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. 

Update on my low idle problem. Basically it is still the same after fitting new spark plugs, distributor caps and new battery.

A mate at work thinks it could be the o2 sensors. Does anyone know of a good mechanic in the east midlands (Peterborough) area would could properly diagnose this fault?  

Hi eta. can't seem to find Bavarian Autoparts on the web. Do you have any other info?

Thanks

Hi Wiseass, thanks for the advice. I have had no "CHECK ENGINE" light or any other type of warnings on the dash.

If it was an air leak, how do I check this?

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

Looks like BAP is no more Patrick..  Martin has put up an announcement on the 7 register this morning...

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.

I beleive he went to Jayson at ChippedUK.com and got the work done.

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.  

Here is jays reply.

I trully think that the problem is with an electrical fault on the lpg conversion, i spoke to a company who do this conversion and the current accross the injectors can send the wrong signal to the dme causing rough idle.

I also think taking it to bmw may be a problem as they are almost certainly going to blame the lpg conversion and you will end up with a massive bill and no fix.

On the other hand it is possible it is just a bad earth on the engine (again given by advice from another company)

I will get his number for you and hopefully you can have a chat with him and see how it goes.

Thanks guys for trying to help.

eta. I would like to take up the offer from your wife's cousin and don't mind the drive upto York. Could you please let me know contact details and a convenient date/time.  

Latest news – Yesterday I took the car back to where I had the LPG conversion done about a year ago. The LPG system is fine; however their diagnostics did pickup that the right hand (looking at the engine from the front) lambda sensor was not working. There was no signal on the screen for this sensor, whereas on the left bank there was a nice square wave.

So, could this be the cause of all my trouble?

Does anybody know where on the car these are located?

How difficult are they to change?

Is it worth doing both at the same time?

Thanks