Flat spot between 2-3000 rpm

Post Sat Jun 21, 2014 7:37 pm

Hi,

I need a bit of help.

It idles fine, and accelerates well above 3000rpm, but until then is hesitant and lumpy. However, not so bad if I put my foot down.

Any ideas? Thanks in advance

(Oh, its a 1987 M10B18 Auto)

Post Sat Jun 21, 2014 8:01 pm

Not trying to hi-jack the thread, but I've got a similar issue, except with a m20b25.

The car judders and shakes, when in 2nd or 3rd gear, exactly at 2000 rpms, more apparent in 3rd (driving 30 mph). If I go slower or faster, it goes away, but it's there exactly at 2k rpm. Weird, I know...

Jona, I've been told it could be AFM, so might be the same thing in your case?

Post Sat Jun 21, 2014 8:47 pm

Hi,

I've got a new AFM, so still puzzled...

Post Sun Jun 22, 2014 11:08 am

Jona1971 wrote:Hi,

I've got a new AFM, so still puzzled...

Any air leaks, rubber boots, vacuum pipes?

Post Sun Jun 22, 2014 5:43 pm

Just checked rubber boot and its fine.

I'll have another check for air leaks elsewhere, should I be squirting carb cleaner or WD40 at the joints? (when engine cold I think?)

Thanks,

Post Sun Jun 22, 2014 5:51 pm

Jona1971 wrote:Just checked rubber boot and its fine.

I'll have another check for air leaks elsewhere, should I be squirting carb cleaner or WD40 at the joints? (when engine cold I think?)

Thanks,

Have a look here for common air leak sources > http://www.e30zone.net/e30zonewiki/inde ... #Air_Leaks

And yeah, you spray carb cleaner around the intake area and if the revs go up, that means there's a leak at the point of spraying.

Post Sun Jun 22, 2014 6:54 pm

Thanks, will do.

Post Sun Jun 22, 2014 8:23 pm

Right done that, even took the airbox off and sprayed the boot both ends, rocker cover breather, etc. - no leaks.

Idles great, goes ace at WOT, but otherwise hesitant between 1-3000rpm if accelerating gently, sometimes popping noises in the intake area, sometimes revs cleanly.

Took it for a test drive and symptoms worse uphill under load, no probs downhill????

Puzzled...

Post Mon Jun 23, 2014 8:00 pm

No ideas, maybe some issues on the fueling side of things? FPR, pump, injectors?

These kinds of things are difficult to diagnose...

Post Tue Jun 24, 2014 1:23 pm

when did you last have your fuel filer changed ?..equally have the injectors been serviced lately?

Post Tue Jun 24, 2014 8:08 pm

New fuel filter last month as part of major service.
Never had the injectors serviced.

Post Tue Jun 24, 2014 8:34 pm

Jona1971 wrote:New fuel filter last month as part of major service.
Never had the injectors serviced.

That would be my starter for 10, a zoner overhauls injectors
and did a smashing job of mine.. Think he charged about £10 per injector... his name escapes me for the minute.

Post Tue Jun 24, 2014 9:30 pm

aimlessrock wrote: a zoner overhauls injectors
and did a smashing job of mine.. Think he charged about £10 per injector... his name escapes me for the minute.

Not exactly a zoner as such, but one of our traders. Look in the traders' forum!

Post Wed Jun 25, 2014 6:15 pm

Thanks chaps.

Another puzzler, I've just filled it up with LPG and it runs better on gas than petrol...

Post Wed Jun 25, 2014 7:17 pm

Jona1971 wrote:Thanks chaps.

Another puzzler, I've just filled it up with LPG and it runs better on gas than petrol...

So it should! LPG is a much more suitable fuel for spark ignition engines than petrol.
Chances are that the injectors are clogged since they probably only rarely get used, and would greatly benefit from a rebuild, but since you're only likely to use them to get the engine running, there isn't really much point.

Post Mon Jun 30, 2014 5:47 pm

Removed the throttle body, filthy.
Checked the TPS as per WIKI: TPS working microswitch but not WOT.
Removed TPS and it was full of oil.
Cleaned and replaced TPS and checked with multimeter, not working as per WIKI, but working fine as per Mr Haynes Manual (different pins to check).
Maybe the TPS is different on my 318i than other models?
I don't know if the TPS was working OK before or not as I checked it wrongly for my model, but I can't have done it any harm giving it a clean up.

Result: drives better now. Idles perfect and great at WOT. A bit chuggy and hesitant still at half throttle. Chuggs when cruising at about 2,200 rpm. Drives better on lpg than petrol, more responsive.

Still feels like an air leak, but I can't find one...

Post Mon Jun 30, 2014 10:12 pm

Jona1971 wrote: A bit chuggy and hesitant still at half throttle. Chuggs when cruising at about 2,200 rpm. Drives better on lpg than petrol, more responsive.

Injectors!

Post Tue Jul 01, 2014 5:58 pm

Yes, I think you're right.
But the lpg system has its own injectors and i'm getting the same symptoms (but less so).

Post Sun Jul 20, 2014 11:35 pm

Err apologies,

turns out the "new" AFM I bought, that turned out to be a reconditioned one, (with all the seals intact) from Lucas was sha**ed.

I put the old one in and it runs like a dream.

Sorry to mess everyone around, I assumed this new part was OK and everything else was U/S.

(Another Haynes Lie: the Resistance of the AFM on an M10 is linear - NOPE; the old one I tested, as did my garage that also don't know what they're talking about, was OK to start with.)

On a happier note, my car now runs beautifully and I am a very happy camper once again.