M30 lumpy idle when cold

[youngbimmer]

Hey all, I've been meaning to ask other 735 owners about how their M30 sounds/behaves when cold. Whenever I start her up, the M30 sounds lumpy, as in revving up and down (even though the tach gauge holds steady at about ~750rpm). The lumpiness reduces as the engine warms up. Is this typical m30 behavior? Vac lines have been replaced, but the exhaust manifold gaskets have not (dunno if that makes a difference)

[DarinS]

It's certainly not uncommon on these motors, but it is still indicative of a problem. It's entirely possible that you have a vacuum leak somewhere, or you may have a failing engine temp sending unit (the one in your thermostat housing). You can check for vacuum leaks by spraying starting fluid around the intake boot, intake manifold gasket (at each runner) and any other suspect areas. If the idle jumps and/or smooths, you know you have a leak. The sending unit can be checked with a bowl of hot water, a thermometer, and a ohmeter. Finally you can try cleaning your AFM with a can of MAF cleaner. Finally, have your valves been adjusted recently? That could cause some rough idling at cold temps.

Values for the temp sender are here. IIRC the blue sensor on the m20 is exactly eh same p/n as the blue sensor on an m30: https://www.bimmerforums.com/forum/s...2#post29791282

[Cactuar]

My M30 acts the same way. The idle goes up and down. I suspect a vacuum leak, I plan to get a smoke test done.