'89 300CE Intermittant eratic idling

[GarryO]

Hi all,
My car drives ok, starts ok, engine is silky smooth, 130,000 recorded miles, sometimes idle speed can hit 1500-2000. Sometimes the idle is perfect. Sometimes when idle is high I can blip the throttle and the revs will come down to normal, but not every time. When stopped in gear with engine running idle is around 900, but if moved to nuetral or park it sometimes goes high.
I thought it may be a dodgy throttle cable, but it seems to move ok. The carpets arent fouling the foot peddle.
A mate of mine told me that the injectors fitted to some fuel systems are sometimes mechanical, not sure if the mercs are though, and can stick sometimes. Can this cause the problem?.
The car has just passed an MOT and the emissions test was fine, and I can only assume it didnt over rev when the mechanic tested it.
The fault has been evident for about 6 months or so, but as the car drives ok, and I rarely have to sit in heavy traffic Ive ignored it, but I feel I should get it sorted.
Any help or advice will be greatly appreciated.

 

[GarryO]

'89 300CE erratic idling

Got hold of a workshop manual today and see that there is an idle control valve! Is this an obvious place to start looking? Can this valve be cleaned or overalled? Is it a manual valve or is it an electronic thingy?

Car was great yesterday and this morning, but on the way home from work was over revving again. There is no spluttering, the engine revs smoothly.

Has anybody else experienced this problem?

Help!!

 

[television]

Yes you have a rotary idle valve to test put 12v over the contacts and it will buzz.


As your car is intermittent it could be the butterflies gummed up under the air flow plate. Many clean it all up with carb cleaner

 

[GarryO]

Erratic idle

Spent most of last night browsing the archives and can see that this is quite a common problem, but there does not seem to be a definitve sollution to it.

So been and got a can of carb cleaner today and will clean the throttle body and butterfly valve.

Might as well do the low cost options before I start replacing parts!

Cars been great today, and as such Im convinced it cant be nothing any more serious than a bit of gunge sticking to something.

Also been thinking of using a different fuel, maybe one that has some kind of cleaning agent in it.

Anybody know of one?

Thanks

 

[GarryO]

Erratic idle

Took the air box off to spray the throttle body and butterfly valve and noticed that I had a lot of oil contamination in the air box. Seems the engine oil level was over filled, I must have miscalculated at the last top up, and was emptying itself in the airbox through the breather pipe. The oil was then sucked into the throttle body gungeing up the insides.
I sprayed the carb and injector spray into the throttle body and gave it a good scrub with a soft paint brush.
I put it back together, and "hey presto" it runs perfect.

Checked the idle valve while I was under the bonnet and found its ok.

I'm going to run it for a few more days and I'll take the airbox off and check it again.

Hope its sorted.

 

[Telegraph-Hill]

Hope its sorted.

Hi Garry,
usually you may have a defective potentiometer on the LMM (Air Intake Sensor). As you sprayed inside you may cleaned it a bit if it was not the oil itself. Good luck,
Oli

 

[GarryO]

erratic idling

Cars been great for past week now, so looks like problem is sorted.
I hope my posting will help other owners as this looks like a common problem.
This is what I did.
1, Make sure that throttle cable is correctly adjusted.
2, Clean throttle body with carb cleaner and soft paint brush.
3, Clean idle micro switch on throttle linkage. Mine was caked with hard oil/grease.
4, Check rotary idle valve. Take it off and clean with carb cleaner or WD40.
5, Make sure that all the breather pipes and tubes are good and not cracked or porous.
6, Put an additive in fuel to help clean the injectors.

Im not sure if any one of the above was responsible for the problem but it didnt take long to sort once I got going.

There are lots of tips in this forum , but you need to look for them. All it cost me to sort my problem was an evening researching the forum, a can of carb cleaner and fuel additive, a second hand haynes manual from ebay and 2-3 hours in the garage.

Prior to joining the forum I telephoned 4 different independent mercedes "specialists" who told me my car was too old for them to be able to use their diagnostic equipment, and as such they couldnt help me. Couldnt be bothered more like!