Engine bucking and O2 sensor

Ok my engine from about 35-65 will start to buck or out out then back in, causing a almost violent shudder. After doing some searching it seems this is realted to the O2 sensor. Well I just replaced it a year or so ago, but I do have an exhaust manifold gasket leak. The leak has been there for quite awhile and I havn't noitced this problem before but could it be causing this? Obisouly with the leak the O2 sensor won't be reading correct as the xahust will be elaned out correct?

What year XJ? I've seen a bunch of Renixs with a fried harness that runs down the front of the motor to the O2 sensor, the temp. sensor and the knock sensor. The connectors also probably need to be scrubbed out.
If you've got the 2000 RPM buck, it may be the O2 sensor. It may be distributor indexing depending on the year. There was a TSB about that for the 87-beginning of 88 4.0 models (If I remember correctly).
TPS also has an influence on timing, that can cause the same thing. But generally all through the RPM band, unless you have a TPS with a dead spot. I had one TPS that had a hitch at right around 1/4-1/3 pedal and it would cough everytime I left the throttle in that position for more than a second or two.
An exhaust leak may venturi and suck air or it may not. If the leak does ventrui extra air into the exhaust, it would probably cause high idle or a rich condition.
The bucking often comes from a motor running too lean. Too lean usually cuts way down on your top end and/or near redline operation. Fuel pressure or volume, O2 and other things can cause too lean.
Sometimes a miss will cause extra fuel to build up and when it finally ignites this causes bucking. Usually fairly random.
I've had wet connectors cause all sorts of grief. The moisture seeps inside and can't evaporate again. If the problem isn't obvious, I start cleaning connectors, even if the problem doesn't go away, the motor usually runs better anyway. The same with grounds and the clips between the coil and the ignition control module.
Sometimes it's just a combination of small things that add up.
Idle is a good indicator of how healthy your motor is.

8Mud said:

What year XJ? I've seen a bunch of Renixs with a fried harness that runs down the front of the motor to the O2 sensor, the temp. sensor and the knock sensor. The connectors also probably need to be scrubbed out.
If you've got the 2000 RPM buck, it may be the O2 sensor. It may be distributor indexing depending on the year. There was a TSB about that for the 87-beginning of 88 4.0 models (If I remember correctly).
TPS also has an influence on timing, that can cause the same thing. But generally all through the RPM band, unless you have a TPS with a dead spot. I had one TPS that had a hitch at right around 1/4-1/3 pedal and it would cough everytime I left the throttle in that position for more than a second or two.

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It's a 89 4.0. Not sure what RPM it does it cause I don't have a tach but it seems to do it all over the place. Will do it with the pedal steady like you described above or sometimes will accelerating.

8Mud said:

If the leak does ventrui extra air into the exhaust, it would probably cause high idle or a rich condition.
Idle is a good indicator of how healthy your motor is.

Click to expand...

It does run a bit rich I think. The idle tends to be higher then it should.

Based on that I should be looking at the TPS. I'll check the O2 sensor harness as well.

You might like to check out the throttle position sensor - if it has gotten wet, dusty, or just worn, from my experience, it could definately cause symptoms like the ones you are describing....

Also, you might like to check the speed sensor located on the side of the transmission - if it's oily try cleaning it up and also check the connectors for a good clean connection.

Best of luck with it !

Weasel said:

It's a 89 4.0. Not sure what RPM it does it cause I don't have a tach but it seems to do it all over the place. Will do it with the pedal steady like you described above or sometimes will accelerating.



It does run a bit rich I think. The idle tends to be higher then it should.

Based on that I should be looking at the TPS. I'll check the O2 sensor harness as well.

Click to expand...

Check the hard plastic line leaving the TB going to the MAP sensor for cracks. Check the input voltage of the MAP sensor with a digital meter, the output voltage with an analog meter and a hand vacuum pump (your looking for a lineal increase/decrease in voltage, not intermittent or partial) and the resistance between chassis ground and the remaining ground wire of the MAP.

Check the TPS also for a lineal reading with an analog meter. You should see a smooth rise and fall of the needle (automatic trannies will have a two sensors built into one. To determine the set-point, you will need a good digital meter and the correct percentage of the idle's value. Setting is different for a manual or automatic transmission.

Thanks guys. I just swaped out the TPS sensor. It still had the original after 188500 miles. Defiantly idles better but the bucking is still there. I didn't get a chance to set it to the correct voltage though. Probably have my shop adjust it on Monday.

Not to highjack the thread. I have a 96 xj with the same bucking problem and my mpg is horrible. I only have 65K on the jeep and it has been babied. I was thinking it could be an 02 sensor. I recently had a non oem cat put on about a year ago because the original is shot. Could the new cat have damaged the 02 sensor after the cat? For a while the xj smelled strange but i thought it was the cat breaking in. Maybe that is what is hurting the mpg and making it run so horrible. I was also thinking egr or tps. One more thing worth mentioning is that it has a new computer in it (rebuilt), i had it replaced because one day i went out to the jeep and it just wouldnt start or do anything. I dont know if that is a common issue with these xj's.