Fast idle Speed 1991 325i

I have recently taken on a new girlfriend and she came with a great 1991 325i Saloon.

The car is in great condition and recently serviced, however when it is started from cold it idles at just under 2000 rpm. As the car is driven and the engine warms up, over a period of 15 mins or so, the idle speed slowly returns to it's normal rate - about 900 rpm. During this time there is quite a lot of exhaust. This morning once I had reached work (45 min city drive) the idle speed had still not reached its normal level and the water temp guage was lower than its usual half way mark.

Apparantly this is a new condition.

Anyone got any ideas what might be causing this?

Low water temperature and slow warm-up rate might indicate that the thermostatic valve in the cooling system is stuck open. Normally the valve should be shut until the engine warms up, allowing the coolant to circulate through the engine and thus heat up faster. If the valve is stuck open on a cold day, the engine might never reach normal operating temperature.

It wouldn't ever get to normal temperature on a cold day if the thermostat was jammed wide open. The idle speed is adjusted according to the coolant temperature. Admittedly, however, after 45 mins one would have expected the engine to have heated up enough to reduce the idle to normal.

Hi, this might be of some use. I also own a VW Golf MkII Gti 8v. Sitting on top of the rocker cover is whats called the FUEL REGULATOR. Its about the size of a can of redbull. Now at the moment mine idles from cold at around 1650 - 1900 rpm, until it starts to warm up, and then like yours drops to its regulated 900 rpm. Basically its knackered and needs replacing. A ten min job for the Golf - not sure about you bimmer. But at around £110 i cannot be bothered and as im selling it to pay for parts on my Hartge + is not worth the effort.

I would hazard a guess as you have the same problem. If you can get hold of a Haynes manual it will tell you what voltage it should be producing. Get a multimeter and test it. Im sure that it will confirm my thoughts that its on its way out.

Hope you sort it out.

OK, thanks b318isp. The other day, I was reading about the Motronic EMS in a technical manual and found that there was a whole lot I didn't know about the system, so I'll refrain from making comments about it until I know what I'm talking about!

I still think there's a good chance I'm right about the low coolant temp being caused by a stuck thermostat though. The symptoms (low temp, reducing on open road) are exactly the same as those on my housemate's car.