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Discussion Starter #1 (Edited)
Hello,

I'm experiencing the fish bite / stuttering / flatspot when accelerating part throttle in my S38B38.
When performing diagnostics on all parts that could be causing this problem I checked my Idle Control valve electrically (as described in the workshop manual). The resistance between the pins at the connecter between the throttle bodies has to be measured. It states the resistance between the outer pins has to be 40 Ohms and between pin 2 and 3 or 1 and 2 has to be 20 Ohms.
The best I could find was 25 ohms between pin 1 and 3 and 14 Ohms between 1 and 2 or 2 and 3 (warm or cold engine made no difference).
At this point I am uncertain if the idle control valve is faulty, or if an aged ICV will always show these resistances and the valve still works adequatly. When I took a short test drive with the ICV disconnected the car responded very well and the fish bite / stuttering / flatspot feeling was minimal. After a few km's the car went into a steady 30sec normal, 2 secs flat which i can only assume is the fault mode of the DME. However the improvement could be due to the fact the engine management goes to fault mode and presumes a better (I expect richer) setting. Like I said the stuttering was still there, only minimal.
Does anyone know any other tests I can perform on the ICV? Or is this a common problem and my problems could be fixed with a new ICV.

Geert
 

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Geert

Your problem shouldn't be anything to do with the ICV, which is purely a device which provides a limited amount of air into the intake manifold at idle while the throttles are closed.

I wonder if it's more likely to be caused by a vacuum leak?

If you spray carb cleaner around the intake ducting while the car idles, you'll hear if there's a leak when the cleaner is sucked into the intake.

Edit:

I've just re-read your post. Do you mean the car responds fine, then poorly, then fine, then poorly, even at a constant throttle?

Is it similar to the difference when the AC compressor cuts in?

Good luck.
Andy
 

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Discussion Starter #3
AndyE said:
Geert

Your problem shouldn't be anything to do with the ICV, which is purely a device which provides a limited amount of air into the intake manifold at idle while the throttles are closed.

I wonder if it's more likely to be caused by a vacuum leak?

If you spray carb cleaner around the intake ducting while the car idles, you'll hear if there's a leak when the cleaner is sucked into the intake.

Edit:

I've just re-read your post. Do you mean the car responds fine, then poorly, then fine, then poorly, even at a constant throttle?

Is it similar to the difference when the AC compressor cuts in?


Good luck.
Andy
Andy,
first of all thanks for the reponse.

I'm aware of the function the ICV has but it could cause problems functioning during non idle rpm. If it bypasses air passed the throttle valves it could lean up the mixture.

The behaviour of the car ( the constant on off on off ) is during diving with the ICS disconnected.
Geert
 

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Sounds odd... usually when the ICV goes, the car won't idle or will die out while idling in traffic.

I'd check all the vacuum hoses and test the TPS for good luck.
 

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The ICV opening will not cause lean running, as it is still air that has been measured by the maf.

At part throttle is `might` cause a surge and sag as effectively it is equivalent to the throttle plates opening and closing slightly, but only by a very small amount.

Usually the ICV is turned off completely once the tps idle switch is opened, i.e when the throttle is opened.

I`d check the tps first.
 

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Hi, years ago I had problems with my M5 where it would 'pulse' at low constant speeds although the speed according to the dash remained constant, it was very annoying! Other than this the car ran and idled fine but I metered ICV readings similar to yours, once a replacement was fitted the problem went. With the old ICV a shake of it left the valve rocking, so i can only offer the estimation that the weakened valve allowed extra air to pulse around the throttle bodies which was only noticable at low speeds regardless of whether the valve was powered or not. Hope this helps but the only definitive answer is to borrow or buy another and try,

MIKE.
 
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