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Help me troubleshoot my 91 4.0 HO

Justpunchit

NAXJA Member #1291
Location
Jaffrey, NH
Rig Specs: 1991 4.0 HO mated to an AW4


So this is what is going on..
It takes a long long time to get running, but once it starts she runs fine. Now I shut it off and it just cranks and cranks and won't start fully. It's not a timed wait either, becuase sometimes it cranks and starts right back up other times it takes 15-20 minutes.

Any ideas?

Before someone asks, this is what was replaced. (I wasn't chasing a problem I just replaced things that were rotted, or I thought weren't up to par)

Fuel system:

Pump/sending unit
Filter
new hard and soft lines
pressure regulator


Ignition

cap&rotor
coil
wires
plugs
camshaft position sensor
throttle position sensor
battery
cables



After looking deeper into the problems this morning, I conducted a fuel pressure test and without cranking the engine I'm at 20psi then while cranking it's at 35psi. This may be a new sympton or maybe I'm just noticing now but I'm getting no spark anymore. I haven tested the CPS with an ohm meter yet, but I can hear my injectors click when it's plugged in and they stop clicking when we unplug it. What am I missing here? Is there something really obvious that I looked over?

thanks guys.
 
Hard to say, you may have two issues.

First, the early symptoms suggested a thermal failure or weak crank sensor.

Do you have 12 volts to the coil? If you do, the ASD relay is not the problem, and the crank sensor/PCM/harness/grounds are more suspect.
 
Hard to say, you may have two issues.

First, the early symptoms suggested a thermal failure or weak crank sensor.

Do you have 12 volts to the coil? If you do, the ASD relay is not the problem, and the crank sensor/PCM/harness/grounds are more suspect.

Thanks Joe, I'm looking into the coil voltage as we speak. I've tested the ASD relay and it passed, and I just replaced the crank sensor with no better results yeilded.
 
Coil has no spark also found two connectors that have no home wires to plug into could be nothing, but I just want to cover all my issues. One is on the left (drivers side of the block about 4 inches above the oil pan, in my hayes repair manual it claims to be a coolant temp sensor?

Also another connect I took a pic of that I can show a little later tonight. Also clean the ECU grounds, and other engine grounds. I guess I'll pull the part of the harness that powers the coil and start looking that over.
 
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I took the leap. I couldn't trace a decent reason for why I have no spark so I took a harness from a 94, along with the ECU.
 
I think this is going to do the trick. The only thing is your going to have to update you coolant temp sensor on the back of the head, and the crank sensor as well. The other good thing is it gets rid of that terrible ballast resistor on the drivers fender.
 
A single blue butt splice got rid of my ballast resistor.

If you have specific questions about the wiring harness, I have a 91 FSM sitting on my desk at home.
 
Well I took care of it! The 94 ECU and harness worked great!

A few things that were different; crank sensor, coolant temp sender, oil pressure sensor, wiper motor, and speed sensor wire harness and sensor. I work at a salvage yard so those weren't a big deal for me but for someone else I imagine it could get expensive going this route.

Kastein- That's a really good thing to know I tried using my 95 FSM but on some things it's worlds different.
 
You work at a salvage yard? which one? :eyes: any chance you could keep your eyes out for a few things?

Yeah, 95 vs 91 is a fairly big difference, I have the 91 FSM that covers the YJ, XJ, and MJ. Picked it up on ebay so I could fix a few things on my MJ.
 
ughhhh


I have a new problem, it's not connected to the harness because it was all working until today. 3 things stopped working this evening, Radio, clock and hvac controls. what can cause this? I've checked all my in cab fuses... the blower/heater? (iirc) was blown but everthing else is good.

Other small issues might be the cause is, my headlight switch? it doesn't dim my dash lights, and seems to seem be very touchy. Also I can pull the key out of my ignition in any position. Any ideas?
 
Well, sounds like you have a number of issues going.

1. Need a new lock cylinder, or have that one rebuilt.

2. Locate the short/excessive draw that blew the fuse.

3. When you say the HVAC quit, in what way--no blower, can't change the direction of the air flow, no a/c?

4. I would suspect the headlight switch if you can't dim the dash lights, but it could be melted wiring allowing a cross over of full voltage.

I would fix the key cylinder, and while you are in there see if the ignition switch has issues, then move to the light switch and HVAC/blower issues.

Good luck.
 
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