R107 AAV - surging on idle

Hello, I read Volker's article about issues comming from a failed Aux air valve. I understand that because of AAV being stuck in the lower position and a big amount of air comming into engine revs go high until they are corrected (decreased) by ECU.
In my case I have noticed sawing(surging) as well but in an opposite direction. After engine's start it is idling with higher revs(I do not know how much since my rev counter does not still work) but it is not dramatic, I would say about 1500 rpm which is normal for a cold engine. By sawing the revs fall down to maybe 500rpm and than go back to 1500rpm. It repeats 6 or 7 times and then the engine keeps idling on 1500rpm and revs go normally down as the engine warms up. It happens only in winter.
My question is what can be a reason of this type of "opposite" sawing?
Thanks.
Dodi

Hi Dodi,

it is exactly the same sawing as always, In your case it only happens when the expansion element is already weak and it takes too much time to mive the piston. I had exactly the same situation: jetronic.org/de/forum/jetronic/1557-wint...-und-dichtung-450-sl - sorry for German text.

This is what happens:

1. On cold enigne piston in AAV is pushed very much down.
2. Now ou start engine and ECU should go to 1000 rpm - 1500 rpm is too much in your case. This is to compensate hi friction and fuel condensing on manifold.
3. If engine goes to 1500 rpm, ECU does not yet do over-run shutoff. Limits for that vary with engine temperature.
4. When you drive, your car quickly warms up. But AAV expansion element is too weak and cannot push up piston quickly enough. Friction in engine also decreases and rmp even goes up. I had that when I took my car to winter garage at 1st red light if I switched gear form D to N.
5. But with increasing engine temp ECU decreases limit for enabling over-run shutoff. And now sawing starts.
6. I even found out that my original AAV did not 100% close any more.

I was happy to install another overhauled AAV from my stock. Then I noticed that my AAV was still original and very clean. So I decided to also overhaul my original one built by Bosch 1 month before my car was built. That is why my original AAV is now back in car.

So back to your question: It is the standard sawing caused by weak AAV. Nothing opposite, all the same.

Thanks Volker! It's clear. I read your way of overhauling. What do you think about this one?:

www.benzworld.org/threads/the-comprehens...50sl-thread.1600137/

Or did you find meanwhile another way?

Thanks.
Dodi

Hi,

I do not consider this way a proper one. You risk to deform the orifice and interference fits. I use my own extraction tool.

Thanks again for your help.

Regards,
Dodi

Hi,
I had the same issue with my 1972 350SL.
In 2018 I found these on the internet - www.ebay.co.uk/sch/aavpart/m.html?_trksid=p3692
It still works okay.

Hi Unknown johnr2,

this is the wrong part. It does not fit properly nor does it have the same expansion values as the original expansion element. It does something yes, but your AAV will not open and close correctly. I would not use this part.