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1991 XJ Stumbling, Bucking, Lacks Power Under Load/Acceleration

S

SlipperySquid

NAXJA Forum User
Location
Orange County, CA
Three months ago I changed my U-Joints. During the test drive, the engine stumbled for the first time. I was accelerating hard and the engine briefly cut out a couple of times. This was the beginning.

Now I experience random sputtering, stumbling, and even violent bucking under acceleration. Occasionally the bucking will kick in when I'm cruising on the freeway. This seems to be completely sporadic and I can not find any pattern. Sometimes the Jeep drives fine and you would never know there was a problem. Hot or cold makes no difference. Here is the kicker... it drives fine in reverse. I can floor the pedal and she takes off anytime and everytime without any problems.

Thanks to all of you with similiar posts and a big thank you for all of the insightful responses.

1991 XJ 4.0 HO AW4

What I have checked:
-Air Filter is clean
-No Vacum Leaks. I used a can of carb cleaner and vacum gauge.
-Fuel Pressure is good.
-Replaced fuel filter
-Coil tested bad. Replaced. Same problems.
-Replaced TPS No joy
-MAP sensor is good
-MAT sensor is good
-Unplugged each injector to see if leaky injector was the culprit... no dice
-Replaced cap and rotor. Just because
-Misted distributer cap and plug wires with water in the dark with engine running. No arcing found.
-On a hunch, I replaced the fuel pump. Everything was fine for a few days and then all problems returned.
-O2 sensor is less than 6 months old. I'm assuming it is not the problem.
-Yes, I have also used injector cleaner.

-There are no fault codes.
-No check engine light.

I suspect the Cam Position Sensor (CPS) or maybe the Crank Postion Sensor.

Did I omit any pertinent information?

Keep in mind it runs perfectly in reverse!? Any Ideas?

Please, Please, Please, Help!!
 
Two guesses:

First - Although you checked your plug wires, I had one burn up internally inside the coil wire. It gave me fits trying to diagnose, but listen for engine static on the AM stations of the radio.

Second - Neutral safety switch on the transmission. It reports the shift lever selection back to the TCU and I have seen strange behavior from just this item. We had one that sent the car into spasms occasionally because of this. I would at least take it off and clean it.

Good Luck.
 
You could have a problem with your Idle Air control (IAC) valve. A friend explained very similar problems with his XJ. After trying various fixes he finally replaced the IAC and that took care of the problem, now I dont know what he experianced in reverse but if your O2 sensor is working properly and the cat is in good shape check the IAc to make sure it is working correctly. dont get too frustrated.:peace:
 
might be that fuel pump you replaced...i put a new one in my xj and about a month later it started the same exact description as you are describing. it would only do it if you get down on it or on the interstate or random times around town. so i checked everything you did and even replaced 2 injectors and fuel pressure regulator. and still the same crap was happening....so i replaced that fuel pump with a mopar one. not the crappy advance auto one. and haven't had any problems since.
 
Thanks for responding so quickly.

I am definitely going to check the Neutral Safety Switch on the transmission.

Where is the Idle Air Control Valve located. It is not mentioned in the Haynes manual. Does anyone know the test procedures?

Believe me, I have considered replacing the fuel pump again, but I'm not convinced that is the problem.

Does anyone know where I can find a field service manual? Out of curiosity, would a 1995,6,7 field service manual cover previous years?

As soon as I have a chance to troubleshoot I'll provide an update.

If anyone has has seen this problem before or has an idea to offer please help!
 
Just remember that you always replace the cheapest parts first. Unless you know darn well what the problem is.

I almost paid 600 bucks to have a new fuel pump put in because my jeep was stalling/bucking/hesitating.

Turned out it was a 60 dollar 02 sensor.

So I have 540 left for other things I enjoy in life.
 
jmfelty said:
Two guesses:

First - Although you checked your plug wires, I had one burn up internally inside the coil wire. It gave me fits trying to diagnose, but listen for engine static on the AM stations of the radio.

Second - Neutral safety switch on the transmission. It reports the shift lever selection back to the TCU and I have seen strange behavior from just this item. We had one that sent the car into spasms occasionally because of this. I would at least take it off and clean it.

Good Luck.
Click to expand...
 
My '91 XJ is doing the same thing. Mine is a 5-spd, and it has no cruise control, so I am wondering if it has a nuetral safety switch? I haven't checked the plug wires yet, but will as soon as the sun comes back up.
 
IAC is located on the throttle body next to the TPS. Search "Idle Air Control" and note that three letter accronyms do not work in the search here (See FAQ's forum).

Have you tested the ignition control module underneath the ignition coil, it is another electronic gismo in the ignition circuit, Also check the fuel pressure regulator.

My guess is that it is a loose electrical connection, one of those fun ones to find that could be anywhere, inlcuding a loose ground wire!
 
Next time it does this try unplugging and replugging the CPS (crank pos. sensor), check the plug for corrosion or grease and oil.
Note the condition of the wires going to the plug and make sure they haven't become hard and brittle or burned form the heat of the downpipe.
 
SlipperySquid said:
Three months ago I changed my U-Joints. During the test drive, the engine stumbled for the first time. I was accelerating hard and the engine briefly cut out a couple of times. This was the beginning.

Now I experience random sputtering, stumbling, and even violent bucking under acceleration. Occasionally the bucking will kick in when I'm cruising on the freeway. This seems to be completely sporadic and I can not find any pattern. Sometimes the Jeep drives fine and you would never know there was a problem. Hot or cold makes no difference. Here is the kicker... it drives fine in reverse. I can floor the pedal and she takes off anytime and everytime without any problems.

Thanks to all of you with similiar posts and a big thank you for all of the insightful responses.

1991 XJ 4.0 HO AW4

What I have checked:
-Air Filter is clean
-No Vacum Leaks. I used a can of carb cleaner and vacum gauge.
-Fuel Pressure is good.
-Replaced fuel filter
-Coil tested bad. Replaced. Same problems.
-Replaced TPS No joy
-MAP sensor is good
-MAT sensor is good
-Unplugged each injector to see if leaky injector was the culprit... no dice
-Replaced cap and rotor. Just because
-Misted distributer cap and plug wires with water in the dark with engine running. No arcing found.
-On a hunch, I replaced the fuel pump. Everything was fine for a few days and then all problems returned.
-O2 sensor is less than 6 months old. I'm assuming it is not the problem.
-Yes, I have also used injector cleaner.

-There are no fault codes.
-No check engine light.

I suspect the Cam Position Sensor (CPS) or maybe the Crank Postion Sensor.

Did I omit any pertinent information?

Keep in mind it runs perfectly in reverse!? Any Ideas?

Please, Please, Please, Help!!
Click to expand...
let me know if you have any relief. from what you explained mine sounds to be the exact same. i was told by a few mechanics the it might be the IAC but i dont want to go in and start replacing stuff like you did to have no avail. also i am experianceing high idle. once i start it and it gets warming up. it idles about 800 when i first start it. as it warms up the RPM's will climb while ildeing. once it reaches about 200 it idles about 3k. you have any idle symptoms?
 
SlipperySquid said:
Three months ago I changed my U-Joints. During the test drive, the engine stumbled for the first time. I was accelerating hard and the engine briefly cut out a couple of times. This was the beginning.

Now I experience random sputtering, stumbling, and even violent bucking under acceleration. Occasionally the bucking will kick in when I'm cruising on the freeway. This seems to be completely sporadic and I can not find any pattern. Sometimes the Jeep drives fine and you would never know there was a problem. Hot or cold makes no difference. Here is the kicker... it drives fine in reverse. I can floor the pedal and she takes off anytime and everytime without any problems.

Thanks to all of you with similiar posts and a big thank you for all of the insightful responses.

1991 XJ 4.0 HO AW4

What I have checked:
-Air Filter is clean
-No Vacum Leaks. I used a can of carb cleaner and vacum gauge.
-Fuel Pressure is good.
-Replaced fuel filter
-Coil tested bad. Replaced. Same problems.
-Replaced TPS No joy
-MAP sensor is good
-MAT sensor is good
-Unplugged each injector to see if leaky injector was the culprit... no dice
-Replaced cap and rotor. Just because
-Misted distributer cap and plug wires with water in the dark with engine running. No arcing found.
-On a hunch, I replaced the fuel pump. Everything was fine for a few days and then all problems returned.
-O2 sensor is less than 6 months old. I'm assuming it is not the problem.
-Yes, I have also used injector cleaner.

-There are no fault codes.
-No check engine light.

I suspect the Cam Position Sensor (CPS) or maybe the Crank Postion Sensor.

Did I omit any pertinent information?

Keep in mind it runs perfectly in reverse!? Any Ideas?

Please, Please, Please, Help!!
Click to expand...


Thanks again for the suggestions.

I just checked the Crank Position Sensor and it reads open on all terminals (3 total). According to the Haynes manual it should read open between A and B... There were no letter or number designations so I checked all possible combinations and everything reads open. The wiring, while definitely old, looks to be okay. It hasn't melted or burned and isn't brittle. Let's say the Crank Position Sensor is good.

I noticed there is slight back and forth play on the distributor shaft. I'm not the most seasoned mechanic, so tell me, is this bad???

Additional symptoms I've noticed are:

When slowly moving forward and turning front wheels to their max limit the engine rpm can slow drastically nad engine almost dies. Again, the load is increased because it takes more torque to move the vehicle because of the angle of the front wheels. I noticed my headlights dim the other night and, to me, that indicates this is definitely an engine problem. If it were the transmission neutral switch, engine rpm should not be effected. Does that sound logical?

Yesterday I was told it sounds like the bands in my tranny are going out. So, driving in 2x4 and 4x4. 1st/2nd, drive, OD, the hesitation occurs in all gears except reverse. I think I can eliminate te transmission theory because of the erratic engine behavior and the dimming lights.

Also, last night I watched the RPM needle jump up and down at least 6 consecutive times between 1500 and 2000 RPM while I was driving on the freeway.

Is there a test procedure for the Cam Position Sensor (CPS)?

I'll keep everyone updated as I make progress.
 
My intermitten prob was # 2 plug wire shorting THROUGH the boot on the block.Start the engine in the dark and wiggle each boot around untill it touches the block.If there is a difference in the way it runs then it's your wires.Mine threw an arc to the block that was easily seen in the dark.(sounds like a poem)
HTH
 
In my experience the rotational play isn't really critical, the drive gears are cut at an angle, so this keeps the shaft fairly stable. Excessive side play of the distributor shaft can cause problems.
IAC problems are often more apparent at idle or near idle.
Somebody not long ago posted that a loose flex plate, was causing almost the same symptoms you have mentioned. Except for the reverse thing.
Many of the sensors drop out of the loop during acceleration (except the TPS). This can be an indicator.
The O2 sensor doesn't do much until the motor gets a little warm, this can also be an indicator. If it cruises OK, when it is still cold this can be an indicator.
My best guesses would be an O2 sensor, with the back up thing, possibly a wire touching someplace. The rear O2 sensor is known for the wires getting fried on the exhast.
Next would be the CPS, they can test good and still be questionable. To include the harness and the flex plate.
I've never had a cam position sensor go out on me, so I can't really say what the symptoms are. Same with the MAP on the OBD 1 motors.
Igntition problems usually manifest themselves at idle, they idle rough and at top end when the ignition breaks down.
A stopped up CAT or exhaust is one thing many people forget about. Sure does rob torque, the Cat in my 95 was mostly plugged, what a difference in acceleration when I swapped it out.
 
8Mud said:
In my experience the rotational play isn't really critical, the drive gears are cut at an angle, so this keeps the shaft fairly stable. Excessive side play of the distributor shaft can cause problems.
IAC problems are often more apparent at idle or near idle.
Somebody not long ago posted that a loose flex plate, was causing almost the same symptoms you have mentioned. Except for the reverse thing.
Many of the sensors drop out of the loop during acceleration (except the TPS). This can be an indicator.
The O2 sensor doesn't do much until the motor gets a little warm, this can also be an indicator. If it cruises OK, when it is still cold this can be an indicator.
My best guesses would be an O2 sensor, with the back up thing, possibly a wire touching someplace. The rear O2 sensor is known for the wires getting fried on the exhast.
Next would be the CPS, they can test good and still be questionable. To include the harness and the flex plate.
I've never had a cam position sensor go out on me, so I can't really say what the symptoms are. Same with the MAP on the OBD 1 motors.
Igntition problems usually manifest themselves at idle, they idle rough and at top end when the ignition breaks down.
A stopped up CAT or exhaust is one thing many people forget about. Sure does rob torque, the Cat in my 95 was mostly plugged, what a difference in acceleration when I swapped it out.
Click to expand...
about a year ago i had the cat back up problem but the exhaust man said mine was the worst hes ever seen. i had parts of my cat move towards my moter and literally got molded inplace. replaced all new pipe and cat and OMFG with 35's i was burnin rubber at every stop light. so much more power it was not even funny..
 
I would check your O2 sensor again. You might of hit it replacing the U-Joints. Like someone else mentioned if it is the culprit it should run alright before the engine reaches operating temperature.
 
Today I removed and cleaned the Transmission Neutral Safety Switch. It needed the cleaning. Now it is reinstalled.

I called the local jeep specialist. When I described the symptoms he asked how many miles are on the engine (185,000) and immediately said it sounds like a worn lobe on the cam shaft.

I'm on my way to get a compression test kit. I'll provide updates with my progress. Thanks again for all of the suggestions and advice. Please keep the help coming if you have any insight or experience with this problem.
 
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