MPG meter and dodgy idle

Can someone help? I've just bought a 1988 318i with 130000k. Overall the car is in good shape but it doesn't like to idle. It was idling at about 500rpm - a bit to slow with a flicker on the oil light (to be expected) and at the same time the MPG meter doesn't work. I've tried speeding the idle by adjusting the accelerator cable with the result that having got it to idle at 800rpm the next time I started it the damn thing refused to idle (and still hasn't forgiven me)! I'm wondering if I have a slight leak in the intake system and on this train of thought - how does the mpg meter work. If it works direct via a vacuum hose -..... I have read some of the indepth article by SAM but wanted to eliminate the easy before I get to bogged down. Any help would be much appreciated :eek7:

The mpg meter is controlled by a small computer inside the speedo. To access the unit remove the pannel below the speedo's, this can be done by removing the two nuts from behind the lower pannel below the steering wheel must be removed first.
You should now see a small white box about 1 cm by 3, which is clipped into the speedo on the right hand side. It should say BMW then a number, either 01, 02, etc, below this number is 1377361 or similar.
You do not have to remove the speedo to remove this unit, it slides out the front.
I own an 87 316, my car will run without this unit, but the service indicator board screws up and the inspection light will stay on. I dont quite know about the 318, but my 316 has a small unit attached to the top middle of the bulkhead inside the engine bay. A vacum lead runs from the intake manifold to this unit and then on to the distributor advance diaphram. The unit also has a 2 pin electrical connector connected. A 5 pin relay on the left hand side of the engine bay (as you are sitting in the car) controlls the system.
I have a similar problem as my mpg meter has the wrong computer and as a result the mpg meter does not drop like it should under acceleration, it gives a constant reading in each gear with top being over 50mpg! also my relay buzzes about 15 seconds after turning off the ignition. As a result i have removed the relay and i live without the mpg meter but all other functions are fine.

On the older injection systems there is an adjuster screw on a vacuum diaphram which pushes on the throttle. When the engine is about to stall the diaphram basically opens the throttle, then retracts when the revs pick up.

Hope this is of some help.

thanks for that - I'll have a good look at it this weekend