My M20 won’t idle properly (solved!)

M20’s should idle 700-800 RPM ideally. There are many reasons why it wont:

Bad unstable idle is most likely caused by a vacuum leak. Check all hoses including the intake boot, ICV (idle control valve) connector, the pipe that goes to the rocker cover and the one that goes to the brake servo.

Any leaks in these pipe will cause bad idle. In the case or our Sportster the connection between the server and the brake master cylinder had a large hole – possibly due to the shape of the Marlin supplied metro servo. We decided the best course of action was to use the BMW master cylinder with adapter plate. The BMW cylinder has a rubber O ring to seal it to the adapter. The adapter plate then has a nice flat surface to seal to the cylinder (to be confirmed what we’ll be using to seal this, probably silicon sealer or some gasket cork). Note: in both cases the pedal travel of the brakes is severly reduced due to a gap between the servo and the piston, please Brake Travel FAQ (once we’ve written it!)

Our engine currently has a high idle problem (around 1000RPM) will report back once its solved, but things to do:

  • Clean the AFM (Air flow meter)
  • Clean the ICV (Idle control valve)
  • Clean the Throttle body
  • Check the wiring that connect the engine loom to the injector rail for corrosion – the round connector in the middle of the in take manifold.
  • Check wiring to sensors for corrosion
  • Check the TPS (Throttle position switch) on the bottom of the throttle body. As it closes it should click to tell the ECU the throttle is closed
  • Check the blue ECU temperature sensor and try a replacement one
  • Check the spark plugs, if they are black then the engine is running rich – an injector may be blocked. Take the injectors out and get them cleaned, or if you have an ultrasonic cleaner you can do them yourself! While your doing this get a full set of new o-rings for the injectors (and one extra for the fuel pressure regulator while your at it) It maybe that you have a small vacuum leak around one of the injectors if the o-rings have rotten. On a 20 year old engine it can never hurt to take the injectors out and give them a good going over!
  • To test the AFM:

    "To test an AFM, connect a six or nine volt battery across pins 4 and 3, and connect (preferably) an analogue voltmeter across 4 and 2. Oscilloscope is even better.
    Move the air flap slowly from fully closed to fully open by hand while watching the meter needle. It should move slowly with no jumping whatsoever".

After much head scratching we purchased a Gunstone gastester with build in RPM read out. Low and behold it turns out the engine is idling around 750RPM which is pretty much spot on. Its the gauge that has the wrong read out. It over reads near the bottom of the scale, sits about right at 2000RPM and above it under reads.

Update: Richard spoke to Simon at Marlin today. It turns out the gauge needs calibrating to the BMW ECU signal so we need to send it to the manufacturer with the variation we’ve read of the gastester and they will tune it for us.

Here is a really good link for idle problems with the E30 engines: