280SE 4.5 1973 Idle drop/surge - Mercedes-Benz Forum

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post #1 of 17 (permalink) Old 10-10-2012, 06:35 PM Thread Starter
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280SE 4.5 1973 Idle drop/surge

I sort of posted this previously in the context of an alternator problem, but the alternator issue has been resolved and this one abides.

On starting, the engine rises to the proper idle, then idles a bit too high, then the idle drops and bottoms out, only to pick up again immediately. This continues until the engine is warm or is put into gear.

It only appears to occur when the engine is cool but not cold. Cold starts are fine, hot starts are fine but after the car been sitting for two or three hours after use, this surge (or reverse surge, I guess) returns.

Listen here (the surging begins at about 00:18):

Last edited by ross220sb; 10-10-2012 at 06:39 PM.
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post #2 of 17 (permalink) Old 10-11-2012, 07:02 AM
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Typically, those problems are caused by a vacuum leak.

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post #3 of 17 (permalink) Old 10-11-2012, 11:55 AM
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old topic

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Originally Posted by ross220sb View Post
I sort of posted this previously

On starting, the engine rises to the proper idle, then idles a bit too high, then the idle drops and bottoms out, only to pick up again immediately. This continues until the engine is warm or is put into gear.
I have had this on a Euro 300SEL 3.5, but never resolved it myself, as that car was sold as is.
That car had been sitting a long time.
But I remember seeing a lengthy discussion on the subject somewhere.
Use the search function.
I'm sure You'll find something there.
Alex
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post #4 of 17 (permalink) Old 10-11-2012, 02:08 PM
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old topic

Quote:
Originally Posted by ross220sb View Post
I sort of posted this previously

On starting, the engine rises to the proper idle, then idles a bit too high, then the idle drops and bottoms out, only to pick up again immediately. This continues until the engine is warm or is put into gear.
I have had this on a Euro 300SEL 3.5, but never resolved it myself, as that car was sold as is.
That car had been sitting a long time.
But I remember seeing a lengthy discussion on the subject somewhere.
Use the search function.
I'm sure You'll find something there.
Alex
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post #5 of 17 (permalink) Old 10-11-2012, 07:46 PM
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Wow, that's a pretty significant surge!
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post #6 of 17 (permalink) Old 10-13-2012, 07:45 AM
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That is often the result of a sticky cold air valve. See what it is (or isn't) doing.
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post #7 of 17 (permalink) Old 10-13-2012, 10:14 PM
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The cars of this era seem to run on vacuum as much as fuel. If there are any vacuum leaks, you will have all sorts of weird problems. That being said, checking the function of the AAV, Slide Valve, cold idle valve (actually just different names for the same valve) would be a good place to start. It can even be removed and cleaned with carbueretor cleaner.

Remember: It's not just a car. It's an Adventure.
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post #8 of 17 (permalink) Old 10-17-2012, 04:36 AM
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Ok, itīs an easy one, itīs the cold air valve. This is how it works. The fuel system detects an increase of air going trough the system and sends more fuel but then detects that the throttle valve is not being actuated so it cuts the fuel being sent. this happens at aprox 2.000rpm

When the engine is surging down the system detects that the engine is going to stop so at about 750rpm sends a signal to the system to inject fuel.

The surge is between 750 and 2.000rpm. Thatīs beacuse the cold air start valve remains open. This valve actuates mechanically, itīs like a thermostat, remove it and cleanned internally, how to do it?

Remove the valve, on top of the engine, in front, it has 2 air hoses.

Boil water and introduce the valve, when hot (around 2 minutes in boiling water) the valve will close (in this case open). Pour inside a good degreaser, or even WD40, take the valve out and deep it in iced cold water. Repeat this 8-10 times.

How to know if it is fixed? After finishing the procedure and with the valve hot try to blow up air inside one of the wholes, no air should come on the other end.
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post #9 of 17 (permalink) Old 10-18-2012, 07:57 PM
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That is what I was talking about, but a much better description. I have not had great luck getting these valves to return to their proper behavior, but his ideas all seem reasonable. Might try them myself in the future. If you try them, please post your results. Happy motoring!
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post #10 of 17 (permalink) Old 10-19-2012, 12:16 AM
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The way the valve works is quite simple, the bronze capsule is filled with a kind of wax that expands when heated, this forces a pin to rise and close the valve.

At time passes by the pin gets all kind of dirt, normally is oil fume mixed with dust that burnts out and gets stuck on the pin walls so it doesnīt move anymore.

An used valve will cost you around 100-150$.

How I do it.

1- First, I deep the valve in a special carburetor cleanner for about 24hrs, that getīs rid of all dirt stucked inside.

2.- Then I use the procedure I explainned before, hot-cold water.

I did this last time a month ago on a 350SL, same problem as yours, now works perfectly.
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