how to improve idle on M30

[Robert K]

I posted a few weeks ago that I had just done a valve adjustment on my 535i. Rather than set the valves to .012 cold per spec, I set them to .014 cold. I also gapped the plugs to .035. I just wanted to report that my engine now idles much more smoothly than it ever has. Granted, the valves tick a bit more. But then again, anything with solid lifters is going to have a bit of valve noise. I believe that setting the valve lash a bit loose has decreased the valve overlap (the time both valves are slightly open at TDC when the exhaust valve is almost closed and the intake has already started opening), which smoothed the idle out. If you've got a lumpy M30, you might give it a try during the next valve adjustment. One thing though...decreasing valve overlap gives up a bit of high RPM power. I'd doubt it will be that noticeable though. Of course the other side of that coin is that you may gain a bit better low-end torque and possibly improve your fuel mileage too. Something to think about.

Grace and peace,

Robert K
1991 535i

[BigKriss]

Recently I set my valves to (cold) 0.014+, 0.14, 0.13 and finally 0.11. Everything above 0.11 was too noisy but the engine's idle was definitely smoother.

[Alexlind123]

You will probably have less power with larger clearances as well.

[Dr. evil]

excessive valve clearance is going to give you less duration and valve overlap. this will lower the engines volumetric efficiency lowering the amount of exhaust gas scavenging making more heat in the cylinder head.

i like valve duration

[fujioko]

Low lift, short duration = better low speed torque.
High lift, long duration = better efficiency at high speeds

[shogun]

http://www.pelicanparts.com/bmw/techarticles/JF-Tech/BMW_E30_3_Series_Idle_speed_troubleshooting.htm

[califblue]

I have been running mine a .009 for several years now...way smooth & quiet and a noticable increase in performance ;)