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ok weird problem... thinking fuel issue...long

Tre

NAXJA Forum User
Location
canada
well this is a very intermittent problem.
its a 88 xj 4.0 aw4 np231.

well this has happend 2 times this past week. i'll be driving down the road. then the truck just dies on me. i'll try to restart. and all i get is a little sputter out of it and it won't stay running. the first time i got it towed home. got home tryed it and still wouldn't start. so i sprayed some carb cleaner into the throttle body to rule out fuel. and it started up and stayed running. didn't have a problem for the rest of the week.

now today same thing happens. luckily i was only a block away from home so i got a friend to tow my the 2 blocks back home. before i got it towed though i tried the carb cleaner again. and it would fire it for a second. then it wouldn't stay running. so after some thinking. i decide to check the plugs and for spark. plugs were totally dry. not fouled at all. i get my buddy to ground the plug so we can check for spark. next thing you know the thing fires up and is running on 5cyls. so we shut it down. put the plug back in. and its running again.

so my quesiton is.... what the heck do i start checking?
 
I'd start by doing an ohm check on my CPS. If it tests good it doesn't mean its good though, but if it tests bad it is likely bad. Take a good look at the CPS wire and make sure it isn't cooking on the exhaust manifold. Test the CPS when the motor is hot, they can be temperature sensitive, after the motor cools down a bit, they start working again.
If you have never changed out the fuel filter, now is the time. Do a fuel pressure test.
If everything checks out OK, jump your balast resisitor and change your relays around. If the B+latch relay or the fuel pump relay is acting up, this could be the problem.
The next round gets a bit more involved. Everything from the ECU ground at the dipstick, to various connectors and the harness. Then checking, less likely sensors.
I've got a whole routine worked out, I usually get lucky early. Sometimes it takes days. Intermittant problems can be a bear, it is often much easier if it just up a quits and refuses to run again.
 
i'm not convinced its a CPS issue just yet. cuz i actually tried that as part of the diagnosing. i unplugged the cps connector to see if the "no start" condition would change.

and it did. instead of getting a little studder then shutting down. it would just crank and crank. no sputter at all.
plugged it back in and it would sputter.
 
Mine would occasionally studder on the highway. And would occasionally, die at a stop light. Then it would start again hard, if it started at all. If I let it sit for awhile, it would sometimes start and act like the MAP vacuum line was unplugged.
I decided it wasn't the CPS, because it was fairly new. After checking things for a couple of days, swapping in my spare MAP and cleaning all my connectors and grounds. I decided to do an ohm test on the CPS, just for the heck of it. Tested 278 ohms, hot. It was the crank position sensor. I figure it was oil soaked from my leaky rear main seal.
Mine would act up the worst at a specific temperature, at 140 it refused to start often. At higher temps. it would occasionally go through the extended crank thing, then start but run like crap and would barley accelerate (like I said it acted like the MAP vacuum line was disconnected).
It once refused to start, just for the heck of it, I poured a bottle of water on the CPS, started right up. Like I said they can be temperature sensitive.
Mine never just died on me while driving down the highway, some of your symptoms sound the same though.
Mine would buck and act up sometimes at highway speeds, turned out to be the O2 sensor. It would lean the motor down too much under certain RPM and vacuum conditions.
I did find a fuel pump connector on one XJ (87) that had a pin partially backed out. Drove me nuts, the connector was right where the harness came out of the drivers side rear door sill and went behind the rear side panel.
I measured the voltage at the ballast resistor out, and then again at the fuel pump connector and found a significant line loss. Low voltage to the fuel pump, not the normal 9/10 volts it was closer to 6 volts.
I had one XJ that would crap out at about 1/3 rd throttle. Turned out to be the TPS, it had a dead spot.
Good luck. Been there done that. Hopefully you get lucky, most times it just take perserverance.
 
I have a 88 xj 4x4 4.0 same problem changed the coil and ing.module fixed the problem for good expensive but worth it been down that road with all the cps tps and all that didnt work coil/ing module is the answer!!
 
well today i got some parts. picked up a new cps and plugs. changed the plugs and the CPS (what a PITA). go turn the key. nothing changed. it starts for a second and dies.

any other ideas guy's before i drop the tank and replace the fuel pump.

oh also tryed jumpering the fuel pump resistor. didn't make any difference.
 
well the plot thickens.
went out to it again. checked spark. and yep its sparking.
checked the new plugs i put in. the showed signs of fuel. so i got fuel. plus i could smell it.

so i got spark, i got fuel. why is it firing then shutting down right away?

out of desperation. i start checking the fuse box under the dash. all fuses are ok. also swapped the fuel relay under the hood around. and that didn't make any difference either.
now while i was monkeying around under the driver side dash. for shits and giggles i start poking around by the computer. basically just to push the connectors on and make sure they're seated. now i didn't feel that it was unseated. but right after that step i turn the key. fired up and she's running now.

but now i'm scared to drive it anywhere as i have this feeling its gonna happen again.

what do you think? need a new computer? or check the wiring around that area?
 
next time it acts up try banging on the fuel tank!
Mine wouldn't start to night, thought it was out of gas(Gauge on extra E:doh:
Put 7 gal in it and it still would tray to start but wouldn't! put a fuel pressure gauge on it and it showed 0 PSI!!!! fuel was flowing out of the test port but now pressure.
had my buddy crank on the started will i beat on the tank with a BFH and after about 10 hits, it started and the fuel gauges read 35 psi!

Drove it home with out a problem.........will be buying fuel pump tomorrow!!!!!!!!
 
Flash said:
next time it acts up try banging on the fuel tank!
Mine wouldn't start to night, thought it was out of gas(Gauge on extra E:doh:
Put 7 gal in it and it still would tray to start but wouldn't! put a fuel pressure gauge on it and it showed 0 PSI!!!! fuel was flowing out of the test port but now pressure.
had my buddy crank on the started will i beat on the tank with a BFH and after about 10 hits, it started and the fuel gauges read 35 psi!

Drove it home with out a problem.........will be buying fuel pump tomorrow!!!!!!!!
i wish i had a pressure gauge. cuz like i said earlier. any fuel injected vehicle i've pressed the schrader valve on it squirts fuel pretty good. when i did it to mine when its acting up i just get a little pshh and thats it. no more pressure.
 
When the key is in the start possition (Renix) the balast resistor is bypassed (power comes from the start relay for the pump), when in the run possition, the power for the pump goes through the balast resisitor (from the fuel pump relay). So you can have fuel pressure during start, but not during run. Hook a volt gage up to the ballast resistor out spade connector and watch it while the key is in start and then when the motor is running. The power during start, is spliced into the fuel circuit, so you will see the voltage during start at the ballast resisitor out also and during run.
The splices into the harness for the fuel system have been known to get corroded and flacky, a real pain to find and repair. The harness near the relay block is real spagehetti.
What you said about the ECU connections is worth a look even if it isn't the problem. The last time I had my TCU out, I found two contact connections backed partially out of the connector. Pull gently on the wires where they go into the ECU connectors. No big deal to spread the contact open a little and stick it back in the connector (all the way).
Most all of the motor functions, ground at the dipstick holder.
A fuel pressure gage isn't very expensive, last one I bought was about fifteen bucks. it was actually an old R12 refrigeration low pressure gauge and hose. The refrigeration hose has a built in depressor for the shrader valve. Gas has never hurt my refrigeration gauge (or hoses) any, I've had it forever.
When the CPS (or wiring) is bad, you will get no *spark* or *fuel* to the injectors, injectors won't cycle. If you are getting spark, the CPS is likely OK.
Check the block to firewall ground, near the rear of the head drivers side. Pull the ground bolt from the firewall and clean it down to metal. Same with the grounds at the dipstick holder. Check the gorund rings at the dipstick holder for corrosion or wire fatigue. This is the gorund for the ECU, the injectors and most everything else for the motor control circuits.
It's sounding like you have a bad connection someplace. No easy way around it, it will take time, you may have to do a resistance test on many circuits to nail it down. Alldata could be helpfull, they do a good job of breaking things down into sub curcuits and make troubleshooting some easier. I have an extra harness in the garage, that I spread out on the floor and use to help me find possible troublespots and help sort out the connectors. It makes things a lot easier.
You could hook a hot wire directly from the battery to the fuel pump wire at the ballast resistor and let the pump run continously just for a test.
 
Last edited:
8mud thanks for all the help and suggestions.

i'm hoping it was a loose connection at the ecu. after unplugging it last night and making sure all the wires where in nice and snug. i put it back in and probably put about 60miles on it today without a hitch. knock on wood.
 
Tre said:
8mud thanks for all the help and suggestions.

i'm hoping it was a loose connection at the ecu. after unplugging it last night and making sure all the wires where in nice and snug. i put it back in and probably put about 60miles on it today without a hitch. knock on wood.
Been there done that, I feel for ya bro. I've found myself on many occasions wishing an intermittant problem, would just fail completely, so I could find it. What will really throws you, is when after you start checking stuff, the problem goes away and you never figure out exactly what you did to fix it :).
I spliced the harness side CPS connector into the CPS wires very near where they exit the firewall. Solder and shrink tubing. My CPS only goes through two connectors now, the CPS connector and the ECU connector. And has at least three feet less of harness to travel through.
I don't know wether you have the C101 connector, CPS harness bypass in your 88 or not. Mine does, but the connector for the bypass, didn't look all that good either (whoever installed it bent some pins), I just wired the whole works as directly as possible.
 
well i did some more maintaince on it last night. i put in a new fuel filter. and well i noticed that when i unclamped it. there really wasn't much pressure in the line. i put the new one in and started it up. checked for leaks. and one of my clamps was leaking a bit. i noticed it seemed quite pressurized now. so i went to the rail wth a rag to depress the schrader to relieve some pressure. and it shot out quite alot now and for quite a while longer then it used to.

regardless if this is related or not. for the 11 bucks the filter cost me. i think it was worth doing.
 
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