Should I fix this fast idle or leave it alone?
 

Car: 1989 260E, 198,000 miles


I noticed my car was idling at approx 1000-1050 in gear, and 1500-2000 in neutral/park.

I immediately searched Mercedesshop for causes. The consensus was to check the following:

Throttle Linkage
Idle Control Valve
Idle Control Unit
Decel micro switch

Since it was late, and dark I focused only on the linkage.

After removing the air cleaner, I blasted the linkage components with carb cleaner. They were fairly dirty and a bit grimy. I did not disassemble anything since I was not sure how.

While cleaning, I noticed the micro switch with its little roller, and saw that the flat part of the metal linkage that is supposed to open and close it was *under* the switch arm, rather than against it. The switch also appeared to be the grimiest part of the linkage.

With the engine running, I manually moved the linkage to rev the engine. In the “rest” position, the engine slowed, and idled normally (approx. 550 rpm) when the linkage engaged the switch properly. It also seemed to idle normally when the linkage was not in contact with the switch, i.e. when it slid under it.

Also, the little “button” the switch arm is apparently supposed to push in and out did not appear to be moving. In fact, it appeared to be pushed all the way in.

When manually tripping the switch with the throttle linkage moved off of the switch arm, open, the engine did not drop to idle speed. Then again, I was not revving it that high, so it may not have had anything to drop to.

My assumption is that the switch is so filthy that its moving parts have either jammed, or are very gummed up and move much slower than they should.

After replacing the air cleaner, the car idled normally (550 rpm) in neutral and in gear. This continued after a test drive as well.


As of this morning, the car idles at 950-1000 – both in gear and in neutral/park. My assumption, again, is the switch. Perhaps some of the gumminess was loosened by the carb cleaner, but is now starting to stick again?

I am not unhappy with this condition, however. I am actually quite pleased with it. My Economy gauge is at Zero when I am idling. Shifting is a bit smoother and quicker. Engine is running cooler.

So, if the idle says in this range, should I leave it alone, or would there be potential risks involved?

I realize I need to replace the switch, and will do so after the weekend. Unfortunately, if it really is the culprit, there goes my acceptably fast idle.

But if the problem is not the switch, am I correct in assuming the linkage is too sticky (whether from dryness or grime) is the culprit and I need to properly disassemble and clean it?

If so, what is the correct way to disassemble the linkage parts? There appears to be one or more loose metal sheaths with spherical ends covering what I assume are the linkage arms and (ball?) joints. Do these sheaths simply lift off?

To your list, add the airflow potentiometer. If it wears, it will make the engine idle high.

I removed mine to inspect it, and when I reinstalled, my aligment was a tad off, resulting in high idle. Obviously your pot. has not moved, but it may be worn out.

Several forum members have discussed the function and failure of this part at length.

I’ve been on holiday for the past few days. Thanks for the info.

The airflow potentiometer – is that a mechanical part or electrical? I could not find it on Fast Lane.

I plan to do the micro switch first, since it needs replacement regardless, and since the car’s fast idle behavior is variable – from 1000 to 2200rpm in neutral, and normal after the initial spray-down with carb cleaner. (The linkage itself, after blowing off the surface dirt, is actually very, very dry. But that could cause friction stickiness as well.)

Also because the temperature of the engine appears to be a factor – a hotter engine seems to lower the idle from 2000 to 1500, suggesting the micro switch movement is freer under high heat, and that my earlier comments about the switch being stuck may be valid. I hope so – I love easy repairs.

I will look up the airflow potentiometer threads next and see what’s up with that.

I have read through all of the posts with "airflow potentiometer" in them. I am not quite getting the feeling that the potentiometer is the cause of my problem. I am, however, thinking I should check for air hose or vacuum leaks.

My main problem is the fast idle. I have also noticed, on reflection, that in the past 8-12 moinths or so the first startup of the day, or secondary startups after the car has been sitting for a long while, seemed to indicate some form of air/fuel starvation. After flooring the accelerator the engine would not respond as quickly as it should -- or would experience a significant lag, especially in very cold weather. The fuel economy guage always seems to react immediately, but the tachometer does not, which suggests an airflow problem, more than fuel flow, if my armchair, "working with insufficient technical knowledge" analysis is correct.

Also, when cruising along at highway speeds, and lifting off the accelerator, there is not the typical drop in engine sound that indicates fuel reduction or cut-off. Instead, it is clear that fuel is still being pumped, and the car does not seem to know my foot is off the accelerator.

My limited knowledge of this still points to the micro switch, but I will check the things that the rest of you recommended.

Also check the throttle position switch. It's located under the flow sensor at the end of the throttle shaft. You can pull the pigtail off at the harness connector on top of the manifold. There is a WOT and idle signal line, plus a common ground.

This switch is often overlooked when dealing with idle speed control problems. It was the cause on my car, and once I cleaned around the switch and got good continuity on the idle signal the high idle went away.

Duke

Duke,
have you removed the switch for cleaning /inspection?
I intend to do it on my 260E 1988 which has the same engine as 190 2.6.
How did yo adjust it back, any procedure?

fahrgewehr2,

how did you do to remove the small caps that are used to hide the screws that fix the potentiometre.

I would like to inspect mine as the plastic is old ( from 1988) i owuld not like to break the cap .

any procedure how to remove them safely? pictures?

The switch would be very difficult to remove due to its location and use of phillips head screws to mount it. The first thing to do is disconnect the pigtail on top of the manifold and check that the switch provides the correct WOT and idle signals. Mine was intermittent at idle.

The "fix" for me was to liberally spray the switch and surrounding area with mineral spirits, then a shot of WD-40 when it dried. I also disassembled the throttle linkage and gave it a thorough cleaning and lubrication. I used white lithium grease on the ball and socket joints and a shot of silicone spray lub on the roller joints.

Duke

Dumb Question:

How would the car behave if you removed/disconnected the Throttle Micro Switch (or "Sender Unit" as the dealer calls it) altogether? Would it idle normal, fast or what?

I purchased a new one and will probably not install it until the weekend since I am curious about the idle pattern I am observing and want to see if it holds:

First drive of the day: Normal (550) to 750 idle in drive, 750 in neutral.

Lunch time drive: 1000rpm in drive, 1500 in neutral.
Restart after lunch: 750 in drive, 750 to 1000 in neutral.

After work drive: 1000 rpm in drive, 1500 rpm in neutral.

This has been the pattern since last Friday. (5/28)

Thanks Duke,

I checked with an ohmmetre the switch and I am not sure it is OK.
The switch behaves like and resistance readings were like this:
at idle: contact between 1 and 2 on connector,
between idle and full opening: sometimes hesitation on reistance reading,
change at almost full opening of the throttle butterfly: contact between 2 and 3 on connector.

What should be read inbetween idle and full opening , should there be no contact between 1 and 2, neither 2 and 3b? or something else

Here is the pict of the position of the butterfly when the contact changes.

It's just a two pole SWITCH (open or closed on each of the two circuits - NOT a potentiometer) to tell the ECU that the throttle is either at idle so it can manage idle speed or WOT so it will go into open loop and enrich the mixture. Obviously, if the ECU never gets the idle signal from the switch, the idle stabilization system will never take over idle speed control, and you end up with a high idle speed.

The idle side should read closed (zero ohms) at idle and infinity when the throttle moves off idle. The WOT side should read zero at WOT and infinity at less than WOT. Your problem sounds like what I experienced. The WOT circuit was fine - got continutiy between the proper pins, but when I closed the throttle the idle circuit portion of the switch (repeated this test several times) did not always show continuity. Snapping the throttle closed would usually give continuity - just as kicking the throttle would usually bring the idle speed down to the proper level when the problem started occurring.

The microswitch on the linkage can be checked in a similar manner by disconnecting the wires and manually exercising it and watching the meter swing between zero and infinity.

I think a good cleaning/adjusting/lubrication of your entire throttle linkage will solve the problem. Pay particular attention to cleaning the area around the throttle valve switch and the idle stop on the opposite side of the throttle valve shaft with a spray bottle of mineral spirits or Gumout or equivalent. Also make sure there is enough slack in the lower linkage to let the throttle shaft lever fully rest against the stop.

Based on my own experience and anecdotal evidence from others I think the throttle valve switch is the MOST COMMON CAUSE of high idle problems, but most advise to look at everything else, BUT the throttle valve switch. I don't think most even know it's there.

One other thing. According to the MB documentation that I have on the KE system the airflow potentiometer signal is ONLY used by the ECU to provide acceleration enrichment when the engine is cold - before it goes into closed loop operation. It has absolutely nothing to do with the idle speed control function. This is from the "Model Year 1984" manual that describes the new 190 and its various systems. There were some changes to the KE system in later years, but I don't think the airflow pot/idle system relationship was one of them.

Duke

Duke,

Thanks for the info. I have the replacement switch and it is much beefier in construction than the one in the car. I wonder if failure of these things is common.

The existing switch seems stuck in one position, or not moving properly between open and close, and does not make clicking sounds like the new one. I hope it is indeed the culprit. Sure sounds like it.

Also, it is not a "Sender Unit" as I said earlier. The dealer made a mistake. It is a "Thrust Cutoff Switch" which I believe it has been referred to here in one or more threads.

I assume you are talking about the "microswtich" on the linkage as opposed to the throttle position switch (TPS) on the end of the throttle valve shaft.

The mircoswitch is easy to test and if it misbehaves you might even be able to clean it. In my case the problem was the TPS and both the TPS and microswitch must operate properly to activate the idle stabilization system to control idle speed.

Duke

Yep -- the microswitch.

I recall your earlier comments about the TPS, but don't know where the throttle valve shaft is (I only examined the throttle linkage for the first time last week.)

I know for sure microswitch is faulty, and will check the TPS next, once I find it. :)