m6 rough idle

Hi Rohan

Sychronising will help but do these first.

1. New plugs - proper ones

2. Check resistance of leads including columns.  Should be around 300 ohms I recall.  If much higher or open circuit then repair as necessary - all individual connectors all available from BM.

3.  Check valve clearances - if out (tolerance is .3-.35mm) then you can get poor idle due to this.

4. Reset vacuums on each throttle bodies.  This is not synchronsing the throttles by the way.  Sychronising is where you set each throttle plate to open at exactly the same time.  This should not change much from the factory setting unless things have badly worn or been hit by something.  It will be the vacuum difference at each throttle port that will be causing rough idle.  Reason being is that with time the bleed screws (under the blue/yellow caps if they are still on) get crudied up with gooey black carbon deposits.  This unsettles the air feed past each throttle - but only at idle.

4.  Reset vacuums by doing the following.  Remove bleeds screws.  Clean with carb cleaner inluding the ports and make sure evertying is nice and clean.  Replace screws all the way and then open 1 full turn.  Now go and by a cheap vacuum gauge from the local car shop.  You will need one that records up to 400mbar.  Most record much more than this.  Also by a fuel filter which you place in line with the vauum gauge to reduce oscillations as each port only sees vacuum for 1/6 of the time.  Now with each bleed screw open 1 full turn record each port vacuum replacing the black/red rubber covers after recording each one.  Add all six up and this will give you the total vacuum.  Divide by 6 to give the average port vacuum and then set each port vacuum to the average.  When you are happy that the vacuums are set more or less equal then set idle speed via that big brass screw in the middle of the throttle bodies to 850rpm.   Job now done.   Idle should be much improved.

If not then synchronising the throttles and repeating the above would help but this is time consuming and expensive if your don't have DTI's

Something else worth trying as well is cleaning the injectors.  Over time the injectors get clogged and hence fuel flow and spray patterns are affected particularly at idle with low air flow.  Getting them cleaned does cost a bit (about £60 each) or use my method or chucking them in the wifes jewelery cleaner.  Made a big difference.

Best of luck with it.

Ian