oscillating idle

[adssmuri]

I have a 1990 300SE and was changing the air filter and cleaning the carb by spraying carb cleaner as per the instructions on the bottle. I later went to work and noticed that when I put the car either in parking or neutral the engine idle fluctuates from 1000 to 1500 rpm. is it a vaccum problem. any thoughts??

[wbain5280]

The car is fuel injected and does not have a carb. There may be excess cleaner in the intake manifold or CIS-E internals.

[G-Benz]

We in the forum call this symptom an "idle surge". A number of things can cause this, from a bad MAF to an ailing temperature sensor. Most commonly, a vacuum leak.

Search on "idle surge" for further info...

[adssmuri]

Thanks for the help guys. Ill check it out and keep you posted.


Jose Muriente

[just-n-time]

So what was the idle like before the carb spray was use? Why did you use it? If you had a problem and this increased the situation,or made it worse,we can go in one direction,and if this just came on as an after effect,we can look in another area.JNT

[adssmuri]

the idle was a little bit above 1000rpm but worked fine. I removed the air filter cover to change the air filter and noticed the area was dirty and had some gunk. I thought it would be a good idea to clean it with carb cleaner. After shooting a couple of shots of it throught that throat while revving the car, the idle increased to a steady 1500rpm and then it started to surge from 1000 to 1500 on continuous wave.

[just-n-time]

Originally Posted by adssmuri

the idle was a little bit above 1000rpm but worked fine. I removed the air filter cover to change the air filter and noticed the area was dirty and had some gunk. I thought it would be a good idea to clean it with carb cleaner. After shooting a couple of shots of it throught that throat while revving the car, the idle increased to a steady 1500rpm and then it started to surge from 1000 to 1500 on continuous wave.

This is to high for idle,first do a smoke test,find the leek,you do have a vacuum leek,the idle valve you might get away with cleaning.though I did not have any such luck.I did how ever get one (used) from another member.I would suggest,replace all the rubber parts of the intake system,unless you know,that the parts are new.After some age the rubber gets hard and will lose sealing ability,and it is cheep insurance,look up some of my post you will find some with pics on the intake rebuild. JNT

[adssmuri]

Thanks for all the information. Luckily I have a 4 day weekend from work and will work on the car then. Thanks again
JRM

[deportes]

whats a smoke test?

[just-n-time]

Originally Posted by deportes

whats a smoke test?

this test is done by pressurizing the intake system with smoke,it will leek out through the places that do not seal.

[adssmuri]

sorry for not posting earlier, but incredibly the car fixed itself. I checked for a vaccum leak by inspecting all the hoses and intake parts. I wanted to do a smoke test but the next day the car started fine and the idle remained at 700rpm steady. no more surging.

maybe there was dirt lodged somewhere in the system and loosened itself. I dont have an answer but it runs great.

[cc260E]

hi adssmuri

As you may read from numerous posts, most of us are seeking for a full understanding on this idle syndrom,

May be some dirt glued itself to the throttle flap edge(modifying the air flow/ idle air circuit) and then after a while went away allowing the flap to go to normal position at idle.
I am happy for you.

[gmercoleza]

If this problem comes back, do a search on cleaning the TPS (throttle position sensor). It worked like magic for me. Basically you clean it by hosing it down with brake cleaner, then you spray WD40 on it.