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318i idle woes ...

Jay,

To fix the idle ckt. on the 318i, first let's go over what it does and 
how it does it ..

The Idle Control Unit (or module, hence called ICU) is an analog closed-loop
controller based on some op-amps that outputs a PWM (Pulse Width Modulated)
signal to the Idle Control Valve (called ICV) ..

A current of 0mA will cause the ICV to be fully open, and a current of about
500mA (approx) will fully close it ... 

The ICU gets the following inputs ..

1) 12vdc [power]
2) Ground
3) RPM Signal [from coil primary]
4) Idle Switch input
5) Auto Trans Park/Neutral Input 
6) Engine Coolant Temp Switch input (closes at 45degC+ coolant temp)
7) Air Temp Switch (on firewall, behind battery; closes at OdegC- air temp)

The output is a two wire ckt. to the ICV ..

The PROPER operation is a follows ...

AT in ANY gear, 700 rpm

AT in P/N or MT vehicles ...

Engine cold (< 45degC) 950 rpm

Engine warm (> 45degC) ...

	Ambient Air Cold (< 0degC) 850 rpm

        Ambient Air Warm (> 0degC) ...

		A/C ON  850 rpm

		A/C OFF 700 rpm

Get either a Bentley, or a factory (318i and M3) service manual and check
all of the inputs to the ICU ... you'll have to look close to see the Idle
Switch value, as it is only there for a split second unless the engine is
running .. both Bentley and factory manuals have good ICU input tests ..

Next check for proper setting of the throttle plate position ... it should
be closed almost completely, a few thousandths is the proper opening!  ...
also check for vacuum leaks ..

You can check the ICV with a 9v battery .. if you remove the ICV and
connect it to a 9v battery, it'll snap closed if O.K. ...

BTW, you DO have the silver metal ICV, and not the black plastic one, right?!?

If you have found nothing else, then it is a good bet that the ICU is dead or
dying .. NOW .. if you are brave, you might be able to fix it!

Remove the ICU, unplug it, and bring it inside .. open the case and the
connector (12 pin) will slide out with the 2 small circuit boards attached

You open it up by prying the connector out of the end of the case with 
2 small flat-bladed screwdrivers ...

If you look at the circuit board, you may see a burned area near a large
transistor on the board that is directly connected to the 12 pin connector

Check those burned traces with a multimeter & magnifing lens, chances are
one is cracked/fryed ... use a small piece of wirewrap wire to bypass the
burned area, from transistor solder pad to the next solder pad on down the
line ... (usually, this is one of the 12 "connector pin" solder pads)

This MAY fix it, but if not, get yourself a new ICU AND ICV .. you should
change them BOTH at the same time ...

The last hope, is that one of the connectors in the harness side is a bit
stretched out and is not making good contact with the pins on the ICU ..

If you ARE REAL CAREFUL .. you can slide the back off of the 12 pin harness
connector and AFTER NOTING THE POSIION OF EACH WIRE BY COLOR IN RELATION TO
the INDEX PIN (plastic) .. you can remove the front of the connector, and
using needle nose pliers GENTLY crimp/close/tighten each of the 12 female 
socket pins .. replace the connector front, assuring that the each wire is
in the correct place in relation to that INDEX pin, and put the back of the
connector back on .. hook it back up and try it .. a lot of times, the 
connector is loose, and that's the ONLY problem ...

Jim Conforti