M20B25 High Idle

[Eddie1990]

I have a high idle on my M20B25, it's sitting at about 1200 irrespective of if it's warm or cold.

I've checked for vacuum leaks, and the blue plug is new, so it shouldn't be either of those.

The engine is in my 1977 BMW E21, so there has been some modification to the wiring, but it is all done right, I'm sure of that.

I'm running out of ideas, could it be the TPS, or the MAF or the ICV? If so, what's the best way to be checking them? It failed the MOT on this, so I want it sorting sooner rather than later really.

Any help would be greatly appreciated.

[jimbom30cab]

Normally A TPS which isn't plugged in can give a high idle.

There are instructions on testing the TPS in the wiki but I would check it's plugged in and not soaked in oil first.

[Eddie1990]

Yeah I'll have a look and see tomorrow, thanks.

[Brianmoooore]

Simple things first! Check that the throttle butterfly is physically hard against its stop at idle, and isn't being held partly open by a stiff mechanism, a throttle cable that is too tight, or similar.
While you're there, look for any signs that the throttle stop has ever been adjusted since the car left the factory.

[Eddie1990]

I noticed this yesterday, the throttle isn't actually stopping against the stop, however I can't be certain the screw is in the original position.

It's not the cable as I've removed it and it still idles a little high.

As I say, I can't be sure on the throttle stop screw.

It's only idling at about 1200rpm when cold, it's not massively high, just a little.

[jimbom30cab]

Does the TPS make a quiet "click" as soon as the throttle is opened?

Is the TPS connected?

[Eddie1990]

I'll have a check tonight and get back to you.

[Brianmoooore]

If the throttle isn't closing against its stop, the first thing to do is find out why. It may be the linkage holding it open, it may be an obstruction inside the throttle body fouling the butterfly, or it may be that the butterfly is closing fully, and some idiot has been adjusting the stop.
Whatever the reason, this is something that is clearly incorrect, and should be sorted before doing anything else, even if it makes the situation worse or has no effect.

[Eddie1990]

Okay, so assuming the throttle stop is wrong, does it need winding back to touch the linkage stop plate? It might not have been moved, but as I say, it appears to be just off the linkage plate, by about 1-2mm

[Brianmoooore]

Find out why and report back.

[Eddie1990]

Still can't find anything.

Removing the oil filler cap reduces the idle to about 1,000 but it sounds ever so slightly lumpy. Don't know if that means anything to any of you in the know?

[Eddie1990]

Is it possible that the engine is over pressurising because it's plumbed into the E21 brake servo's? This creating a higher pressure in the intake system, and raising the idle?

Seems like a long shot, but I'm just brain storming with you guys

[Eddie1990]

When I disconnect the MAF it runs poorly, idle drops and raises rhythmically, almost like a really slow pulse, up and down of the revs.

[Eddie1990]

When I very slightly blip the throttle (with the linkage, not the pedal) the revs drop a lot, and it has caused it to stall on one occasion. When I say blip the throttle I mean slightly tap it, sometimes the revs climb momentarily and then dip very low, sounding like its about to stall.

[Eddie1990]

Just taken the vent hose off the rocker cover and that lowered the revs, in a similar manner as when I removed the oil filler cap.

[BenHar]

Does the TPS make a quiet "click" as soon as the throttle is opened?

Is the TPS connected?

[Eddie1990]

Yeah the TPS clicks when the throttle moves.

Yeah the TPS is connected correctly.

[BenHar]

Try taking out the ICV. Block the holes it connects to. If the engine still idles at 1200 the throttle is not fully closed.

If the idle goes down to very lumpy then the throttle is probably correct so clean/replace the ICV.

Ben

[Eddie1990]

Engine has now developed a running issue:

http://www.e30zone.net/modules.php?name ... 64#2834064