Rough idle on C180. Petrol. December 1994

[Dec]

I had a full service last week, the car had a slight rough idle, I forgot to mention this when I left it in to main dealer for service, however I expected them to spot the fault and fix it, but the failed to detect it.

The idle has got a lot worse over the past week and I am concerned about bringing the car back to them as the will possibly start fixing it by elimination i.e. replace every thing one by one until the hit the fault. Very expensive. Any ideas?

Symptoms. (Engine normal working temperature)

• Very slight rough idle over past few months, but not enough to bother me.
• Idle got a lot worse since service. Fluctuates between 500-800rpm, should be about 750rpm
• Rough idle in Drive - 2 –3 and Reverse
• Changing to Neutral or Park eliminates the problem; the revs increase slightly to a steady 900rpm approx.
• Excellent starting, car drives perfect

.

I’m waiting for a kit to replace center bearing on the propeller shaft, could this have any relevance to the problem as the rough idle is only apparent when in gear, i.e. Drive - 2 –3 and Reverse

Dec

 

[Bobby Dazzler]

A few questions...

Were the spark plugs changed?

Given it only happens in gear (not neutral), when was the transmission oil last changed?

 

[Dec]

Hi Bobby
I asked about plugs, the said the were ok, the would have changed them if there was any problem, the said the are probely 50,000 plugs? new one on me.

Gearbox oil and filter changed at service at my request (milage 84,000 )

Dec

 

[Bobby Dazzler]

Maybe the plugs are going off, but unlikely they'd get noticeably worse following a service. Transmission oil sounds like that can be eliminated.

It seems as though you have the problem when the engine is under load, ie when D-1-2-3 is engaged, and you would expect rpm to dip a little compared to N or P.

My only thought is that the idle speed is set too low, or the fuel lines may be partially blocked. Clutching at straws here though. Someone will be along with some proper advice shortly...

 

[BlackC55]

You are probably looking at either

plug caps, leads or coils.

A set of leads and plug caps are cheap enough for you to start there. Get some from Euro as they are reasonbly priced.

 

[Dec]

Thanks Black, will have a go with the leads first, I have a timing light so will put it on each lead to see if I can isolate any particular cylinder that might be missing.
By the way good luck with your enterprise, I am in Ireland, only for that I would be round to you with this problem.

Dec

 

[LastMinute]

I expected them to spot the fault and fix it

In my experience dealers will never do anything you don't explicitly ask them to. All too frequently they don't do things you do ask for, either...

 

[Dec]

I agree. Experienced technicians with know-how on MB cars are very scarce where I am, only for that I would not go near MB dealers.

Dec

 

[Dec]

Update;

When I removed the spark plugs cover I found a small pool of brown water, the only possible explanation is that it is condensation.
I can only presume the condensation originated on the underside of the spark plugs cover and then runs down the inside surface of the cover, collecting at the lowest point, which is at spark plug #1. See picture.
(i.e. Hot engine + cold night = condensation forms on the inside of the cover).

I cleaned all 4 leads and inside the spark plug compartment and cover with WD-40 this has eliminated the rough idle completely, the revs are now steady with no fluctuation.

I don’t know if cleaning the leads or disturbing them while cleaning is responsible for the successful outcome but the idle is back to normal again.

Has any one come across this situation before regarding condensation?

Dec

 

[Dieselman]

I can't see it being condensation unless either you only do really short journeys or the engine is running very cool. Obviously water has got in there somehow though.

 

[Dec]

Hi Diesalman

I do quite a few short journeys yes but I did not consider that posibility, but it is a plausable explanation alright, in fact it must be the only explanation as there is no way water got in as the seal is very good and there has been no power washing of engine. It must be a common problem with anyone driving short journeys, never seen any posts on this matter before.
P.S. engine tempature normal a little over 80c

Thanks for your reply
Dec

 

[GazCaff]

Had the car steam cleaned or valeted recently?

 

[Dec]

No Gaz, must be short trips as Diesel man has said.
Dc

 

[recycled]

I think i will go with diesel man for the short journeys. But they must be really short cos as far as i know ,that part of the engine goes boiling hot in seconds.
When i power washed my engine bay, i had rough idling and vibration for all of 2-3 mins and then after some serious letting off of steam the idling was back to normal and the top of the engine compartment was bone dry.

 

[kth286]

Dec

Have you thought it might just be coming UP from the plug hole ?

Was it water ?

Is that plug screwed in tight ?

 

[Dec]

Recycled

I agree, that part of the engine dose go boiling hot, the walls of the plug compartment are quite thick, you can't touch it because its so hot BUT the compartment cover is only 2 or 3 mm thick, warm to touch and so cool quite rapidly. I think it is this that is the origin of the condensation, and there were signs of condensation on the under side of the cover (forgot to take a picture)

I'm only half a mile away from work and I cant change that but if the problem returns I do have a Plan B.
I will drill 2 large holes either end of the spark plug cover and fix some kind of heat resistant fabric to filter any dust, this will leave the compartment open to the atmosphere and should eliminate the condensation problem.

Most other cars have there plugs and leads open to the atmosphere, the other halve of my 4 leads are outside the plug compartment anyway and open to the atmosphere.

Kth

No I don’t think it could be coming UP from the plug hole because of where the water is lodged in the picture and anyway the plugs are tight.
It’s definitely water, stained brown probably from past spraying with WD-40


Regards
Dec

 

[recycled]

No problems . i think you are spot on there. Easy with the drilling though

 

[BlackC55]

I have come across this problem before on the 111 engine. 9/10 a small amount of condensation does no harm at all but sometimes it can make the engine missfire. Try replacing the seal on the removeable panel that covers the plugs. As far as I am aware the seal is available from MB parts

 

[Dec]

Thanks Black

The seal looks pretty good but I will try that just to eliminate it from my inquiries.

Dec