That Jeep Guy XJMJ

Ive got a 99 XJ 4.0

starts up and revs just fine, however when i put it in drive i cannot drive over 10 mph without the engine sputtering and coughing sometimes it stalls out and other times it just struggles to run. I cannot even break 20 mph. Now if i put it in reverse i can go as fast as i want no issues...

There is:
NO check engine light.. nor any pending codes (ive done 3 different code readers and none of them had anything to help.)

Brand new TPS. I did find the distributor "shaft" for the rotor was sloppy, Thats been replaced and still no difference.

its had regular oil changes and a tune up.

Fuel pressure at idle is 50 lbs. i haven't been able to check the fuel pressure while driving.

Any tips or experience will help.

Radi

Well there's an oddball. Turn your seat around and drive in reverse?
Here's a real longshot, but what happens if you unplug the TCU and drive it manually? How about if you power brake it in reverse? Does the engine sputter?

That Jeep Guy XJMJ

Haven't really thought about the TCU trick yet... though i have tried driving it in the other ranges *od, 1-2* and there was no difference.

Ive picked a number of brain's and i bought a new Air intake temperature sensor and it made no difference.... I also swapped the computer out from another solid running XJ and it also made no difference

One of my buddies said it could possibly be the neutral safety switch being faulty.. I've typically heard more of a starting issue (mainly wont start in park and will only start in neutral) when it comes to the NSS.

Radi

I doubt it's the NSS, but you can test it easily enough. Start the jeep then unplug it and take a drive. The NSS switch connections are all open with the transmission in Drive, unplugging it will mimic that.

tiwvr1

pull the 02 sensor out in front of the cat. Dont unplug it, pull it out of the exhaust. Sounds like a clogged cat. If it runs a ton better then the cat is clogged.

That Jeep Guy XJMJ

does that still apply despite the fact i can do 20mph in reverse without issue?? I've been told also that if the cat is clogged it should get excessively hot and after letting it run for 10 mins and putting around the property its not that bad.

i spent a good 5 hrs this morning tinkering with different sensors and wires to no success. ruled out: CPS, distributor cap/rotor, spark plug wires, computer, Its not low on fuel nor is the motor blown/bad headgasket (as per some guy who was interested in taking a look at it, talking on the phone.)

we've figured it will only cut-out when the vehicle is under load. I was romping around his back field in the snow with not much issue, then once i pull the jeep onto the pavement the cut-out begins, but still not in reverse.

coloradowheeler

Maybe it's a transmission sensor

Radi

The problem happens under load- under load in reverse too? It's a bit difficult to compare because reverse is geared differently....what happens if load up the motor in reverse with the brake held?

That Jeep Guy XJMJ

No there is no issues at all when in reverse At All. not in the grass/snow or on pavement. Reverse drives like there is nothing wrong.

yes we tried drive in all selections and reverse with the breaks held and the motor over 3K rpm. In drive it was very intermittent...

Say out of 10 times in drive holding the breaks and the motor up to 3K rpm it would sputter maybe 2-3 out of the 10 times. (these break tests last prolly 5 seconds each time)