Mercedes: high idle, vacuum leak, plastic vacuum

Question
QUESTION: I have a 1974 450SL I have a problem with high Idle when I let the motor warm up fully and then I adjust the idle and lower it to about 900-1000 RPM (bringing it down from the 1500 RPM) but when I do that it stalls when driving around town and when I snap the throttle in park it stalls. I will take any advice Thanks

ANSWER: Joshua,
High idle can be associated with a vacuum leak. The computer senses the intake air volume controlled by the mass air valve and provides the proper amount of fuel ratio to achieve the appropriate rpm.
If the vehicle is drawing excess air the computer gets a wrong reading and high rpm can result.
No amount of adjusting will solve the problem because the vacuum is changing with the rpm.
The rubber couplings on the plastic vacuum lines get brittle and have crack that are very hard to see. Some time you can spray cleaner fluid or either on the joints and the leak will suck in the fumes and cause a change in engine sound and rpm.
Good Luck
Give us an  email to say how it turns out.
Rob



---------- FOLLOW-UP ----------

QUESTION: I have changed all the vacuum line which was very expensive and put a small zip tie on the end of each one to make sure it is sealing properly and still have had not success I was wondering if there is some type of computer system that controls the idle like on the 1975 450SL it has an Electronic Idle speed computer to keep the idle at exactly 850-900 RPMS

Answer
Joshua,
Here is some info on your problem taken from another website.
Rob
Don't go crazy looking for the idle control unit, you don't have one Your air regulator is mechanical, instead of electronic. It works similar to a thermostat, and IMO, it's a flawed design. It'll idle the car too fast for too long. But it should work.

I have a 450, too. I hosed mine down with brake clean, and that got it working again. Lots of black crud came out.

If the car is new to you, you likely have other issues. It's crucial that there are no loose hoses. The purpose of the valve is to create a massive air leak past the throttle plate, which increases idle speed. If there are other leaks in the system, the valve can be closed and it'll still idle too fast.

To check for leaks, I like to use propane. Propane is much better than ether or carb cleaner, and it won't make a mess. Remove the air cleaner. Turn the gas on (on your torch) but don't ignite the flame. Move the propane around the top of the engine near all the hoses. If there is any response to the engine (other than near the intake itself) then you likely have a leak there. You can try squeezing the hose near where the leak is detected to see if any change in RPM results. Just don't ever remove a hose with the engine running.

There's a large screw to the right of the valve that adjusts idle RPM. You could try that, too.

When I got mine out this year it was idling fast again. I was pretty annoyed. Here's something else to check btw- This year it was idling too fast because the return spring had gotten weak and it would not return all the way!!! I doubt that's too common, though.
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-Aaron