Author Topic: Idle seems off (Read 3188 times)

bad_robot · « **on:** August 30, 2011, 04:02:56 AM »

BMW 318i 1990 E30c M40
Idle speed not good - when cold revs high then drops - engine shudders threatening to stop but picks up again for a repeat - gradually getting slightly better as engine warms.
When it warms a bit, revs stay high and I can drop them by adjusting the screw below the black plastic cover at top of engine. Helps but is not a long term fix, particularly when engine gets hotter and revs increase.
When Aircon turned on revs rise higher.
Cleaned the idle fuel control at top back of engine to no avail.
I think it may be the air flow control (black plastic box at top of air filter housing) with the screw-in sensor connector. There appears to be a tamper proof opening hole to the top side of the control with, I suspect, some sort of screw adjustment behind it, perhaps.
Is this a source of adjustment and if so how do i remove the centre of the opening? Drill a hole and insert a probe and slide it out, perhaps?
A rap with the hand to this black box momentarily helps adjusting idle speed, if I can say that.

DesktopDave · « **Reply #1 on:** August 30, 2011, 07:20:41 AM »

Sounds like you have a cracked intake boot or vac hose. Check your intake hoses and especially your idle control valve. It's mounted under the manifold by the firewall on the M42, I suspect a similar place on the M40.

Re-adjust the idle stop screw on the TB and the CO pot adjuster in the AFM once you've determined the primary culprit.

bad_robot · « **Reply #2 on:** September 02, 2011, 08:07:36 PM »

Tks for all of that - after screwing up every connector I could seen and disconnecting the screwed electrical connector going into the air intake and stopping the engine - reconnecting it, I adjusted the idle screw, the engine seems to be running a little smoother.

However, where is the co pot adjuster in the AFM?
Is it here -
hidden under a tamper-resistant plug on the Throttle body (has the air filter below it) To access the screw, first drill a small hole (typically 1/8") in the tamper-resistant plug and pry it loose.
ie in the black plastic rectangle which has the screwed electrical connector going into it.

DesktopDave · « **Reply #3 on:** September 02, 2011, 09:08:23 PM »

Yep, that's the CO potentiometer. It won't work with the cat on newer Motronics though. There's a connector on the firewall wiring loom to plug together if the car is running CO and no catalyst, then the CO pot will function.