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I got my Tacho wired up last night (along with the oil pressure, temp & volt meters), and started her up.
As normal she fired up fine, but the tacho was showing it sat at idle at 2,000 rpm. Isn't that a little high?
I couldn't find a quoted 'normal' figure for a M20B25 in my manuals - So I don't know what I'm aiming for.
I've know that the blue ECU temp sensor can cause issues - would this make her idle at a higher rate?
My Idle bypass valve appears to be working (it buzzes when the ignition is on before starting) - and I don't believe I have any vaccuum leaks, as all the hoses are tighter than tight things.
Anything else I should consider before getting a new sensor?
Would the TPS cause something like this?
I wonder if mine is doing the same? I believe I have a tacho setting on my multi-meter. I'll have to check it out with that and see if there is any difference between the two.
hmm that it's quite right. Should be 750-850 normally. Ours it 950 now. Vacuum leak at the servo cause idle issues for me but that was unstable rather than high idle.
Well, I've got it idling at 1K rpm now. Turns out that when I stripped and cleaned the throttle, I didn't "calibrate" the TPS (read: bolt it back on in the right position ) when I put it back on.
As it was a bit skewed, the throttle closed / idle switch was in the TPS was never closing, and the ECU thought the throttle was open all the time even when it wasn't.
It's still a bit high, but a big improvement.
I've also checked the blue temp sensor, and I think it might be buggered. It doesn't match any of the ranges in the Bently manual. I guess I'll be calling the BMW parts desk again on Tuesday.
It's also worth checking the ICV. They tend to get gummed up and don't regulate well. My idle speed was all over the shop 'till I changed mine. (£5 from Ebay). Now it's fairly steady at about 800 rpm - hot or cold.
I believe that the ECU does re-learn the engine after it has been off for a long time. Mine idles well now when it is warm but a bit grumpy when it is cold. (Llike its owner....)
I wouldn't worry too much about it but I think it's worth taking it in for an MOT well before the IVA if only to have the emissions test.
Good find Pete! Mind you, I don't like the idea of sticking bottle caps in the ICV as a fix...
The throttle stop adjustment-I've checked using a feeler guage and it appears to be ok... 0.015 of an inch. How tight down should the butterfly be on the guage? It can slip between it and the throttle body with relative ease on one side, and with a bit of a push on the opposite side.
Have just orderd a new intake boot, ICV hose and 'air pump' (the black plastic thingy that connects to the throttle body to the vac line), as these are all original parts and may be leaking.
The ICV hose is just push a push fit and may be letting air through there, the Boot looks ok on the outside but is looking worn on the bellows portion of it on the inside and the air pump has been on and off so many times I may have worn the insides...
Wasn't that expensive - £40ish for all of them inclusive of VAT from BMW.
Thinking about it, if you have a leak causing unmetered air to enter wouldn't that also make it run lean - which would make the running rough at idle....
Also did you check the vacuum feed to the servo? It's possible (or even likely?) to get a leak between the servo and the master cylinder as they were not designed to work together. As a test you could just try blanking off the feed to the servo.
Robin
PS I have a spare AFM if all else fails and you want to try it.
No - it's not rough. It's not even 'hunting', it sits at a fairly constent 1K rpm, give or take a couple of revs.
I modified my master cylinder a while back to make it sit properly in the metro servo:
The rubber sealing O-ring squishes up nicely and looks like it seals - when I checked earlier on in the build it seemed OK (checked with compressed air) - but I'll give it another look.
Try checking the elbow of the vacuum line where it goes into the metro servo body. The grommet on mine was a bit loose so I stuck the elbow in with some silicon sealer.
Right. I've changed the intake boot, the little pipe that goes from the ICV to the throttle body and the 'air pump'
I can tell that the little pipe needed doing, as the old one slipped on and off the ICV fairly easily. The new one was a right pig to get on! Guess that means it's nice and tight.
The old air pump slid appart when you took it off the engine. The new one most definatly does not, so I guess that's another leak sorted. In fact I know that's working very well, as I tried top remove the Vac line again after running the engine up, and the pressure differential made it very difficult!
The thing is, after changing these bits, the idle is still 1K - and I can't find any leaks, after checking with a can of carb cleaner on all the joints in question.
I wonder if that throttle stop is still a little too open?
Some of the AFM's have an idle contol adjustment screw which should be covered with a plastic plug.
The plastic plug was missing on mine but i've never had to twiddle it.
Maybe it had been 'adjusted' before you got the car to compensate for some other issue that you have now fixed?
The screw on the afm (hex) adjusts the idle mixture not the idle speed in the case of the M20.
If you take the big plastic cover off there is a spring system inside which can overtime become less strong so it needs adjusting a few notches. I never got anywhere with this myself but some people on E30Zone have sorted AFM related problems out this way.
Ours idles a bit higher now with the 2.8 and spikey cams 
) when I put it back on. 
