Low RPM hesitation - The Unofficial BMW M5 Messageboard (m5board.com)

Lantz · 4th July 2008, 00:43

Evening Gents!

I know this has been discussed a billion times before and seems to be a common fault on our cars, especially 3.8's. I spent about 45 mins searching and reading before posting but am still a little unsure about what to check.

So here it goes:

My car really hates driving at low revs ( 1500-2000 rpm ) ie driving around town in a civilised manner. It hesitates and the car will sort of buck at times in this sort of rpm range. Above 3000 rpm she is as happy as can be. The idle is also very lumpy, something I dont mind, but feel is a little OTT if you know what I mean, especially for a BMW ( the RPM needle however doesnt move at idle).

Valve Clearances are all good, rubber boot is new, sparkplugs are fresh. Fuel consumption is about 15.8l/100km according to OBC (A little high in my opinion compared to when I bought her; as low as 12l/100km on the motorway)

I read something about cleaning the crank sensor. Where is it located and how does one clean it?

Also what else should be checked?

Any input is as usual highly appreciated.

Lantz

PS BBS LM are fitted and they look great, will post pics as soon as possible!

nebpor · 4th July 2008, 01:15

Hi mate, it might be that or the idle control valve - you can clean both of them without TOO much effort!

The crank sensor is at the front of the engine, between the fan and the block - I'll upload a picture of it tomorrow as we cleaned mine recently.

The idle control valve is under the air intake - you need to take the rubber intake hose off (at least) to get at it.

Hang in there

nebpor · 4th July 2008, 01:22

Here is the crank sensor installed, with the black cable running off it.

Note that I had removed the viscous fan and the thermostat housing - I'm not sure if you NEED to do that - I wanted to clean everything up anyway, you can see how messy it was!

Here is the crank sensor once it is removed - just clean all the rubbish/oil off it using brake cleaner or WD40.

When you put it back on remember to leave 1mm gap between the sensor and the crank - it works using magnetism (I think).

Ross D · 5th July 2008, 00:07

Lantz
have you had a diagnostic check done to see if any fault codes are stored.
This could help you pinpoint the fault more easily and could save you time and money by not having to replace or clean components that are working fine.

farrell · 5th July 2008, 08:54

Hi
It's not engine timing sensors, I'm pretty sure.
You have timing calibration on camshaft & crankshaft.
If it runs fine at higher rpm, then I would move on to testing other
fuel & throttle based primary sensors.

Cheers
Farrell

Steve Mac · 5th July 2008, 09:35

Have you checked you have a good earth/ground from the block to the body?

just another thing to check

Lantz · 5th July 2008, 11:25

Thanks for the replys, and Nebpor thanks for the useful pics!

Ive had the fault codes read and according to that everything works as it should.

I'm thinking it could be bad intake gaskets, LMM, vacuum lines under the plenum.

Earthing is all good!

Lantz

Ross D · 6th July 2008, 17:54

Lantz
To test your intake gaskets,with the engine running spray brake/carb cleaner at the gaskets if you hear the engine speed raising by itself this will show you where it is drawing air.
this test can also work with vacuum hoses.

286bhp · 6th July 2008, 23:16

Try checking the lambda sensor - sometimes the output voltage 'freezes' indicating a lean mixture, which results in the dme over-fueling to compensate. if you disconnect the lambda sensor plug ( round plug just below front of starter) see if the problem lessens ( the DME will substitute a default value for the sensor. Get a local garage to carry out a live data session to see if the lambda voltage is fluctuating correctly between the following readings at least once every 2 seconds on a warm engine. - 0.1volts -> 0.9 volts. If the voltage sticks at around 0.45v then the sensor is defo u/s. A duff sensor doesn't always flag a fault code on 3.8 DME's

Phil

Lantz · 7th July 2008, 12:10

I think I solved the problem, rather cheaply too for once.

There is a black cover inbetween cylinder one and the fan, with a think cable running into it. I took the cover off and one of the 3 sockets was not properly connected ( the mechanics probably forgot to connect it all back properly ). I cleaned the sockets with some electrical cleaning and connected it all up. However, I have yet to see if it made a difference as its pissing with rain here.

Anyone know what these are for?

Lantz