Intermittent Bucking. ('87 4.0L XJ)

This problem only just recently started (seemingly after a couple of snow storms), and the symptoms aren't reproduceable. When driving down the road, the bucking happens sometimes while maintaining speed, is completely fine during hard accelleration, but is most prevailant during partial or closed throttle deceleration.

It feels as though something in the driveline is binding, but I can't understand why that would only happen intermittently, or why it wouldn't happen under acceleration.

I'm hoping this is a fairly common problem, with a fairly easy fix and that one of the gurus here can point me in the right direction. So far you guys have saved me a ton of money.

Sounds like the TPS. Its function depends directly on throttle position, and it's far more likely for it to have a dead spot at part throttle, where you spend the most time, than at full throttle, which is used less often.

There is plenty of info on the board on checking, replacing, and adjusting the TPS.

Could be the O2 sensor, though O2 sensor bucking seems to be fairly RPM specific. Around 2200 RPM plus or minus 200, usually during constant throttle (cruising), sometimes in the same RPM band in lower gears, but less severe than high gear lock. How old is your O2 sensor?
There are some other reasons for bucking, most any lean burn condition (low fuel pressure etc.) can cause it, a vacuum leak or theoretically a MAP sensor (though I've never seen it caused by a MAP).
As can something in the timing, a TPS adjustment or worn distributor gear. And as a last resort distributor indexing. It can also be an indication of valve trouble.
Foot off the pedal deceleration is supposed to turn off the injectors under 1200 RPM.
I had a dead spot in a TPS, it didn't buck, but the motor would just sort of crap out for a second or two.
What you said about being able to accelerate through it, sure reminded me of when my last O2 sensor started acting up, it got progressively worse. In the beginning it was sporadic, in the end almost constant at cruising speeds. My Max RPM was around 3500, after the new O2 sensor around 5,500.

In the past month, I've replaced the cap, rotor, plugs, wires, air filter, CCV system, cleaned the throttle body, adjusted the TPS to spec, changed all fluids and cleaned a bunch of electrical contacts. AFAIK the O2 sensor is the original, with just over 120k miles on it. I'll replace that ASAP, along with the fuel filter (I have one sitting right here, but it's been too cold here in Ohio to do it as I don't have a garage).

I do hear what sounds like a vacuum leak coming from near the gas tank, but starting fluid has turned up nothing, so I assumed it was the tank pressurizing. Also, it idles fine, so I doubt it is a vacuum leak.

If all that does not fix it, your fuel pump sock might be very dirty or the pump may be on its way out.

8Mud said:

What you said about being able to accelerate through it, sure reminded me of when my last O2 sensor started acting up, it got progressively worse. In the beginning it was sporadic, in the end almost constant at cruising speeds. My Max RPM was around 3500, after the new O2 sensor around 5,500.

Click to expand...

I have noticed that it doesn't feel quite as peppy as it has in the past. In fact, it feels like it bogs down quite a bit when it shifts to 3rd gear. I'll get the new O2 sensor in and hopefully that'll make everything better.

Just for info *if* the O2 sensor is the original (the original had a little heat shield on it), it's a recall.

Any word on this yet? I am suddenly experiencing bucking and suddering at 60+ mph up to about 70 mph. i have newer TPS, less then 6 months, new CPS'S and newish less then a year old O2 sensors..