Is this a D Jetronic idle air valve problem? - Mercedes-Benz Forum

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post #1 of 4 (permalink) Old 10-11-2011, 01:05 PM Thread Starter
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Is this a D Jetronic idle air valve problem?

Trying to fix a no cold idle on a 71 W114 280CE with M110 and D Jetronic. Looking at all the great D Jetronic information on the site, the fault would appear to be with the thermal expansion activated idle aux air valve.

1. No problems starting
2. No idle when cold, but ok with throttle just a little open
3. Tick over fine one normal temp reached.

The valve when cold seems to be stuck open and supplying air around the flap valve - which is what I hough to be correct. I am expecting the valve to close when normal temp is reached so only air path is through the throttle flap valve

Checked by removing air pipe from the valve to the inlet manifold and blocking this up so there is no aux air supply, and engine idles perfectly when cold, and also compensates when auto is moved in to drive.

So should the valve be open when cold, and close when hot, and I am chasing the wrong fault?

The valve is deep down on the side of the engine under the manifold, and not easy to get to from either top or bottom, so hauling it out is going to be difficult, and I am loosing sleep trying to figure this out!
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post #2 of 4 (permalink) Old 10-15-2011, 11:40 AM
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The AAV on a D-Jet system is supposed to be open at cold, closed at hot(engine temp), just like you guessed. But don't get fixed on the AAV, there are plenty of other pieces that could cause your trouble. I chased dirty breaker points for better than a month one time..

Remember: It's not just a car. It's an Adventure.
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post #3 of 4 (permalink) Old 10-17-2011, 04:33 AM
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Use a DVOM and see if your cold start injector is working at start-up. If the valve is open, it only supplies air and without the injector supplying fuel, you will not have fast idle when cold.
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post #4 of 4 (permalink) Old 10-17-2011, 07:25 AM Thread Starter
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Thanks for your help and confirming things are currently working backwards!

Certainly didn't want to have to sort out the AAV if I didn't have to - looks to be designed to be an engine out job! Given ignition a quick visual, but I don't think it has been touched for a long time, so would be good practice to go through this anyway.

With the DVOM, what should I be looking for? I take it that The DVOM will tell if the injector getting a switch signal, but should I also remove the injector and check for fuel spray as well when cold in case this has become blocked?

Feedback much appreciated.
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