idle speed high? - PeachParts Mercedes-Benz Forum

krjt · #1 11-22-2003, 11:17 PM

I have a 89 300E newly acquired. When it is in drive, the car creeps forward as soon as the brakes are released. When releasing the gas pedal on the road, the idle speed appears to be pulling the car slightly. Looking at the tach, at idle, the car is close to 900rpm in gear. If in N, the idle jumps to 1000-1050rpm.

Is there a simple adjustment to bring the idle down slightly? I am not familiar with the injection system, but it feels like a old carburated car .

Thanks.

Mike Murrell · #2 11-22-2003, 11:28 PM

http://www.peachparts.com/shopforum/search.php?action=showresults&searchid=633258&sortby=lastpost&sortorder=descending

stevebfl · #3 11-23-2003, 10:54 AM

Lots of reading...

I might suggest that you disconnect the wiring to the idle valve and see what happens. The idle is controlled (is not adjustable) electronically by the valve. In its unpowered, "limp home" setting it should idle at around 900 rpms with no load. It should drop as load is applied (trans, AC, PS, etc). If it were functioning it would start a little more closed at say 750 rpms and as load was added the valve would open to maintain the speed.

With the valve disconnected the only way to get higher rpms is if the valve is stuck open or if there is false air getting into the motor somewhere else (like a throttle sticking or a vacuum leak).

If, with the valve disconnected, the engine still idles at 1500 then remove the valve and stick a cork in one end with a 1/8 inch hole drilled through it and reinstall. If you still have the high idle the problem is not in the valve or its control!

csnow · #4 11-24-2003, 10:19 AM

Make sure the linkage is returning to 'full idle state'.

Seems to be very common for the linkage to get 'hung up' off of idle for want of a clean & lube.

Make sure all of the little plastic wheels are turning, and the microswitches are actuated.

cc260E · #5 11-24-2003, 03:44 PM

Quote:

Originally posted by stevebfl
Lots of reading...

If, with the valve disconnected, the engine still idles at 1500 then remove the valve and stick a cork in one end with a 1/8 inch hole drilled through it and reinstall. If you still have the high idle the problem is not in the valve or its control!

thanks stevebfl
this is a clever and simple way of checking the idle valve.

I have an erratic idle on my car, does it work also on a 260E 1988 with manual gearbox?

I will try this method although there is a lot of writing and causes for an unstable idle.

Is it correct to understand that the "cork" placed in one of the hoses is intended to obstruct in order to simulate the correct airflow at idle when the valve is stuck open?

stevebfl · #6 11-24-2003, 04:13 PM

Yes, as weird as it may seem the air going into the car through the valve is only a little more than that which can flow through a 1/8 in hole.

When idling normally the engine gets some of its air through the throttle and some through valve. This means of testing just confirms how much is going through the valve.

dakota · #7 11-24-2003, 09:17 PM

Steve, is this idle valve you speak of the same as the throttle position sensor (on the M103)? (or I think S29/2 'full load/idle speed recognition throttle valve switch' in the manual?) I get confused with all the different terms.
Thanks.

stevebfl · #8 11-25-2003, 08:02 AM

As you think of terms you should associate them into two groups. These are things that give info to the computer and things that accept commands from the computer, otherwise known as inputs and outputs.

The idle control valve is an output. The variety of closed throttle, open throttle, airflow potentiometer, and decel switch are all inputs.

dakota · #9 11-25-2003, 08:12 AM

Thank you, Steve.