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Old 10-04-2012, 08:08 AM
Billwill Billwill is offline
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Location: South Africa
Join Date: Oct 2009
Posts: 529
Mein Auto: BMW E30 318i 1991
Originally Posted by casino1 View Post
my poor bm when i start the car the engine idle surges up and down then cuts outs 6/7 times i start it same thing drive the car cuts out when i stop but after 5 min when engine warm its ok but still have erratic idle and still feels like it wants to cut out took icv off cleaned made no diffrence any one got any ideas

OK you obviously have the M40 motor.....same as me!

Firstly for cold start idle problems you need to check out the Blue Temp sensor...located under the intake manifold next to the Brown sensor which drives the temperature gauge.
With a bit of fiddling you can get to the sensor but it is best to actually measure it first at the ECU connector. What you are looking for is that with an Ohm-meter you need to measure about 3,000 Ohms on the sensor when cold which then should go down to about 300 Ohms when hot. This sensor tells the ECU what temp. the engine is at so that the ECU can vary the fuel mixture.

Next you need to check all the vacuum hose for air is best to spray some Carb Cleaner around all the air hose while the engine is at idle to listen for a change in engine note. Also spray around both sets of intake manifold gaskets as you may have a gasket leak.

Then...make sure that no idiot has tried to adjust the idle by turning in the throttle return stop screw....this is a screw with a locking nut on it that is factory set and sealed with paint.

You need to back out this screw while observing the butterfly inside the throttle body...the butterfly needs to slightly close all the way visible gap.

Now with the throttle relaxed...make sure the throttle cable is not too tight...move the throttle linkeage should hear at slight "click" as the throttle come to rest...clicks again as the throttle is moved slightly.

This is the Throttle Position Switch or TPS which is a micro-switch located under the throttle body. This tells the ECU that the throttle is at rest and so the ECU sets the idle speed correctly.

Likewise you need to test this switch is working at the connector at the ECU.

Best place to get more information on the M40 and the ECU pinouts is at the UK club:
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