‘91 YJ Crank with No start


New Member
Oct 30, 2023
Pleasant Grove, Utah
Hey guys, so I have a 1991 jeep YJ with 140,000 miles on it. Recently, i woke up and tried starting my jeep in the morning, and it would simply crank over and over but never ignite. It ran great the day before. I checked the battery, I tried jumping it and it didnt do anything. Later, I was testing things on it and I found that there is absolutely zero fuel pressure from the Schrader valve. Went back a bit on the fuel line and found absolutely nothing coming out of the fuel line behind the fuel filter. So, from my understanding, it seems the fuel pump just died on me. So, i changed out the fuel pump, and the Jeep started right up. Drove me to school, then after school it was doing the same crank with no start it was doing earlier.
Tried jumping it again, no success. Towed it home. Checked fuel pressure and there was nothing again. I put a jumper pack on the battery, and it started again! It would start for the whole day, then the next morning it was back to cranking. Jumper pack isn’t doing it anymore either.

I checked the fuel pump fuse, it is fine. The fuel pump relay seems in good condition and I get the same crank no start when I put a new relay in.
I have NOT checked the relay terminal/circuits or anything. What do you guys think? What could be going bad?

The battery IS old, about 5 years not, and slightly corroded. I cleaned it this morning, and that did nothing to the crank no start. I tested it, got about 12.2 volts out of it, about 10.5 volts when cranking. Any help would be appreciated!
no engine told. EFI sure, no carb. last RENIX Aug.1990 mfg build date.
1991 model had huge changes. (EFI ) can I guess 2.5 liter 4 banger?
the 91 has 4 injectors now, calleed, MPI not TBI and carb , nor is it a french, PCM, 91s are good jeeps.
warning #1 if you crank the battery too low, at 10vdc the PCM can not operate, 11vdc cranking is the limit,(my rules)

fuel dribble test fails, next is smack tank with rubber hammer, see if pump wakes up, even tow truck drivers know this trick,. EFI cars .(pump motor funky)
the next test is swap horn and fuel pump relay. next.
then swap ASD for HORN

what I do is this , not guessing
all other electrics work head lamps not dim or dead. dash blower blows on high fast.
radio works
all cluster lamps glow. keyed on.(as it did for 30 years )
CEL , is next: check engine lamp.
I key on,does the cluster lamps all work right. for sure CEL glows key on,if not that means CEL lamp bulb is burned out , fuses blown or PCM is dead.
1,2,3 all cars EFI do this , the check engine lamp must work key on.(only)
as you see, the pump only turns on and runs if he PCM allows it!. and via the fuel pump relay and Fuel pump fuse. and ASD relay.

next is ASD checks. (meter time) DMM meter, tests. a $15 tool.

"jumper pack? (this battery in a box things? hat when needed are now bad? ( got it 3 Christmas, ago. as a gift or older)? cute sure if they work, but?

your tests are wrong or more accurately not complete. sorry. (the ASD must be checked) on newer jeeps 96 up are more easy, cheap scan tools work then.
PCM logic 101: ( to fix PCM issues learn how it works)
the PCM cuts all fuel for many reasons. (to prevent crash and burn as seen on TV)<<< ASD does that.
rule #1 the ASD and FUEL pump relay are wired, all in one, the control line is 1 line to both, so the both die or work in parallel.

if spark is dead, all injectors are cut dead. if the PCM cuts ASD the fuel relay cuts at the same instant.
the FSM tells you in 2 seconds, A142 overloads cut both in up to 2 seconds flat.

you didn't try spray can test fuel or spark, so don't know if spark is good and EFI not detecting failed spark or failed precursors to spark.

we all know you cranked the battery to death and the funky charge pack thingy.
nothing new there, everyone does ,desperate. (own a battery charger to do diagnosis, page 1 EFI FSM says keep the battery charged or fail..
what i do first is check spark (if dead, ASD is tested next or soon)
then if spark is good try test fuel (in blind hope engine really is good)
then if those 2 tests pass , then fuel pressure, 41psi good. (not 0, not 5 not 10 not 20 psi) see?????????

The PCM must have good inputs or there will be no spark and or no fueling.
the PCM is only a car computer, to do EFI.
even the pump ground can be bad, or other grounds. to PCM or its devices. inputs and outputs.

when an engine dies we put the battery ON charge ( a real tool and charger) or risk wrecking good battery , draining it dead many times.

91yj, is good year, older not so with french Renix PCM , the biggest changes, are ending RENIX>(or worse carb engines) and goes to Chrysler, PCM

the 91. has DTC codes, ever do the key on 3 times tests? "diagnostic trouble codes" that kids call codes now. DTC errors is better name.
The PCM can even warn of ,
dead CMP (distrib sensor)
dead CKP crank sensor
the PCM has ASD relay.
this relay can also be swapped with Horn.

the best act is to monitor the ASD relay with a meter. (using tricky back,probing method) I use tiny wire to pin24
pin 24 must go hot and stay hot 12vdc (or 11v)_ cranking for over 2 seconds the test is 5 seconds it must stay up. the release the key.

see photo below.
ASD goes off line if CMP ,ASD fails, or injectors short or the 02 sensor heater shorts (unplug it ) the HV high voltage primary coil shorts.
ASD pin 24 is monitored by the PCM if this pin ever overloads, (car battery weak?) the PCM drops the ASD off line.
that is how it works.
the ASD and fuel pump relay run off 1 wire on this car, if one drops both drop off line.
the free schematics will prove that.

there are lots more cause, but doing all tests, we learn what to do, next. ASD is first. or you will never fix it.

power feed A142 must not be ever overloaded, the PCM monitors A142 and all times (voltage too low it shuts down all fuel and spark)
yes I said spark and is confusing the jeep folks.
if spark fails, ASD drops and kills spark and fuel both. (sounds wrong and is NOT)
what it does is give spark a chance to be good for 2seconds, if not ,the PCM cuts it all dead.
yes tricky but is crash protection all modern jeeps,1991 up or even dodges

my ASD log

So you found spark dead? (and sure fuel too)_

ASD RELAY drops after closing 2-3 seconds, or is dead (swap horn relay to ASD relay slot)?
The ASD drops out of SPARK is or went dead,(detected) by law and by NO CRASH AND BURN ,events
Fuses not blown or rusted to dust.gee 30 years old?

see FSM page 8w-72 (yj sheet 3,11,14)

1:CKP dead (crank sensor.)

2:CMP in distributor, (on newer cars its on the CAM shaft sensor,my 2010 jeep 3.7L/4.7 yes)

3:the PCM, ( ASD monitor pin)detected ASD output overloaded (too much current, amps)A142 feed yes.
A: spark COIL , primary shorted out (shorted means gross overloads) (a new coil might be a cure now)

B::------one or more injectors shorted (the wires on them love to.)
(unplug them and does spark come back?) and ASD now stays up cranking for 5 full seconds?
C:02 (oxy sensor) shorted heater or its wire to it shorted to exhaust pipe, easy
well unplug 02 and drive, cured?(it's only a test)

D:Some newr years 91+ years have Alternator pin ,A142 that shorts and same deal happens,. Alt, pin C shorted.
so ,unplug all small wires to the alternator and see if spark comes back. (omg engine starts?)

E: yes the fuel pump can in fact short our or just overload to like 15amps and bad and causes ASD A142 feed to overload.
and gee if you pull the FP relay and ASD now holds up and spark comes back? wow!, the pump or its wires are shorted.

read your chapter 8w on your car, and year and engine
see all pages with A142, power feed ASD and learn what can short.

not saying I know what is wrong, that be silly
I only do the tests and the results puts me on the correct track every time.
for sure ASD drops out. cranking.

I do all his before a bath in fuel.

again the battery must be good and kept charged doing all tests. even at the same time. saving the battery from destuction.

Last edited:
checking fuel first.? is a fail.! sorry is.
it's last.
battery good and fully charged and kept so.
engine good
spark good cranking for full 5 second tests.
if yes, spark is good, not just for 2seconds, okay see why? ASD drops out at 2-3seconds spec. (huge clue 1)
if all this is good,spray can test fuel under air filter element. (tube to Throttle body) ,called instant start in a can at walmart
runs now? yes
check fueling last.( pump psi is 41psi) cranking,

watch scotty

the dribble test only proves dead pumps,not why. do not knee jerk pumps bad. Do the ASD tests.
at 5psi dribble passes. and is useless for EFI 100% this pressure.
scotty assumes engine is still good,but that is ok. (until it's not)

ever get a tuneup in 30 years?
spark parts and filters
rotor, cap,HV wires sets and spark plugs gaped correctlY?
hood sticker on car tells the gap, looking at it.

based on test results you do , and get, tell us what to do next.
car diagnosis is a serial process,.