Strange behaviour... - The Unofficial BMW M5 Messageboard (m5board.com)

jack880 · 12th February 2009, 15:26

Hi

When I start the car (1990 3.6) it ticks over fine, but often after driving it when I stop and let the engine tick over the revs drop to almost nothing, then back up to 1000rpm, then down to almost nothing again, then back up to 1000, etc about every second. Sometimes they drop so low the engine cuts out completely. Other than that the car drives fine - no loss of power or anything.

Anyone know what it could be? Some sort of sensor or something?

I don't know if its related, but the car was standing without being driven for about 3 months and the problem was only there after that. The battery was knackered so that's been replaced but the problem is still there...

Cheers!

Crash & Burn · 12th February 2009, 15:46

Don't know for sure but it could be the idle valve regulator which is situated under the plenum close to the engine block. Check the connectors and the 2 hoses. One comes from the rail under the throttlebodies and one goes to the plenum.

Cheers

jack880 · 12th February 2009, 16:00

Thanks a lot mate i'll have a look

IvanDias · 12th February 2009, 18:01

Hiya,

How many miles have you driven since using the car with its new battery?

If it's between 0 and 40 miles or so, it's completely normal behaviour. I have posted before warning owners that after anything but a short disconnection time of the battery the car is prone to stalling whilst the ECU learns the fuel and engine again.

This occurred mainly when at the split second the revs stumble very low eg 400rpm to 500rpm then if at that exact time the clutch is down and the steering turned (loading the steering pump with work to do), the car will cut out leaving you with no power, difficult steering and little brakes, a potentially dangerous situation especially at night when the lights die too!

(Typical issue is approaching a roundabout whilst on a fresh test drive with car and you dip clutch and turn wheel going from 2nd to 1st gear)

I find after battery disconnected work, a good spirited 20 mile blast gets it sorted!

If it has been doing it for longer than this, then something else is amiss, please report back when you discover what it was, the above poster gave good advice on where to start.

Kind regards

Ivan

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Crash & Burn · 12th February 2009, 18:09

Quote:

If it has been doing it for longer than this, then something else is amiss, please report back when you discover what it was, the above poster gave good advice on where to start.

"The above poster" would be me

Willem Kloppenburg from the Netherlands

IvanDias · 12th February 2009, 18:36

Quote:

Originally Posted by Crash & Burn

"The above poster" would be me

Willem Kloppenburg from the Netherlands

I'm at work and not supposed to be on the board, so no time to scroll up and get names!! lol

But your advice is good!!

Regards

Ivan

jack880 · 12th February 2009, 19:39

Thanks to both of you. Ivan, I've done over 100 miles with the new battery but probably no single trips more than 40 miles. I'll have a look at what Willem suggested, and see if a longer trip sorts it out.

Thanks a lot for taking the time to reply!

DrSakke · 12th February 2009, 22:04

air leaks in the intake manifold, check the rubber items thoroughly, as well as for loose vacuum hoses under plenum.
My buddys 750i did this (mainly with cold engine) and the cause was air leaks at intake gaskets on one cylinderbank.

Sakke