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what could possibly be wrong

J

j99xj

NAXJA Forum User
Location
Colorado Springs, CO, USA
I was out driving and I almost killed the engine while pulling up to a stop sign.

Here's what happened:

While slowing down the engine bogged down, almost like putting on brakes w/o the clutch (but mines an auto).

The rpms dropped to about 300-400 rpms and ran rough.

Then after the idle got back to normal (800 rpms), I give it throttle and it bogs again and makes poping noises.

After a few seconds all is well and the Jeep runs great (as any tuned up 4.0 should).



Any ideas about what to check? There's no check engine lights yet.
 
the same thing happened to me when i ran a "fuel injection cleaner." something clogged one of my injecters.

this is how i tested it......

when it started running bad i would lift the hood and unplug one injecter at a time if it died or ran worse that ment the injecter was working. When i unplugged me #2 injecter nothing changed......bingo that was my faulty injecter. I just kept it running and it worked itself out.....havent had a problem in about 1 yr.

also your CPS (crankshaft sensor) will do something similar if it get filled with crap or connector is bad.

GOOD LUCK!
 
I had that happen in my '88. I was going down a mountain road and when braking at the end of the road the XJ died. After waiting a few minutes (after cranking it over and having it NOT start) it fired up. But now, it runs on 5 cylinders with the #5 cylinder not working. The motor sounds like it has fuel pulsing into the cylinders as it rolls downhill with no throttle. It goes, "bwaaaaap (pause) bwaaaap, (pause) bwaaaap" like it is dumping fuel in a rythmic pulse into the cylinders. Why, I don't know.

I changed the idle sensor, throttle position sensor, crankshaft sensor, cap, rotor, wires, and checked all vaccuums. I found the rear main was leaking and changed that. I will unplug and check the injectors next, and then check cylinder pressure, as I think that I either have a stuck valve, or bad rings, or have a bad injector on that cylinder.

Thankfully, I got another '88 XJ for $1 from a guy that hit a deer and then a tree. The motor was perfect in that jeep and now I have all of the parts. So, if my motor needs overhauling, I can just swap in another motor and still have my XJ while I rebuild the original motor.

Gary
 
For the original post, it sounds perhaps as if the torque converter lockup did not disengage when it was supposed to. I believe there's a brake actuated switch that is supposed to ensure that it lets go as soon as you hit the brake. I'd keep an eye on it, and if it happens again, see if popping the tranny into neutral allows the engine to recover. If so, I'd look at the circuitry for the TC lockup.
 
YosemiteXJ said:
I had that happen in my '88. I was going down a mountain road and when braking at the end of the road the XJ died. After waiting a few minutes (after cranking it over and having it NOT start) it fired up. But now, it runs on 5 cylinders with the #5 cylinder not working. The motor sounds like it has fuel pulsing into the cylinders as it rolls downhill with no throttle. It goes, "bwaaaaap (pause) bwaaaap, (pause) bwaaaap" like it is dumping fuel in a rythmic pulse into the cylinders. Why, I don't know.
Gary
Click to expand...

For a Renix system as yours (and my '88) when the engine senses a closed throttle with engine rpm above 1200, the ECU shuts off fuel delivery. Delivery is supposed to be restored when the RPM falls below 1200 or you press gas pedal.

Maybe you have an ecu problem. Here's a link to the Renix manual. Scroll down to the Jeep section.

http://www.pirate4x4.com/tech/billavista/PDFs/index.html
 
j99xj said:
I was out driving and I almost killed the engine while pulling up to a stop sign.

Here's what happened:

While slowing down the engine bogged down, almost like putting on brakes w/o the clutch (but mines an auto).

The rpms dropped to about 300-400 rpms and ran rough.

Then after the idle got back to normal (800 rpms), I give it throttle and it bogs again and makes poping noises.

After a few seconds all is well and the Jeep runs great (as any tuned up 4.0 should).



Any ideas about what to check? There's no check engine lights yet.
Click to expand...

I had a similar problem with my 96. We just figured that it was from it sitting while I was in Iraq, or a bad tank of gas, or both. It was the cps. We pulled one from our parts rig, and the problem went away. Something to look at. BTW, it didn't trip the check engine light.
 
mrrbby said:
I had a similar problem with my 96. We just figured that it was from it sitting while I was in Iraq, or a bad tank of gas, or both. It was the cps. We pulled one from our parts rig, and the problem went away. Something to look at. BTW, it didn't trip the check engine light.
Click to expand...

Today I took a video of my fuel pressure gauge hooked up under the hood while I was driving. Fuel pressure was always above 45-46 psi or so, even at WOT. At idle it was around 47-48. Even though the spec is 49 psi, I'll say I'm close enough because the gauge I was using was a cheap gauge.

So I think my pump and filter is fine, as far as I can tell.

Another note, I know for a fact its not ANYTHING on the throttle body. (I have a bored throttle body with all the sensors and I swapped back to the stock tb with the stock sensors and it still bucks and hesitates sometimes.)

So I guess the cps is the next thing to take a look at.
 
Matthew Currie said:
For the original post, it sounds perhaps as if the torque converter lockup did not disengage when it was supposed to. I believe there's a brake actuated switch that is supposed to ensure that it lets go as soon as you hit the brake. I'd keep an eye on it, and if it happens again, see if popping the tranny into neutral allows the engine to recover. If so, I'd look at the circuitry for the TC lockup.
Click to expand...

I'd go with this.
 
Well the jeep started having an intermittent bucking/hesitation/stalling problem when accelerating from a stop in addition to acting up when pulling up to a stop.

I'm getting concerned that the jeep will pull this crap when I'm trying to make a left turn when there's oncoming traffic! Very scary.
 
Mine was doing something like that a few months ago.

Had done everything I could think of and had Mike Rollins (local jeeper, dealership shop forman) look it over without luck. Problem went away when I replaced transmission and motor mounts.. of all random things.
 
Having almost the exact same probs. I haven' had a chance to diag anything as weather is alasy bad on my days off. Please keep posted of your findings.
 
Have you checked for vacuum leaks averywhere?
 
Generally yes (high idle) but as I recall from my own experience a loose, INTERMITTENT vacuum leak at the throttle body connection hose (rubber peice that fits into the throttle body and attaches to the hose that goes to the MAP sensor) that goes to the MAP sensor can cause your problem.

Also it is not unusual to have two or more problems at the same time partially masking each other making the diagnosis almost impossible and some of the assumptions wrong.
 
Ok just a recap of what I have done in my diagnosis of this bs. I'm checking/replacing the free/cheap stuff first, then I'm going to the next expensive part.

I've checked the fuel pressure, its fine. That rules out the pump and filter. I've run fuel injector cleaner and still no change. My engine runs great at WOT so I'm 99% confident the engine is getting proper fueling.

A few posts back I said I switched back to the stock throttle body with sensors (from a bored tb w/ sensors), and still no changes. So that rules out the MAP sensor, throttle position sensor, and IAC (Which I have cleaned and lubed really good) and vacuum leaks.

This morning I changed and properly gapped the spark plugs. Also I inspected my 8mm plug wires and rerouted them. Jeep runs the same.

So that leaves me with:

upstream oxygen sensor
crankshaft position sensor


I'm off to the parts store....again. I'll check in later.
 
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