86 420 sel high idle - PeachParts Mercedes-Benz Forum

wagonboy · #1 01-15-2009, 01:00 PM

Good morning, my Dad's friend has a problem with his MB. Just happened recently, 1500 rpm idle. We opened the air filter housing and when we started the engine, the round flapper for the injection system went down. Is this normal? I though it should stay closed and the idle controller was the one to give air for the engine. Any tips are much appreciated. Thanks in advance.

Erick

hey_allen · #2 01-15-2009, 01:36 PM

Since the idle air valve picks up it's air from below the fuel metering plate, the plate still has to move to pass the air. That is so the fuel mixture is adjusted to feed the higher fuel requirements of having the idle valve open.

In your case, and as a stab in the dark based on my car's behavior, you might try unplugging the throttle position switch and jumper the connection from the fender that it plugs into.

Mercedes set the throttle switch to open the idle valve whenever you were off idle, even when the engine was warm, apparently to smooth engine behavior when the throttle is quickly closed.
If the switch fails, or gets unplugged, the logic behind it assumes that you have the throttle part way open, and cranks the idle up to ~1200-1500 rpm. Shorting the connection tells it that the throttle plate is closed, and the valve closes almost entirely.

Randy · #3 01-16-2009, 10:46 AM

When I bought my '86 420 from the original owner, it had the same problem; replaced the idle control box underneath the front passenger toeboard, and problem solved; perfect sweet idle for 5 years now.
At this age, there are lots of possibilities like air (vacuum) leaks, but this is what worked for me.

wagonboy · #4 01-16-2009, 11:50 AM

to both your replies. will keep them in mind. My dad said he wiggled wires underneath the air cleaner housing and the idle went down. I'll update this thread if the problem comes back. Thanks again.