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HELP!! 91 XJ Starts right up then Dies and I need this for work!!

D

dshaff24

NAXJA Forum User
Location
pa
Hello,

My 91 jeep cherokee XJ 4.0 automatic - 242 - ran perfect, I parked it outside of work and then 8 hours Later IT STARTS then Dies! So far this is what I tested

Starting Fluid will keep the jeep running which = fuel issue which also means Good SPark!

Fuel Pressure - 39psi prime and 35 psi Cranking = fuel pump good

Bypassed the ballast resistor with no Change

It has a new distributor n cam sensor - fuel filter - cap / rotor - CPS - plugs / wires - NEW BATTERY - new starter - fresh oil n filter [ Most of this stuff was just for maintence b4 this happend ] The ecu last three digits are 707,

IM LOST and need this to run!! I been working on it for a few days with no success, Maybe ecu? NAXJA
 
im going to pick up this Noid light tomarrow b4 work, Do I just unplug the injector and hold it on the terminals or is there another method?
 
Unplug the connector, plug the NOID into the harness, crank engine.

There are different "modes" of fuel pump power supply:

Key ON, power to pump from the ASD/B Latch relay bypasses the ballast resistor, lasts about 3 second.

Key to START, power to pump from the starter relay bypasses the ballast resistor.

Key ON, engine at idle through cruise, power to pump through the ballast resistor.

Key ON, engine at WOT, power to pump through the oxygen heater relay.

Definitely check for the firing signal with a NOID, but also try swapping your a/c or oxygen heater relay with the fuel pump relay.
 
Not to thread-jack, but i'm curious about some of the different fuel pump 'modes',
Mainly the engine WOT, power to pump through oxygen heater relay. I knew of the power through starter and the ballast bypass modes, but what would be the benefit of power through oxygen heater relay during WOT?
Is it b/c the O2 heater relay is a larger circuit? is this 'mode' determined by the TPS through the ECU?

my apologies if my questions seem dumb, but whenever i see a chance to learn something new i tend to jump at it!

thanks!

/threadjack
 
Not sure about the wiring for an XJ but in general fuel injection ECUs will start and then shut down if: your mass air flow meter isn't working, or there is no signal from oil pressure or fuel pressure sending units. All of those safeties bypass for a few seconds to let the engine start running and then kick in. Just an FYI.
 
hightime81 said:
Not to thread-jack, but i'm curious about some of the different fuel pump 'modes',
Mainly the engine WOT, power to pump through oxygen heater relay. I knew of the power through starter and the ballast bypass modes, but what would be the benefit of power through oxygen heater relay during WOT?
Is it b/c the O2 heater relay is a larger circuit? is this 'mode' determined by the TPS through the ECU?

my apologies if my questions seem dumb, but whenever i see a chance to learn something new i tend to jump at it!

thanks!

/threadjack
Click to expand...

Curious, ain't it? Could have something to do with the ECU and open/closed loop modes. The ECU recognizes WOT from the TPS position (a**uming correctly functioning/adjusted TPS), WOT is open-loop, gotta be a tie-in there. Maybe one of our resident engineers/electronics wizards will jump in with more info. There is a lot of things about the Renix system/ECU that remain a mystery, note the near total lack of aftermarket DRB-equivalent tools, Snap-On being the notable exception.
 
I'm not some expert or anything, but try spraying the throttle body down with carb cleaner, especialy in the hole above the plate,,, it sounds like the idle air controler is malfunctioning. Will it run with your foot on the gas?
 
Last edited:
activeshooter1 said:
I'm not some expert or anything, but try spraying the throttle body down with carb cleaner, especialy in the hole above the plate,,, it sounds like the idle air controler is malfunctioning. Will it run with your foot on the gas?
Click to expand...

Actaully IF you try to hold in the gas it will barely Start and shuts down even faster!

hopefully tomarrow I can get this noid light out and check it,

Anyone else got some ideas to try tomarrow? ill post more info tomarrow Thanks everyone for throwing some ideas at me
 
Did you try swapping relays--quick, free, no special tools required?
 
My 00 did the same thing about a month ago. long story, but in the end I found moisture in the engine ecm connector. Dried out and all better.
 
NEWYXJ said:
My 00 did the same thing about a month ago. long story, but in the end I found moisture in the engine ecm connector. Dried out and all better.
Click to expand...
my ecu is located up by the headlight on the driverside fender, I actaully removed the big ecu plug and sprayed electrical cleaner and plugged/bolted it back in with no change
 
puzzling, I'm of no help but anxious to hear the outcome, please keep us posted. You're in good hands here, keep at it. Never ceases to amaze me how little I know, what the hell is a "noid"?!? heck and for that matter WOT? cryke!
 
tkjeeper said:
what the hell is a "noid"?!? heck and for that matter WOT? cryke!
Click to expand...

A "noid" light is also a test light with a ground clamp on one end and and a sharp probe on the other end to back probe connectors with. "WOT" is "wide open throttle"

Hope that helps explain some of the nomenclature on here
 
WOT = wide open throttle (other wise known as flooring it!)

Noid light is an inexpensive light that plugs directly into the fuel injector harness to test and ensure proper electrical power is going to each fuel injector.
http://www.noidlight.com/checking-you-cars-fuel-injection-system-with-a-noid-light/


I've learned an amazing amount just reading through threads exactly like this from the likes of folks like Kastein, Joe_peters, 5-90, Winterbeater, the list goes on!
 
cant believe I didn't know that, thanks all
 
"Fuel Pressure Readings in prime and crank."

Does it hold pressure in "run" and thru "engine dies"?

Same for spark?


Gotta be losing something.

(ignition switch? - start , run switching?)

Good Luck,
Orange
 
Sounds like loss of fuel pressure in run-mode to me. Try bypassing the ballast resistor... it's the white ceramic-looking thing on the driver fender wall behind the near the ECM that is supposed to cut fuel pump voltage during run mode. Unhook the wires and put them together to bypass.
 
ehall said:
Sounds like loss of fuel pressure in run-mode to me. Try bypassing the ballast resistor... it's the white ceramic-looking thing on the driver fender wall behind the near the ECM that is supposed to cut fuel pump voltage during run mode. Unhook the wires and put them together to bypass.
Click to expand...

"Bypassed the ballast resistor with no Change"

He did that, first post.
 
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