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Trying to go 2 for 2

K

Kyle Petterson

NAXJA Forum User
Location
Raleigh, NC
Alright, you guys helped me with my last issue, so lets go again. I just rebuild my 4.0 liter (89) and it ran fantastic. For 4 days. Now it runs great as long as you keep it under 2K RPM. Above that it's got a hearty misfire. Any ideas? I dig into it tomorrow morning.
 
This thing is kicking my ass. Runs great upto 1500RPM (cold) - 2000RPM (hot). After that runs like the exhaust is clogged. (No, the exhaust is not clogged) No power, runs like garbage. Checked timing: dead on, checked TPS, Crank sensor, IAC, O2, ECU: good. No vacuum leaks. Plugs, wires, cap and rotor in good shape.
 
can you tell which one is missing..
im assuming that one cylinder is not firing right.
put it under load and see which plug it is.. either with a plug checker or by pulling wires..
to put it under load put it in 2nd and brake torque it..
i taught the local mechanic that after i told him i had a missfir but only when i pulled a heavy trailer( trailer w/ bobcat)( oh this was in my superduty) he wanted me to bring my trailer and bobcat so he could diagnose.. :D that was funny i laughed at him for that one..
see if its just one plug if it is maybe you had a ring go bad check compressions on all the cylinders...
you could have a cracked plug..
when my superduty was missing thats what it turned out to be. he changed the plug and it was fine....
just random thoughts..
are all the plugs colored the same?
i guess it needs to be narrowed to either an all cylinder problem like fuel pressure being off or a 1 cylinder problem. plug ring valve rocker broke?

narrow it down...

put it in the dark garage and load the motor see if you see spark jumping anywhere..
 
EGR Valve sticking or ripped??
 
could be fuel pump or fuel pressure

same happened to a friends boat. thought it was ignition but was lacking enough fuel at high rpms
 
89xj said:
same happened to a friends boat. thought it was ignition but was lacking enough fuel at high rpms
Click to expand...

That's where I'd look first. But more likely an O2 sensor or the wiring. Maybe try a run up the interstate, coast into a rest stop and pull a couple of plugs, look at the color. Try not to let it idle any or very little so you get a true reading.
Put it in a lower gear and see what it does WOT, if it accelerates through the problem area, it is likely a sensor. WOT bypasses many of the sensor inputs.
 
Lew360 said:
Clogged catalytic converter?
Click to expand...
Brand new. Try bypassing the exhaust just to be sure, same.

jeepman121 said:
having an 89, your not running an EFI how's the Carb?
Click to expand...
Wait....THE CARB IS MISSING!!! No wait, it's injected.

8Mud said:
That's where I'd look first. But more likely an O2 sensor or the wiring. Maybe try a run up the interstate, coast into a rest stop and pull a couple of plugs, look at the color. Try not to let it idle any or very little so you get a true reading.
Put it in a lower gear and see what it does WOT, if it accelerates through the problem area, it is likely a sensor. WOT bypasses many of the sensor inputs.
Click to expand...
New and double checked.
 
Try checking the coolant temp sensor, if it is open then the computer is thinking the engine is -40 degrees and will overfuel. This may not be noticed at low RPM.
 
I'm leaning (pun intended) toward fuel problem. The fuel line can be pinched and not flow full volume for higher speeds, but enough for low RPM opperation.

pressure would still be good at Idle
will induce lean misfire
easy to pinch during engine installation


If supply lines and return lines are not pinched, fuel filter is new, and the ignition system is perfect, I would check voulme output of the pump. Pull line at tank. jumper the pump relay and pump (according to FSM) appropriate length of time into a glass jar. Measuer volume of fuel against FSM spec.

--Shorty
 
just a thought, is there any way to run a fuel pressure gauge temperarily so you can see it while the jeep is running/driving so you can see what the pressure is when you are having the problem?

new pumps, especially aftermarket pumps can go bad.

how about the fuel pressure regulator?
 
Looks like a rock or something came up and beat the tar out of my Crank Sensor. Read fine at idle, high RPM....not so much. So I replaced it. And trimmed my fenders while I was there.
:party:
 
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