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DISCUSSION => Engine + Driveline => Topic started by: anisotropy on October 31, 2007, 05:34:24 PM

Title: Jerky when I lift off the throttle
Post by: anisotropy on October 31, 2007, 05:34:24 PM: I've just replaced centre bearing (preloaded properly too:)), diff, subframe bushes, diff bush etc. AND i STILL have a jerky car at low speeds. Double checked there is no play in the driveline. Guibo fine etc. Does anyone know are there any other causes of a jerky lift off?? AFM? :confused: are ISs just jerky??
Thanks
Title: Jerky when I lift off the throttle
Post by: dude8383 on October 31, 2007, 05:39:23 PM: Spark plugs and wires?
Title: Jerky when I lift off the throttle
Post by: anisotropy on October 31, 2007, 06:01:47 PM: hmm haven't checked them yet but could make some sense as I also have an annoying idle which gets bad under load -ie. a little worse with lights on, worse with full beams, a pig with full beams, fogs, rear demister etc. on. But never stalls -could this be related?
Title: Jerky when I lift off the throttle
Post by: christophbmw on October 31, 2007, 06:23:52 PM: sounds like a fuel problem, check fuel pressure, and the fuel filter first. check the spark too (as stated above).
Title: Jerky when I lift off the throttle
Post by: anisotropy on October 31, 2007, 06:53:07 PM: Doesn't seem to be a fuel problem -new fpr, new rail, injectors cleaned (from when I developed a leak under the intake manifold -at the time I hoped that would sort things:rolleyes:), new filter, new hard lines lines, recent pump. Been busy.;) What should I look for with the leads, plugs etc.?
Title: Jerky when I lift off the throttle
Post by: gearheadE30 on November 01, 2007, 02:15:30 PM: The alternator creates a lot of load for the engine to turn with electronics on, but it shouldn't be that dramatic. With lights and everything running, I only lose about 100 rpm if that. I'd guess a vacuum leak, as that seems to be the usual culprit.

As for jerkyness at low speeds, that is caused by drivetrain backlash. There is a good bit of slop in any car's drivetrain, and if you are driving at >10 mph in first or are idling around in second, the car will jerk. It shouldn't be noticeable on liftoff unless you let of the throttle abruptly, and bushings are the normal culprit, mainly the rear subframe. you already did that though...
Title: Jerky when I lift off the throttle
Post by: Royalratch on February 22, 2008, 04:32:52 PM: I also have this trouble - would really like some thoughts on it?
Title: Jerky when I lift off the throttle
Post by: nicknikolovski on February 22, 2008, 10:37:14 PM: i got the same jerkyness at low speeds, i think it may be caused from differential backlash. not 100% sure though.
Title: Jerky when I lift off the throttle
Post by: Nomadic on February 23, 2008, 06:45:36 PM: How about motor and tranny mounts?
Title: Jerky when I lift off the throttle
Post by: anisotropy on February 24, 2008, 05:26:16 AM: Mounts all look fine -engine ones are ex-pensive:eek: But it would be worth it if it sorted this out. ANyone have any pics of what newish ones look like?
Title: Jerky when I lift off the throttle
Post by: Royalratch on February 24, 2008, 05:38:37 AM: They're just taller as the rubber has not been squashed over the years.

And stiffer I guess, stops the engine kicking back when which is what you're experiencing.

I put new mounts on my 318iS and it made a difference - just made the throttle response a bit more immediate with no lurch or bobs even when you let the revs die to stall...
Title: Jerky when I lift off the throttle
Post by: anisotropy on February 25, 2008, 05:40:57 PM: I find you feel quite a lot of the engine movement on cranking and then as the revs settle - is that usual?
Title: Jerky when I lift off the throttle
Post by: nicknikolovski on February 26, 2008, 03:55:44 AM: thats most likely engine mounts causing the movement during cranking however there is always some movement due to the engine trying to fire.
Title: Jerky when I lift off the throttle
Post by: Royalratch on May 16, 2008, 08:34:00 AM: I also read somewhere it can be the engine block earth strap. Not the battery terminal but the engine block to inner arch/strut tower. If the engine isn't earthed properly it can have a massive effect on this kind of performance.