rough idle/oil light on -linked? worried

[Darragh1]

Vehicle: SL320 1999
WDB 129064 2F 177672 7
ENGINE 112943 30 333401
GEARBOX- 722607 0 1207121
Ireland

Hi,
My car was serviced 6400miles ago (oil service due in 3600 miles). Today my oil light came on - I pulled into a petrol station and checked the level - nearly empty - topped it up by 1L of oil.
I own the car three years -it does get intermittent puffs of white smoke from the exhaust when in idle, maybe moreso on these cold mornings ,perhaps much more at the moment - I also have always had a *rough idle - my mercedes mechanic agrees the smoke is disconcerting but that the engine is good and he can't find fault with it. He acknowledges the idle too but since I don't have the cash to let him spend days trying to find the problem him fixing it isn't an option. The car is well really well maintained but I haven't been able to sort out the rough idle.
So,
I have read that the M112 is well capable of burning oil but I would like to drive assured that I won't have to buy spare litres of oil whenever the car decides - - Why the low oil level? Why the rough idle? Is there a link? PLEASE HELP ME FIX IT
I feel like , and and a bit of with a side order of but I would like to feel like :bannana:PLEASE HELP ME FIX IT

*Work done in attempt to smooth out the car- NO FAULT CODES SHOW
Oil/filter
Air filters
Spark Plugs
Ht Leads
MAF sensor
Removed egr valve and throttle body and cleaned replaced relevant gaskets
Valve cover gaskets
CPS (went faulty)
Fuel filter
Breather hoses from valve covers to intake
Engine mounts front and rear
three fuel additives and one pre service oil additive

Separately - Gearbox has been rebuilt.

Kind Regards,
DArragh

 

[markjay]

I am not familiar with the SL a such, but on a general note:

If you are losing oil, it could be due to (a) oil leak - check the engine is dry all around, and that there are no wet oily patches under the car when idling or in the morning after it was left overnight, (b) engine consuming oil, but this will usually come out of the exhaust as black/blueish smoke, and you don't mention having this (c) blown head gasket, but this is less likely because you would usually get the other way around, i.e. water in the oil and coolant lose.

About leaks, keep in mind that they do not always result in oil dripping under the car - oil could leak over the exhaust and get burnt (though you will see smoke coming out of the engine bay or from under the car and will feel the smell of burnt oil), and it can go to concealed places e.g. rear crankshaft seal leaking into the clutch.

With regards the idle, not sure about the SL but there is usually an ICV - Idle Control Valve - or something similar - that could do with a good cleaning. Cleaning the throttle body can also help.

As for the white smoke - is it really oil? If you get it when the car is cold, it could be condensation (water) in which case this is normal. It could also be coolant pointing towards head gasket again. If it is indeed oil, then it would usually be leaking valve stems - although if it only happens every now and then, I doubt if this by itself can explain the amount of oil you are loosing.

When you say the oil light came on, do you mean the oil pressure light, or oil level warning? And is your SL equipped with an oil pressure gauge like Mercs of old? If so, what did it show? And now that you have added oil, is the oil light still on?

What I don't fully understand, you say the oil was almost empty and you added 1L. If all you were missing is 1L of oil to get the dipstick back to the Max, then this should not have been a problem and should not have caused any warning light by itself - either more oil than just 1L was missing, or you didn't top it up properly?

Hope this helps.

 

[grober]

When did you last check the oil level BEFORE the oil light came on? You said it was serviced 6,400 miles ago-- was that the last time it was checked? There have been complaints in the past about people being "short changed" on their oil service. If the new oil fill level back then was just above the minimum it would not take much to reach the oil level warning light stage?

 

[Darragh1]

Markjay,
Firstly, thank you kindly for your comprehensive reply.

It was the orange low oil level warning light. The oil isn't up to the Max of the dipstick since I added the 1L but half way up.
The light came on, then went off before filled the 1L. It has not come on since then either.
I have no in-dash oil pressure gauge.

No oil leakng under the car. Head gasket is good according to my mechanic's inspection.
My mechanic has also mentioned that the car is running lean if anything.
Definitely white foggy smoke from the rear exhaust pipe and the idle seems to worsen as the smoke is pumping.
My car has electronic throttle actuator and therefore no ICV. I removed and cleaned the throttle body which is now like new and had minimal blackening and no deposits.

When in PRDN the rough idle is constant from about 5 seconds of ignition - evident when stationery but seems smoother when over 1500rpm.
Rev counter is normal at idle with sometimes some slight movement of the needle 1mm up or down.

When plugged into the merc diagnostic machine the cylinders are all firing properly- mechanic showed me.

Thanks again,
Darragh.




Grober,
Thank you very much for the reply. I was with the mechanic when he filled the oil. I actually helped him count the litres.
Regards,
Darragh.

 

[BlackC55]

Had you checked the oil in the past 4000 miles?

 

[Darragh1]

Hi BlackC55,
No, I hadn't checked the oil in 4000miles because I didn't feel it necessary.I would hate to feel I have to be checking my oil level between services to see if it has all been burned off. This car is really bothering me much as I love it.
Thanks,
Darragh.

 

[steve333]

I always check my oil level at each refuel & before/after long journeys,i even checked it when i went to collect it from a dealer last month only to find it was on minimum,for the time it takes to check & bearing in mind the damage/expense engine repairs can be it's not worth overlooking imho

 

[grober]

Its quite possible your rough idle and oil loss are entirely separate issues The little oil recirculating covers on the valve covers sometimes leak but you would normally get an oily smell or see traces of oil. Here's a HOW TO reseal them if its this. View topic - Resealing the Recirculating Cover m112 | THE Mercedes-Benz & AMG Forum for and by the Enthusiasts | MyMBonline.com

 

[BlackC55]

You MUST check you oil between services. All cars will use a little oil. My car uses 1/2 a litre every 1200 miles.

 

[Darragh1]

Grober,
I replaced the valve cover gaskets- there is a tiny tiny tiny bit of seepage fron the top parts of the valve covers - I will reseal them but the oil loss is virtually nothing...would it cause rough idle?? Thanks for the link.

BlackC55,
I will check the oil regularly but I didn't expect I would have to be topping up inbetween services. So some loss of oil is normal??

Thanks so much for the replies - by all means keep them coming-

Regards
DArragh

 

[BlackC55]

Yes of course. Perfectly normal. It does not lose it, it burns it (unless you have a leak)

Do you know how to check your oil properly?

 

[Darragh1]

BlackC55,
When you say 'properly' - dipstick - wipe clean - dip, then check level?
How much between max and min on the dipstick?

If you refer to my list of things done to rid the car of the rough idle at the start of the thread - what would the next likely step be to check-??

Thank you
DArragh

 

[steve333]

BlackC55,
When you say 'properly' - dipstick - wipe clean - dip, then check level?
How much between max and min on the dipstick?

If you refer to my list of things done to rid the car of the rough idle at the start of the thread - what would the next likely step be to check-??

Thank you
DArragh

Not from cold,at least 30 minutes after engine is switched off,wipe dipstick clean,re-dip & read level?

 

[corned]

I think the next step would be to get a compression test done.

A problem with low (or no) compression in a cylinder could conceivably cause both of these symptoms.

 

[markjay]

Didn't check what MB say, but most manufacturers specify oil consumption as up to 1L for 1000 miles. It could be less, obviously - some car don't seem to consume any noticeable amount of oil for some reason - but the point is the even when they do, then, like Black55 said, it is quite normal.

Old-school line of reasoning says that the oil that gets past the oil scrapping and compression rings and gets burnt, actually helps lubrication and reduces cylinder bore wear... so according to that a small oil consumption (through burning, not through leaking!) is actually good for the engine. But this is what the wise old mechanics used to say some 30 years ago - not sure if it is still applicable for modern engines... and as everyone else said, oil level should definitely be checked from time to time between services.

 

[markjay]

And as other said, oil needs to be checked when the car has been stood for a while with the engine off, giving the oil a chance to drain back into the sump. And - very important - on level ground! Not always easily achieved when parked on the roadside or on sloping drive.

 

[Darragh1]

Thanks all for the replies - i will get a compression test like i have been advised - if it's burnage of about 1L per 1000 miles - but the interval per service is 10000 miles and there are but 8 or 9 Litres of oil max- ???

 

[BlackC55]

Not too sure what you mean??

 

[Darragh1]

sorry-
when oil is changed 8L? of oil fill it
if it's acceptable to burn 1L per 1000 miles and the service interval is 10,000 miles - then it means that the car could explode potentailly if a driver didn't check the oil - I don't get it

 

[BlackC55]

Yes 8L

This is what the oil level warning light is for. Have you read you manual? Have you ever owned a car before?