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Renix start, rev, die

blenny

blenny

RRC President
Location
Surfside Beach, TX
So I have had my 1990 4.0, ax-15 on stands for about 3 months doing various upgrades, the only one under the hood being wj booster. To do that I removed the air box. I ran it yesterday and it ran fine, I then put the air box back in and went to start it this evening. It starts after normal renix cranking time, then revs up to 2500 (normal) then dies, no sputtering, just dies. After a few more tries I found if I kept the key in the "start" position it will keep running and rev up just fine. As soon I let the key go from "Start" to "Run" it dies, again no sputtering, just dies. I did search and found a lot of no starts, but nothing like this. Ran out of daylight so cannot do much diagnosis right now so I was hoping for an easy answer. Thanks
 
I'm having the EXACT same problem with my 88 mj 4.0/aw4 right now. Lol I Already tried another ignition switch that didnt help anything. I'll let you know If come up with anything else. Another jeep guy I know recommended trying a crank sensor but mine looks brand new, I just got the jeep so idk for sure.
 
Glad you figured it out, that's exactly what I was going to suggest.

People will tell you it'll make your pump burn out faster, but I've been running my MJ with the ballast resistor bypassed since two days after the PO bought it (friend of mine) at around 200k miles, it's now at over 250k with the same stock pump in it. No issues.
 
MTBracer367 said:
Tried that on mine already but I also put new relays in it since, so maybe I'll try bypassing/replacing it again. Thanks for the help though
Click to expand...

Check the ground and wires, wire connector at the fuel tank to the pump. They may need cleaning.
 
The ballast resistor is right by the air box. I bet you accidentally disco'd the ballast resistor or possibly broke it.
 
There are three or four (A/C equipped) relays located on the right (passenger's) side fenderwell, and the starter relay/PDC.

When you first turn the key to "RUN" the ECU sends a ground to the fuel pump relay and energizes the relay for one to three seconds to "prime" the pump. When no CPS signal has been received in that one to three seconds the ECU cuts the ground and the fuel pump shuts off.

When you turn the key to "START" the fuel pump relay does SQUAT--power is sent to the fuel pump from the STARTER RELAY/PDC during engine CRANKING. If the engine will start when cranking but dies when you return the key to "RUN" the fuel pump relay is the first suspect, followed by the ignition switch.

Key in the "RUN" position, engine running from IDLE to WOT the fuel pump relay powers the fuel pump.

Key in the "RUN" position, engine at WOT and the o2 heater relay powers the fuel pump.
 
MTBracer367 :

If you are getting a brief run followed by the stall, the fuel pump can be safely rulled out. I'm trying to think of this from memory here, and someone else may be able to chime in but I believe there is a fusable link between the fuel pump relay and the power off the battery (the big lug on the starter relay)

Fuel pump gets power during start as previously said via the starter relay (orange /blk tracer wire if i'm not mistaken ) all from memory... During run as other folks have said power is coming from the relay stack .. That power is fed from a fuseable link I think..

Others feel free to chime in... When I had this problem 15 years ago, mine was a fuseable link that had gone out (from teenage stupidity (another long story)
 
Still no luck. I tried hard wireing the fuel pump on a switch just to get it to run and it wouldn't even fire. Put it back to stock and it still does the same thing as previously
 
Have you tried replacing the ignition switch, and checking for damaged wires at the ignition switch?

Also, check the vacuum line at the fuel pressure regulator for signs of leaking fuel, that would flood the engine and kill it right after start up.

You might try a fuel pressure gauge to watch it while a second person starts it.

Joe said earlier:

If the engine will start when cranking but dies when you return the key to "RUN" the fuel pump relay is the first suspect, followed by the ignition switch.
Click to expand...

I would also check the spark plugs and cap and rotor, next.


blenny said:
So I have had my 1990 4.0, ax-15 on stands for about 3 months doing various upgrades, the only one under the hood being wj booster. To do that I removed the air box. I ran it yesterday and it ran fine, I then put the air box back in and went to start it this evening. It starts after normal renix cranking time, then revs up to 2500 (normal) then dies, no sputtering, just dies. After a few more tries I found if I kept the key in the "start" position it will keep running and rev up just fine. As soon I let the key go from "Start" to "Run" it dies, again no sputtering, just dies. I did search and found a lot of no starts, but nothing like this. Ran out of daylight so cannot do much diagnosis right now so I was hoping for an easy answer. Thanks
Click to expand...
 
MTBracer367 said:
Still no luck. I tried hard wireing the fuel pump on a switch just to get it to run and it wouldn't even fire. Put it back to stock and it still does the same thing as previously
Click to expand...

That makes me suspect flooding from a bad fuel pressure regulator.
 
Ignition stuff (plugs wires cap rotor) is all brand new including the ignition switch. I'm at the point of giving up on it and its gonna get scraped or a HO obd1 swap
 
Try my suggestions first. Also use a fuel pressure gauge, to see if there is a fuel leak, FPR to the vacuum line of leaking fuel injector flooding the engine.

I once had an ignition switch install that did something similar, it was a location issue, linkage was off 1/4" just loosened the switch mounting hardware and moved the switch about 1/8" and fixed the problem. Since you now say the ignition switch is new, perhaps it is not located in the sweet spot yet!!!!

I really think it is wiring-switch issue or a leaking fuel injector or FPR, flooding the engine. The fact you hard wired the fuel pump and got a no start screams of leaking fuel flooding the engine to me!!!

You can verify this with a leak down pressure test on the fuel rail with a fuel pressure gauge, free rental at a parts store.
 
The ignition switch isn't even mounted. I've been working it with a screw driver to eliminate the linkage possibility. The only thing I have checked is the pressure regulator. But I work on this thing by myself almost always so it gets difficult diagnosing it single handedly. I'll try it out today and see what happens
 
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