High Idle 300E -88 (Have preformed search)
 

Hello.

I have a problem with my 300E. Cat and lamnda, no EGR.

The engine is sometimes idling to high, 1050-1500 rpm in "N" or "P" and little lower in "D". The idle is often slowly increased from the lower rang to the upper range...

I have tried to disconnect the idle control valve, and then normal idle is achived.
Also if I connect the mass airflow sesor plate potentiometer the idle comes down to normal after a few seconds.
When I tried to disconnect the throttle switch the idle went up even higher and did not come down after reconnecting it...

So to my question(s) ;-)
I did a forum search and found a few probable causes for this problem, however nobody stated that disconnecting the airflow sesor plate potentiometer would give normal idle, however there was talk about worn potentiometer could cause high idling... Nevertheless, someone stated that this potentiometer would have no function at all in closed loop cycle... I'm confused ;-)

Still I'm suspecting this potentiometer to be the problem... How can i check it?!?
Also Is there a problem with driving without this potentiometer connected?

Seccondly, this problem started after I changed my fel filter (which by the way somewhat solved my cold start problems)

Thanks!
/Ola, Sweden

Start by checking for proper function of the throttle linkage microswitch and throttle valve switch.

Duke

If you are lucky, you have the exact same problem I just fixed: a failed Thrust Cutoff (Micro)Switch under the air cleaner. Snap out the old one, insert the new one and the problem is solved in about 5 minutes.

When driving at higer speeds, and you lift your foot off the accelerator, do you feel a noticeable reduction is engine speed? If not, the microswitch is probably the problem.

I tested both of them this weekend, and they seemed to work fine... That is a low (none) resistance at the switch at idle and no contact at throttle more than just above idle...

Strange...

I would really like that to be the problem ;-)
However, I have tested the microswitch and it tested ok... I have been seriously interested in investigating this, however, since I, as you mentioned, have noticed a slight loss in pedal sensitivity (to the release of pedal) at highway speeds....
the problem was there wen I tested the switch I must say to...

Could it be bad connection perhaps??

I'd look back at that airflow potentiometer. Review how it affects idle control, not fuel control. They are two separate considerations.

On visual inspection, my microswitch looked perfectly fine -- but it wasn't.

How did you test it?

What's so strange? Which one are you referring to? The TVS has a set of contacts that should be closed at idle and another set that close at WOT. That's why the pigtail has three wires - idle switch, WOT switch, and a common ground.

The microswitch should open and close as you hear it "click", but I can't remember if it is normally open or normally closed. If continuity flips between zero and infinity as it clicks, it's probably okay.

If both the microswitch and TVS don't send the proper signal to the ECU the automatic idle stabilization system will not engage to control idle speed, and the result is usually a high idle.

Duke

Seems likely it's the airflow meter pot, if disconnecting improves idle. It is part of loop control in that it tells the ECU when airflow demand is changing and in need of enrichment. If this is a false indication due to malfunction (poor contact of wiper), then enrichment will cause the ECU to lean the mixture back out - increasing airflow by opening the ICV and speeding up the idle.

Steve

I don't buy the argument that the air flow meter pot has anything to do with idle speed. The Mercedes documentation I have on the KE system says that it is ONLY used for acceleration enrichment when the engine is cold, prior to going into closed loop operation.

Duke

I should have been more specific. Replacing the airflow meter pot on ours solved exactly these symptoms. The Bosch manual on the KE system says the pot is used all the time for acceleration enrichment, and that makes sense. The Lambda sensor cannot give information about needed enrichment. Other FI systems use typically a throttle position sensor to supply similar information.

Steve

So if my air flow meter pot is faulty, would that possibly explain my cold engine acceleration delays and sluggishness?

It feels as though the engine, when cold, is starved of fuel or air. I depress the accelerator and nothing happens for a second or two. Also, cold idle can be quite low -- 100 to 300 rpm on the first start of the day -- after the weather goes from hot to cool, i.e. if it was 95 degrees outside on Tuesday, and then 65 degrees on Wednesday morning, the car will exhibit this unusually low idle. If, however, the car sits for more than a day or two, that does not occur. And idle always becomes normal after a few minutes of driving.

See below...

Yes, and that was what i stated ;-)
I looked into the wiring diagram on the CD and according to that one i checked the connection. The wire that was idle signal was in contact with the common one at idle and the other one was in contact at full throttle. Everything was ok at that switch as I see it.

Yes it improves idle at operating temperature. However, cold start is impossible with the pot disconnected so I cant just disconnect it (would otherwise be a good way to solve the problem until I have a new one in my hand to replace it)... Nevertheless, is there a good way to ensure that this is the problem?
Also, is it very hard to remove/replace this one? Must the whole fuel distrubutor be taken apart/removed? I have not found the screws that hold it in place so far :confused:
Does anybody know the approx. price for this part?

I disconnected it and measured resistance over it with throttle at idle and "off idle"... At idle: contact, "off idle" no contact...

You need to prise off the plastic covers at either end of the front face of the potentiometer. They protect four screws which you can loosen to allow adjustment of the unit without removing it. I've just been messing with mine in a bid to sort out a rough idle and swapped it with a known good one. Turned out it wasn't the unit that was faulty but the two prongs inside were not making contact with the inner face of the pot. So, although the idle problem sees to have gone away (again) I suspect my Idle Control valve is faulty as I have been unable to make it move inside, even when applying current to the contacts. Anyone have any experience of testing these things?

As for replacing the pot, it is apparently available as a separate unit as a Bosch part, but not here in Indonesia where you would need to either buy secondhand or replace the entire air flow sensor unit. I've read in other forums that you can get the replacement part for $47 from Halsey Imports in the US, who apparently are only reachable by phone. I've got the info at home so if you need the part number let me know.

Good luck

bosch part number (not OEM)
 

here you go

3437224035

Ok, thanks. I will try to remove it and see if a can get some better function in the one that I've got... :)

Thats about the same price that i got from my MB dealer here in Umeå, Sweden, today when I went to check the price for an new one... Quite expensive i would say :rolleyes:

Probably solved now...

If seemed that the problem was the contact of the connectors at the microswitch, not the switch itself that was the problem. Now the car idles fine! :-)

Thanks everybody! :-)


Edit: English and spelling errors... ;-)

The microswitch or the TVS?

I don't understand what you're saying. Previously you stated that you checked both the microswitch and TVS, and the test results you posted indicated they were functioning properly.

Do you mean that the problem was at one of the wire connectors to the microswitch?

Feedback is important on this issue because idle problems are common, and the "things to check" responses are all over the map.

Duke

With all the technical imput here, I know this answer sounds silly, but I had a problem like that once on a client's 91 300E. It always idled at about 1100 in park. I too checked everything and finally solved the problem with the advice the Germans hammered into me when I was serving my apprenticeship. They told me that sometimes one drop of oil can take the place of 3 mechanics. I solved my problem by taking ALL the linkage apart and lubing it. That means removing the levers from the shaft. The car idle was returned to normal. It's worth a shot.

Peter

Yes you understood me correctly. And pardon my english. But it was the CONTACT of the wire connections at the microswitch that I eventually found to be bad. I cleaned the wire connections and with a little force I made the contacts narrower so that they would not loose contact again. The switch itself was ok, and while testing it the first time I never noticed that it was bad contact at the connctors. I guess you understand now(?) :-)

Thanks for the clarification. I cannot in good conscience critize your English, because I don't speak a work of your language. ;)

Duke

Oops. I guessed I overlooked the closed with fix part, before I blithered on about the lubrication.:rolleyes:


Peter

No harm done! ;-)

If the fast idle returns in the next few days, then replace the switch altogether.

Mine idled normally after cleaning the switch, but the problem returned after a day or two. A new switch permanently corrected the problem.

Of cource I will keep that in mind since the location of the problem was found! ;) :D