random hesitation

hey,all.. got a 97. 4.0 5-speed. the prob is random, but more so before it hits operating temp of 190-210.it started about 2 weeks ago. when i take off it will pull good whether at part or full throttle til about 1800-2400 rpms then bad hesitation almost a complete stall untill i back peddle it.sometimes while in 1st next time may be 2nd or even go into 3rd before it does it. may go 2-3 days & not do it.thought it may be watered down fuel. but since its random & i follow the same path to & from work i'm thinking not.since it started doing it ive added dryer to the tank, changed plugs, old ones looked ok but since i had new ones there i put them in.& cleaned & re-oiled the air filter..Oh & no check engine or other lights have come on either when it coughs or after.....anyone have a idea what the grimlen is???

I will be very curious to see what suggestions you get for this because my 98 4.0 AW4 does something very similar. I also have new plugs, distibutor cap, rotary button, plug wires. Sorry to Hi-jack your thread.

at least i'm not alone,.... anyone?

I would finish your tuneup. You put in plugs already but you should spring for new plug wires, distributor cap and a rotor. Also try a bottle of quality fuel system cleaner (BG44K, Techron, etc) to help optimize your fuel injectors spray pattern.

Then see where you are at.

Simple stuff first and you won't be wasting your money on tuneup hardware.

My old '92 5 speed XJ did this and the answer was an O2 sensor with slow response. No codes of any kind, it was just "not good enough". If you can get real time data it would tell you.

BrokenSockets said:

My old '92 5 speed XJ did this and the answer was an O2 sensor with slow response. No codes of any kind, it was just "not good enough". If you can get real time data it would tell you.

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this may be it. check engine light came on a few months back, had autozone check it & said it was o2 sensor, they cleared the code & has not come back on since.i had forgot about it.. il change it this weekend and post back next week if it is still happening.

Mine does this same thing but at a little higher rpm, once it's up to operating temp. no problems at all. I'll be interested to see what you come up with!

The O2 sensors, Throttle Position Sensor, or Coolant Temperature Sensor all can cause similar symptoms. All of these can be tested for proper function. All your tune-up parts are fresh, right ?

regarding being up to temp, that was my experience. once the sensor warmed up it worked better. It did have a significant impact on mpg. Even when it seemed to be working at temp and the hesitation was gone, the fuel economy was quite poor.

Obviously, it happens in various gears but at similar rpms.

I have already replaced the coolant temp sensor and the TPS. Which o2 sensor? Before the cat or after, or both should I look @ replacing if I'm getting no codes?

mine did the same thing. and it ended up being the throttle position sensor. It had a "dead spot" in it and it would just kinda hesitate and buck once it hit somewhere in the RPM range you described. I think a new one was 60 bucks. just remember that after you put it on, disconnect the battery for about an hour so the computer clears itself and relearns everything once you start it back up.

nvm just read the post above that you replaced it already.

Samson04 said:

I have already replaced the coolant temp sensor and the TPS. Which o2 sensor? Before the cat or after, or both should I look @ replacing if I'm getting no codes?

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The "upstream" o2 sensor(s) effect engine operation, the "downstream"--after the CAT--are like a little sister, it is there to tell on you if the CAT isn't doing its job.

Seen a lot of bad o2 sensors that never threw a code, best to follow replacement guidelines, usually every 60k or 80k miles.

Buy a Bosch, they invented it, all the rest are imitations.

I think I'm gonna try the upstream O2 sensors. Will I need to do the "computer reset" trick?