Occasional jerking/bucking when accelerating?

[jasonbk]

Hi guys,

my m20b25 occassionaly would jerk/buck once or twice when I apply gas slightly at cruising speed (around 3k rpm at 5th gear) - once I floor it it would go away but appears only when I gently apply the gas.

The issue was disappeared during the summer but came back again as weather got cooler. Now it's got slightly worse as it stutter/jerk/buck in lower speed as well.

Funny thing is - it happens more likely when I stop the car after driving it, then restart it. (it doesn't happen when the car initially started from cold)

Any other guys have similar problem? I searched and top things to check was AFM, spark plugs&wires and injectors - but stated problem wasn't exactly the same.

[IRON-E]

Hi guys,

my m20b25 occassionaly would jerk/buck once or twice when I apply gas slightly at cruising speed (around 3k rpm at 5th gear) - once I floor it it would go away but appears only when I gently apply the gas.

The issue was disappeared during the summer but came back again as weather got cooler. Now it's got slightly worse as it stutter/jerk/buck in lower speed as well.

Funny thing is - it happens more likely when I stop the car after driving it, then restart it. (it doesn't happen when the car initially started from cold)

Any other guys have similar problem? I searched and top things to check was AFM, spark plugs&wires and injectors - but stated problem wasn't exactly the same.

Fuel related dude, get ready to replace your fuel pump.

[Bullhead]

I was having the same issue. I replaced the main pump and filter yesterday because it was wining something fierce but it still doing it slightly. I think the guy I bought the car from didn't put a new pickup in tank pump in like he says so I am going to change it next week too because my new pump is now wining as well.

[jasonbk]

I see. Is there any way to make sure it's fuel pump related issue? I probably have to get one for 55L fuel tank. http://www.realoem.com/bmw/showparts...43&hg=16&fg=05

[jasonbk]

BTW I had a fuel filter replaced 6 months ago.

[Bullhead]

Well if the tank is cruddy (it rusts internally over time from condensation) you can literally destroy a new filter almost instantly. I have actually went ahead and decided to do Eastwood's tank refinish kit next week because orange rusty gas is pouring out of the filter when I take it off. This is getting my injectors nasty and burning my pumps up. Your problem could just be the need for a new fuel pump. I have done all my vacuum testing etc... and it didn't fix the problem and my FP on the rail varies too much so I know my problem is fuel related and would strongly assume yours is as well. Change the filter (they have one for $6 at Oreilly's made by Wix) and if the gas that comes out is orange you may have a nasty tank too.

[DesertBMW]

I have the similar problem with my M20 and have driven few E30 with similar problem. I still haven't found a real cause.

[jasonbk]

I'm picking up used fuel pump from running 318i to see if that improves (meantime I can rebuild my fuel pump with better aftermarket pump), if not I'll report back.

It almost feels like drivetrain problem as it only jerks/bucks when it's in gear. When I apply throttle in neutral, I don't notice anything unusual.

[TurboJake]

Would it buck the whole car? Or is it something you can just feel the engine bouncing a little bit but not losing combustion?

Because if it's the whole car, like it's losing combustion, I'd say fuel or ignition.
If it's just like a rattle, and doesn't feel like it's losing power, Engine mounts.

[frolf]

When I purchased my car it has a hesitation at two points in the rev range. If I feathered the throttle around those points it would buck like nobody's business. I chased it for a few weeks (AFM, fuel, etc...)

The fix? Spark plugs.

[jasonbk]

I haven't had a chance to check anything yet - engine mount was in very good condition when I got the m20 swapped in less than a year ago. I had a new set of spark plugs 6 months ago but will check that as well. I think I would try spark plugs - spark wires first, then see how it goes.

[hooken m]

I am having the same damn issue... I have replaced almost everything. refurbished afm, refurbished injectors, new FPR, new plugs, new ignition coil, all new vacuum and fuel lines, fuel filter was replaced a few months ago, ICV is good (have 2 that i tested to be good), plug wires all ohm out fine, its not engine and tranny mounts as i have poly ones in there now. Its driving me fucking nuts! and my idle is at a steady 800-1000 all day. i've replaced so many damn things. subb'd to see where this goes

[3vilE30]

When I first picked up my seta it occasionally bucked hard if
I pushed anywhere near WOT. If I remember correctly I drove it hard few times and it pretty much went away, I eventually changed the plugs and it never happened again.
I would try getting a new distributor/rotor just to rule those out and they most likely are done by now.