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Mixture / Idle Troubleshooting

Posted by Flyboy 
August 31, 2012 04:29AM
Hi All.
It seems a lot of posts are about lumpy/erratic idle and mixture problems, I see this coming up time and again.
I have had my fair share of head scratching, you will remember the saga with my little 318i after the valve train overhaul. :redface:
On the way to e30 happiness I have had to go through a lot of experiments.
Now I guess most of the senior members here will know most of this stuff, the newer guys, maybe not so much, so I hope that my findings can help someone else and save a few grey hairs.

Vacuum leaks - I had a crack on the emissions breather hose that goes from the tappet cover back to the throttle body - this caused the idle to be too high by about 800 - 900 RPM, with a slight regular surging. Check your vacuum hoses, even if you're sure they are fine (as I was, until I found the leak) – took me ages to find this.

Throttle stop screw - If you're messing with this to correct idle, you're masking another problem. This should be adjusted to be pushing on the stopper by only about 0.5mm, just enough to prevent the throttle butterfly from sticking in the throttle bore. It is not for allowing more air into the system. If your idle is too low when this is correctly set, you probably need to adjust the idle mixture at the AFM.

ICV – This is activated by the DME, when it receives a closed throttle signal from the TPS. I cleaned mine inside with carb cleaner and tested it with a 9V battery on the connector pins on the valve itself. The valve should close and open completely and move easily, when alternating the battery polarity between these pins. Resistance should be 40 ohms between the outer pins and 20 ohms between each outer pin and the centre pin.

Coolant Temp Sensor – The blue connector on the thermostat housing. Resistance between the pins should be around 3000 ohms when the engine is cold, and around 200 ohms when the engine is warm.

TPS – There are actually 2 switches inside the unit. This sends 3 different throttle condition signals to the DME.
Closed throttle – DME activates idling map.
Part open throttle – DME adjusts mixture and timing based on info from AFM.
Wide open throttle – DME activates map for power.

Check that you have continuity between the first and middle pin, with throttle closed, and that the switch is deactivated as soon as you begin to open the throttle (no continuity) – you should hear a click. With the throttle around midway, you should have no continuity between any of the pins. You should have continuity between the middle and end pin when the throttle is about 10° from fully open, when the WOT switch is activated (My WOT switch turned out to be faulty). If the idle switch is de-activated too late, you have to adjust the switch positioning on the butterfly shaft by loosening the two screws below.

Bypass screw on the AFM – Stock default depth setting is stamped on the AFM body, close to the screw hole. This is the distance in mm from the top of the screw head to the flat surface above. Mine says 16.9 - exactly 3 full turns out from fully screwed in. This only affects the mixture at idle – the bypass air passage is too small to significantly affect the mixture when driving.

AFM spring tension – If yours has not been tampered with, leave it alone. Mine had been adjusted by a previous owner, so I had no idea where the stock position on the cog wheel was. With the bypass screw at default, and all other conditions OK (no vacuum leaks, switches, valves all good), gently push the wiper arm in either direction, and note where the idle is highest / smoothest. Adjust the toothed cog so that the wiper contact points sit at the area on the resistor track where the idle is best. Once that's set, tweak it with the bypass screw by up to ½ a turn in either direction as needed (richer = in; leaner = out). This should get you fairly close to a stock AFM setting... but having a CO² analyser will help, especially to check mixture at mid-throttle.

I hope this helps some people, Feel free to add or correct where necessary.


When in doubt, use full throttle,
it may not improve the situation, but it will end the suspence.
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