Poor running on start-up M40

crashthecar · 10-23-2006, 09:51 PM

I have a Jul 1991 318i with an M40 engine,

Lately it has been giving me grief intermittantly on HOT start up.
It starts fine and idles but when you go to take off it is dead in the water. The revs stay static and the car moves off very slowly.
Give the throttle a couple of good stabs and build the revs, it will back fire lightly a couple of times and then it's ok! It will drive fine for the entire trip and when you come back to it may or may not do it again...

I've checked leads for connection and condition, I've checked and insulated the induction pickup on lead No1 which plugs into the box under the inlet manifold as i thought it may have been shorting. Also I've checked the Air Flow Meter as i sort of feels like it's trarving for air? but the vane moves smoothly through it's travel and does not appear to get stuck.

Has anybody come across this problem before? Any ideas on what to check next?
I know it could still be the AFM but i've got nobody local with the same engine that i can swap AFM's with to test, and paying AUD$850 on a hunch is not going to happen...
Luke.

crashthecar · 10-25-2006, 03:41 PM

Anyone?

Wayne 962 · Wayne 962's Garage

Hi there. Yes, vacuum leak in the boot behind the air meter will cause this. This is very common, and these leaks may be very difficult to see with the boot still on the car. Basically, if there is a vacuum leak, the engine will be drawing air into the intake through the crack/leak and bypassing the air meter. This will cause the "starvation" problem that you were referring to, seeing no real air getting past the air meter.

Check it out and let us know what you find out...

-Wayne