engine 'shudder' aka no balls from idle < 4k rpm

[Kozworth]

I have a 325is that had a fuel pump issue back in may, so it sat while I picked up my eta sedan and drove that. I'm trying to get this a daily driver, I put a walbro 255 in it.

Its really hard to describe, but feels like the whole engine and driv etrain is shuddering up and down. Lack of power from idle to about 4k and anything above it cleans out and seems fine. The car has the bav auto chip.

It doesn't seem like a typical missfire from my experiences (of all times it was ignition failure) so I am wondering if the new pump is causing fuel starvation in the low end. The car basically has little power up to 4k rpm.

I read a bunch of missfire causes, but the car never drove like this before it sat. All injectors are clicking from what sounds the same pulse width but I will check if they do under a load at neutral. But even reving at neutral does not seem to affect how the tach operates.


Please pitch ideas, I need it as a reliable daily as my current sedan goes thru a major uphaul of body work. Thanks

[TG Goose]

check fuel pressure, could be too much fuel too. pull a couple plugs and take a look, white = lean, black = rich for the most part.

[Kozworth]

Ttt

[TG Goose]

What have you checked out so we can cross it off?


Sent from my iPhone using Tapatalk

[DisgustipatedAP1]

Have you retuned for the higher flow pump?...

[cabriodster87]

Check mass air flow meter for smooth signal, (from what i have read)

[jlevie]

A problem that goes away above 4-4.5k could be a bad DME or bad AFM.

[CorvallisBMW]

Or bad FPR

[Kozworth]

I never heard of anyone retuning for a walbro fitted pump.

Car drove fine, pump fried, replaced with 255, drives bad low end. I was wondering if na leaky injector would be an issue, because if the duty cycle is longer at wot then leaked gas wouldn't matter, where as when the duty cycle at idle is much less

[jlevie]

Leaking injectors will have a greater effect at lower engine speeds and are particularly noticeable at idle. The symptoms sound like something else is going on.

Below 4500rpm the DME is using intake & engine temperatures, AFM, and RPM data if the WOT switch is closed. Above that it only uses intake & engine temperatures and RPM data if the WOT switch is closed. So a problem with the air flow measurement can make the engine be down on power below the transition point. I've seen DME's that had a bad input circuit for the AFM data do this as well as AFM's that were producing bad data.

[barry]

Smoke test for vacuum leaks, ICV, CPS, AFM, Filter, spark harness arcing, Distributor, ETC
GL

[Kozworth]

I forgot to add, car is bav chipped, and the wot was not in spec when I tested it, but the idle is (on tps). But when I tested it and noticed wot was out, I did not have this issue

[Kozworth]

I was wondering if I upgraded injectors to 17/19 lbers with m30 maf and software if this would be a decent upgrade for the upgraded fuel pum/p, or if the problem is not related to this.

I am going to try to swap afm and check the plugs tolmorrow

[rhynolite]

Subscribed - having this same issue intermittently. Car will "shudder" below ~3K RPM's but then run fine at any higher range.

Problem will usually go away after a hard pull all the way through 2nd gear, which leads me to suspect fuel pressure/delivery. Checked wires/spark plugs/vacuum lines so far, all look fine to me.

[Kozworth]

Its weird, i drove it the other day and it wasn't nearly as bad, but recently it seems almost as bad.

at wot it is the worst, but even half throttle i notice it.

I will be checking the ignition system tomorrow, and possibly adjusting my tps. Keep you posted