Car Bucking (?)

[IrishGuy]

Maybe some of you can help - the j-body forum members aren't much help.

My car has this weird problem that I cannot figure out.

On some days (seems to be when it's very cold out) my car will begin to constantly buck  while traveling a constant speed. The last time this happened I was traveling about 70-75 mph with the car warmed to its normal operating temp. All of a sudden it began to jump in RPMs and then drop back down again. I could feel the car shudder/buck. It was if it was switching between 3rd and 4th gear. If I down-shifted to 3rd gear while it was doing this it would be fine (although I'm not a fan of driving 70-75 at 4,000 RPMs). So I would put it back into 4th and it would do it again anywhere from  20 seconds to 15 minutes later.

Once again:

Seems to be only in cold temps (sucks since it is the NE).
Car warmed up to normal operating temp.
RPMs will jump then fall back to normal operating range repeatedly.
Doesn't happen if driving in 3rd gear - although this is not an option.
Car shudders/bucks multiple times while doing this.
The CEL does not come on when this happens - nor do any other lights.



If anyone has any sort of idea what it may be I would appreciate it. I really don't want my transmission to be all over the NYS Thruway. I'm just curious about what I should be looking for.

[Raza]

Sounds like the problem is the fact that you have a Sunfire.

Hehe.

It sounds like a transmission issue to me.  But I don't know that much about it.

[Run Away]

Stab in the dark, Throttle Position Sensor?

What rpm in 4th does it buck?

If you drive along at the same rpm in 3rd (or anyother gear), will it still buck?

I'm guessing this in an auto since you cruise in 4th?

[IrishGuy]

Stab in the dark, Throttle Position Sensor?

What rpm in 4th does it buck?

If you drive along at the same rpm in 3rd (or anyother gear), will it still buck?

I'm guessing this in an auto since you cruise in 4th?

Yes, it is a four speed automatic.

Okay, I was just thinking about it during class trying to picture exactly when it happens. And I remember that it has also done it in the past while lightly accelerating. But only does it on cold days.

What does the TPS do exactly?

[93JC]

What does the TPS do exactly?

The TPS monitors, well... throttle position.  :P

It keeps tabs on the throttle plate's opening. Information from the TPS is correlated with information from other sensors by the main computer, which will adjust the fuel injection and ignition systems to suit the operating conditions.


Sounds to me like it's a transmission problem though, not an engine problem.

[footoflead]

What does the TPS do exactly?

The TPS monitors, well... throttle position.  :P

It keeps tabs on the throttle plate's opening. Information from the TPS is correlated with information from other sensors by the main computer, which will adjust the fuel injection and ignition systems to suit the operating conditions.


Sounds to me like it's a transmission problem though, not an engine problem.

I agree^

good luck  

[IrishGuy]

What does the TPS do exactly?

The TPS monitors, well... throttle position.  :P

It keeps tabs on the throttle plate's opening. Information from the TPS is correlated with information from other sensors by the main computer, which will adjust the fuel injection and ignition systems to suit the operating conditions.


Sounds to me like it's a transmission problem though, not an engine problem.

Sorry to bother you guys once again, but why would it only do it in cold weather?

[93JC]

Well, temperature has a profound effect on the operation of many of a car's systems, notably hydraulic systems like automatic transmissions. Cold weather could be screwing with the viscosity of your ATF, or it could be buggering up one of the electronic components. Who knows?

Certainly I don't know exactly, but temperature can do a lot of things to a car.

[IrishGuy]

Thanks for the replies everyone. I really appreciate it.  

[Run Away]

Hm, does the J body automatic only lock up in overdrive (4th)?
Could it be that it isn't locking up properly or something?

[93JC]

Hm, does the J body automatic only lock up in overdrive (4th)?
Could it be that it isn't locking up properly or something?

I thought of a TCC lockup solenoid malfunction, but it locks and unlocks under normal operation anyway. I doubt it would produce engine speed variations to the degree he's describing. And if it was malfunctioning it would probably affect all gears: he'd probably have problems with stalling at low speeds in first or second, for example.

[Run Away]

I didn't think it would have the large varitions in engine speed either but I don't know much about autos.
I knew older autos (like TH350s and 400s) only lock up in overdrive (right?) so I thought maybe that since the problem only occurs in overdrive it might be related....

Maybe try changing the tranny fluid and see if it goes away?
EDIT: J body autos are "no maintenence" arn't they, they have no dipstick....can you still change the fluid conventionally?

[IrishGuy]

I didn't think it would have the large varitions in engine speed either but I don't know much about autos.
I knew older autos (like TH350s and 400s) only lock up in overdrive (right?) so I thought maybe that since the problem only occurs in overdrive it might be related....

Maybe try changing the tranny fluid and see if it goes away?
EDIT: J body autos are "no maintenence" arn't they, they have no dipstick....can you still change the fluid conventionally?

I don't believe I can. It seems when I go to get the oil changed at non-dealerships they mention that its recommended to get it done but due to being a closed case (I think that's what it was) that they cannot do it.

The tranny fluid has never been changed and I'm about 5 miles from 118,000. So perhaps I will start there. Too bad I'm planning on getting a "new" car around May...

[ifcar]

If that's the case, I'd just live with the problem until May. It doesn't sound dangerous, just annoying, and it isn't constant.  

[280Z Turbo]

Definately not the TPS. A misadjusted TPS would make it buck, but only when you apply or let off the gas.

Sounds like an electronics thing. If the problem doesn't seem to make sense...there's a good chance it's a electronics issue.

[IrishGuy]

Thanks for the quick replies everyone  :wave:  

[Tom]

Unlikely, but maybe you need new plug wires or spark plugs.  Less likely, maybe air or fuel filter.  Of course, this doesn't explain why this only happens at high speeds in 4th.  Does you engine run kind of shitty or take an unusually long time to warm up?

[IrishGuy]

Unlikely, but maybe you need new plug wires or spark plugs.  Less likely, maybe air or fuel filter.  Of course, this doesn't explain why this only happens at high speeds in 4th.  Does you engine run kind of shitty or take an unusually long time to warm up?

Nope - no shittiness. I mean - its rough but thats how it always has been. Not anything out of the usual.

As for warming up - it warms up right away. And the air/fuel filter - just recently did that. The cold weather part is the confusing bit.