Yes, this is set at the factory and they say it should never be messed with, but as the youngest M40 engined BMWs are approaching 30 years old now, chances are the stop-screw and throttle-plate will have been adjusted at some point in its life.
Here I will explain what happens when it is adjusted, the problems it may cause if it has been and how to reset it back to stock.
WHY HAS IT BEEN ADJUSTED?
The stop-screw should have a paint mark on it from the factory to show if it has been messed with, but after so many years the mark may no longer be visible. If you feel yours has been adjusted in the past, the main reason this has been done is likely to falsely raise a low idle caused by another issue, say an induction air leak. If an over lean mixture is causing a low idle, the tendency is to tweak the idle stop-screw to get the car to tick over at higher revs and stop the engine stalling out or running lumpy. This is a great short-term workaround, but will cause other issues with the AFM / DME and fuel delivery further down the line, particularly when the underlying issue is worked out.
SYMPTOMS:
The issues you will get with a poorly adjusted throttle stop-screw are:
High idle.
Hunting / pulsing revs.
Poor or no low-end throttle response.
Erratic idle (if throttle position switch TPS is not engaged).
Bogging at high revs (where wide-open throttle WOT switch is not engaged).
There is only one way the throttle can be adjusted, via the stop-screw, though the throttle-cable itself can be adjusted to change throttle response somewhat, mainly with how the pedal / cable reacts to driver input and will not affect the fuel/air ratio and the engine idle.
** The intended job of the idle stop-screw is simply to stop the throttle-plate from jamming in the throttle-body and being difficult to open when the pedal is pressed and not to change the car’s idle characteristics. BMW recommend that this is never played with, so do so at your own risk. **
1. Back off the lock-but using an 8mm wrench.
2. Use a small flat screwdriver to wind the idle stop-screw in and out.
* Clockwise will push the throttle-linkage further from its rest position, holding the throttle-plate open slightly and allowing more air to bypass it while the pedal is not pressed.
*Anti-clockwise will allow the throttle-linkage to close further and will reduce bypass air and choke the engine while the pedal is not pressed.
STOCK SETTING:
You will need a set of A/F / imperial feeler-gauges as this is how the stock throttle-plate aperture is measured.
** If you are only adjusting the throttle-plate and do not need to adjust the throttle position switch (TPS) then the throttle-body itself does not need removing and you can skip to step 7.**
Removing the Trottle-Body:
1. Remove the main air-inlet hose from the AFM to the throttle-body by loosening the jubilee-clip and easing it off.
2. Unclip the throttle-cable from the throttle-linkage and remove the two screws holding the throttle-cable mount from the top of the throttle-body housing using a 10mm wrench. Move the cable assembly to one side.
3. Remove the two water hoses and air vacuum-hose from either side of the throttle-body by undoing the jubilee clips and teasing the hoses off with a screwdriver.
4. Remove the six nuts from the upper inlet-manifold using an 11mm wrench and the two locating screws from the manifold using a 10mm wrench. Lift the upper inlet-manifold so that the throttle body can be fully accessed.
5. Remove the throttle-body from the inlet manifold by undoing the four nuts using a 10mm wrench.
7. Back off the lock-nut of the stop-screw using an 8mm wrench.
8. Use a small flat screwdriver to adjust the throttle stop-screw by the notch in the end of it. Obviously, clockwise will move the screw further out and make the throttle-butterfly rest in a more open position. Anticlockwise will allow the butterfly to close more.
9. For the OEM setting, the throttle-butterfly should be 0.377” from the housing, so use your feeler-gauge between the side of the tube and either side of the butterfly until you find a happy medium.
10. Tighten the lock-nut up again with an 8mm wrench.
11. Now take the throttle-position switch (TPS) and locate it back in the housing so that the switch is depressed while the throttle is in the fully closed position. With the two screws loosened, the TPS can be swivelled left and right as in pic below to adjust the point at which it engages. The ideal placement for the switch is to have it click closed while the throttle is open about 1mm. (You will hear the quiet click from the TPS as it opens and closes.) When you’re happy with the TPS placement tighten the screws to lock it in place.
13. Refitting is a reversal of steps 5-1.
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