Low Idle

Hi all,

I own a 1998 BMW 750i but it has recently developed a very low idle speed. The symptoms are as follows:-

1. Cold Start - Engine starts ok and initially the idle speed is correct at 700rpm. After about a minute the idle drops to about 400rpm and the engine has an intermittent misfire. When driving off the engine can be heard to misfire until warm. Sometimes there is also very low power, which will suddenly cure itself after about 5 minutes. Another way to cure the low power is to stop the car, turn off the engine, then restart. 

2. Hot Start - Engine starts ok and initially the idle speed is correct at 700rpm. The idle then drops to about 400rpm when I select drive or reverse.

3. Driving - Great going down motorways etc when the engine speed is above 1000rpm, however, when in town or slowing down for junctions, the idle is again very low or sometimes the engine will stall. 

The car had an inspection II service about 2 months ago and there are 3 service lights to go until the next service. The car is great apart from this idle problem so I hope someone out there can help. On other forums I've read about idle control valves and lambda sensors, but I've now idea where these are or how to change them on my car. Are they expensive?

 

Thanks AG

Do you know where abouts the idle control valve is located? Is there a photo or diagram that you know of?

On another matter, my battery is very low and I think it needs replacing. I understand there are 2 batteries installed in the boot. If I replace these will I need to reset anything. I've heard some stories that the engine needs re-chipping etc. if the battery is disconnected. I think myself this is a load of bull. 

 

Bavarian Autoparts in the Leicester area. Come back to me if you cannot locate.

Not sure about European models, but isn't your car OBD2?  Usually bad O2 sensors will trigger a "CHECK ENGINE" type of light.

(I am under the general impression, if I recall from basic conversations that I have had with Gpower that 96+ E38s are OBD2 pretty much worldwide).

If I recall it's pretty easy to test them using a multimeter yourself.  Again, I am unfamiliar with the European model cars, but US-Market cars have 4 O2 sensors.. upstream and downstream of the cats, so there could be 4 to check!

Still, sounds like an air leak to me....    

The sneaky way to check air leaks is.....

Get a can of intake/throttle body/carb/etc cleaner.. and lightly spray it on the air intake boots that run past the air flow meter.  If you hear a change in engine idle speed/tone, then you found you leak.

Worth a shot anyways.

Trying to get info on BAP as their website not loading. Will advise ASAP. Hope they are still around in one shape or form as they know the e32 inc 750 inside out.

Try for the air leak!  a very possible cause IMO

check for splits in the rubbers boots connecting the MAFM etc. to the manifiold, you have 2 IIRC

If you give Jayson a call on 07815-501867 he will do you a full, free, diagnostic on the car so you will know for sure what is wrong.

He is based in Birmingham.

at a price

OK guys, here's the latest on the problem. I visited Jayson on Saturday morning and he kindly did a free, diagnostic on the car. The results showed a lot of misfiring on various cylinders and a fault with the EWS. The misfiring is exactly what I get when cold starting, but I’ve never heard of the EWS.

 

He reset the EWS and the idle went from about 300rpm up to the normal 700rpm. I took the car for a quick 10 minutes drive around the block and returned back to Jayson’s. When parked up the idle seemed steady at 700rpm.

 

Unfortunately by the time I had got home (about 80 miles) the problem had reappeared. I’m not really sure what this means. Is the EWS broken and needs replacing?

 

I’m wondering myself whether there are 2 separate faults here. #1 the cold starting problem and #2 the low idle.

 

Another symptom I’ve noticed is when slowing down for a junction or to stop, the engine revs are always above 1000rpm as it changes down automatically. As I continue to slow down below about 35mph, suddenly the revs will drop almost to zero and the battery light will quickly flash before the revs pickup slightly to the 300rpm level. Could someone on the forum please advise what is supposed to happen here? I would imagine the normal behaviour is for the engine revs never to fall below the idle rate of 700rpm as it changes down.

 

I’m now thinking I may have to ‘bite the bullet’ and take it to a BMW dealer. My nearest is Murketts in Huntingdon. Are these any good?

 

Thanks Jayson for your time and effort. Sorry it’s not better news.