Jeep YJ 4.0 help

Mac5005

Mac5005

Well-Known Member
Joined
Oct 19, 2005
Location
Rocky Mount
So got to good evening ranch, went to unload the jeep and it wouldn’t start.

It cranks over but won’t fire. Can easily hear the fuel pump running.

Wiggled the steering column and wheel, and it fired up.

Feels like it’s not getting spark when it won’t run.

Hot/cold doesn’t matter.

Sometimes it’s fine, some times it takes 2-3 mins of various attempts to get it to run.

If I hold the starter to make it crank longer it doesn’t make any difference.

Really seems like ignition switch.

Obviously it did it cold and hot so I’m leaning away from crank sensor,

My thoughts are to bypass ignition switch and tie the red wire and yellow wire together with a switch.

What says Nc4x4?
 
If you wiggled the column and she fired over I'd be looking at the ignition switch. I'd bet it got a loose connection going into the switch.

@77GreenMachine is up there right now, just let him sit in it. Before too long it'll be on its lid and you won't have to worry about the ignition switch.
 
GM tilt column I assume? That is what they came with. The ignition on them is a rod to a switch lower on the column. I'm betting if its the original column the wiggle of the column was just coincidence... Also will it crank over with no spark or not crank over unless you wiggle it?
 
New info lol

@ObeseFieldMouse ran out of talent and his rig got sleepy.

We troubleshot and found nothing really wrong, other than not getting fuel at startup. Once it runs, it stays running.


Also found that if I hold 1/2 throttle or more during cranking, it fires up every time.

No amount of wiggling anything made any perceived difference.

So finished the day by just cranking at wot and leaving it running most of the time.

Good evening ranch is probably my favorite WV spot over bear wallow and ivy branch
 
Throttle position sensor? Random, but manipulation of the pedal may make you think??? I honestly have no clue just spit ballin with provided info...
 
skyhighZJ said:
Throttle position sensor? Random, but manipulation of the pedal may make you think??? I honestly have no clue just spit ballin with provided info...
Click to expand...

I definitely think a sensor is bad / going bad and whatever it is, when I increase throttle to crank it trick computer into getting the injectors going.

Runs fine once it’s running. It weird.

I’ll be sure to update when I get back and get it solved.

The crank sensor controls spark and spark timing, the Hall effect (cam sensor in distributor) definitely controls the injector firing.

It’s baffling all of us how it immediately fires up with throttle opening, but is absolutely not getting fuel during cranking if throttle isn’t opened 1/2 or more. We had an extremely knowledgeable and capable group working on it yesterday, and were all left scratching our heads and laughing, going WTFFFFF.

Fuel psi and flow are good.
 
Idle air control? Though I would think that would prevent it from idling once running also. Unless you have a weak battery, and it "forgets" to open until the voltage comes up.
 
I was thinking IAC, or whatever Jeep calls it. If fuel pressure is good, the pedal controlling the TB and air coming into the TB would be my first guess
 
Any progress?
 
Update and end to this thread.

Finally went to solve this problem yesterday, thankfully it’s now fixed.

It was the throttle position sensor.

The sensor had a massive bad spot in the pot at/near idle position. So bad, that it registered as “open lead” on my meter. Once the throttle opened more than 1/3, the resistance showed up, and swept less and less as the throttle opened.

Changed the TPS and problem solved.

Now as to why this again confirms the injectors were not firing while cranking and throttle closed.

My suspicion is this. On some older obd1 and some obd2 systems, if you hold the throttle wide open during cranking, the ECU shuts off the injectors to clear a flood condition.

I haven’t been able to find that my 92 YJ computer does this in any information, but I suspect it.

This mean with zero resistance at the sensor at idle, the ECU was thinking “ clear flood” and wouldn’t fire injectors.

The clear flood is only active until the engine starts running. This is why the jeep would start off carb cleaner, and then run as normal all day.

Also explains why if I held the throttle out of the dead area, 1/3-3/4 throttle, the jeep would crank and start as normal, but wouldn’t start with throttle closed.

All fixed, just wanted to update for clarity, if someone is troubleshooting, that they may read and remember or search and find this.
 
Thanks for updating. I’ve probably spent a months worth of hours reading forums about someone with my exact issue only for it to be a 3 page dead end with no solution. Infuriating to say the least.

I hope this helps someone some day.
 
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