry for the long story but I think it helps you all understand everything as much as possible.

 

An Xj will not idle if the battery is bad.

It may even start, but especially if jump started, measure the battery voltage when it has been off the charger a while to settle, should be over 12.5V

Dont throw out you OEM crank sensor yet !

Do the older XJs have fault codes available with the "key trick" ?

Th e Cam sensor or its wiring can also be suspect

In a case of overheat, its also worth considering doing a compression check

 

Your profile say 1989 so we'll go with that,

First, no "key trick" or other method to pull codes. Renix ECUs don't store them.

You should check your fuel pressure with a gauge to make sure there's enough. With the regulator vacuum line disconnected (and plugged) you should see 39psi. When you first turn the key to RUN (not start) you should hear the pump prime for a few seconds then stop. Do you? Fuel pressure should be 31psi with the engine running and vacuum line reinstalled.

How "energetic" is the cranking? Renix needs to see about 300 rpm before it's starts injecting fuel. You can try squirting some starter fluid in it to see if you can get it to fire up.

If you haven't already, you should visit and bookmark Cruiser54's (RIP) site on Renix tips here: https://cruiser54.com/?page_id=75. You should review tips 1-5 to start (no pun intended).

Under the technical manuals tab, you'll find the FSM, parts guide and Renix Fuel Injection manual. The tips and manuals will help you test the sensors and diagnose the issue.

 

I do hear the fuel pump engage when the key is in the on position, not start. How would I check fuel pressure? Is there a special type of gauge or would a tire pressure gauge work just fine?

 

Fuel pressure gauges are pretty cheap to buy. You can also "rent" them for free at a local auto parts store. DO NOT USE a tire pressure gauge.

 

Got my battery back from AutoZone, checked the voltage, it read 12.4 or something in that range, not quite 12.5 though. I had my dad try cranking it over while I tested the fuel valve, fuel would spew out. Maybe I should still get a guage? Also, does my alternator have anything to do with my spark plugs, my dad thought the battery may start the spark, and then the alternator takes over, but that doesn't make sense in my head. I'll look into a compression test, but I'm not sure what to do from there.

 

Took the starter out and tested it with jumper cables, does what it's designed to do. Cleaned up all of the grounds and it still won't start. The ignition control module seems to be another issue on some other forums, but I have spark so that should be fine right? Same thing with the MAP sensor. I'm really stumped on this...

 

BUMP!

I found out my problem was fuel. It started right up on starting fluid and died out. Took my fuel rail off and tested if the injectors were injecting and they were weak. I have the KSUSPENSION 4 hole injectors with the stock harness plugs. They ran great for a solid 2 weeks, and then it overheated and wouldn't start. Maybe thats when the fuel pump went out, so I decided maybe I should replace the fuel pump and filter. Removal went smooth. Installation was the hassle. The new pump primes, but no fuel comes out the lines when the key is in the on position or when cranking. Does it have to sit in the tank a specific way? Oh, I also tried it with the fuel tank lowered, so maybe that has something to do with no fuel out the new pump? Any help is appreciated, I really need this jeep running soon.

 

a few things here

I dont know the early vehicles but is the fuel pump receiving 12V in the "run" position (you can back probe the power wire to the FP harness down near the back

Another method is remove FP from tank, sit it in a bucket of fuel and see what it is doing

take safety precautions against fire/explosion/fuel spraying everywhere.

Try to avoid running the FP "dry" for more than 1 second, as fuel lubes them

In the later vehicles the FP and ASD relay interact with each other to ensure all systems work

If the PCM senses something is "wrong", the ASD will command the FP to shut down after priming