Car Jerk - Transmission or Engine??

[bimboy]

I drive a '95 325i (AT) which has around 106K miles. Last year my AT transmission broke and I replaced it with a rebuilt
transmission from an independent mechanic. He aslo gave me one year warranty on the tranny. Now here is the problem I have,

My car jerks ta low speed, when coming to a sudden halt (or suddenly reducing speed), when accelerating from a stopped position etc. This happens mostly during cold start (usually in the mornings when I drive to work). Again this jerking doesn't happen every morning. Since it doesn't occur always it is very difficult to make it happen when I take it to the mechanic. Is this related to the transmission or engine? How do I diagnose this problem?

Any help/advise is appreciated.

Thanks

 

[DSK]

The first thing I would do is check the fluid level and condition of the fluid in your transmission just on the off chance you have lost some fluid since the gearbox was fitted. Replace it if necessary and see what difference that makes. May as well replace the diff oil too whilst you are at it.

If this does not cure the fault, then I'd take it to another garage for a diagnosis, then armed with that, if it turns out to be a fault with the transmission, take it back to the garage that fitted your transmission.

 

[bimboy]

Thanks for your response.

As I had mentioned earlier it has 106K miles on it. I don't think differential oil was replaced ever. So here are my Q's,

Can this problem occur because of old diff oil that was never replaced?
Because of a wrong grade/type of transmission oil?

Do you recommend any particular brand/grade of transmission and differential oils?

 

[Tom]

Since this doesn't happen all the time, it sounds to me like its more engine related than Transmission.

Has your fuel filter/fuel pump been replaced? It could also be a bad coil.

 

[DSK]

If the wrong grade oils are used in the diff/transmission, yes it can happen, and it gets smoother as the oils warm up, but if the wrong oils were used then the problem should have happened all the time ...... As for the grades, ATF (auto trans fluid) is used and I'd say a decent brand like Castrol Dexron III. (My manual g/box uses that too!)

I always change my g/box and diff oil at every service and I do notice a smoothness in the drive train from cold. (This is what has cured my jerks and roughness in the drive train during the first few minutes of cold use in all of my vehicles in the past).

Tom makes an interesting point to with regards to the fuel pump or an ignition coil.

 

[selavia]

I was thinking something to do with fuel line too Tom, but wasn't sure. One time my car was doing something similar; a buddy said use a fuel injector cleaner...solved the problem. Although, the jerks were barely noticeable and if you stepped on the gas even slightly it went away.

 

[bimboy]

I drove the car to work this morning. Very minor drag or hickup (no jerks though !!). But once the car warmed up (< 5 mins) it drove like a gem. It was shifting well when I accelerated from a stop signal. I totally loved it !!

But then when the jerking occurs the feeling is something like

(i) suddenly shifting down to a 1st or 2nd gear when you are driving faster (more like third)
(or)
(ii) releasing the clutch too quick after shifting to 1st or 2nd in a manual transmission. But mine is a AT though.

Also as I had mentioned earlier this doesn't happen always. It happens at cold start and once it happens it doesn't happen for quite some time (atleast a week to 10 days). Pretty strange. But never does the car drive smooth at cold start.

So do you guys still think this is more related to engine than transmission?

Thanks a bunch !!