Oil in Expansion Tank S212 E250 CDI

[jjc2020]

Hi all,

I need help please. My Auto 2010 S212 E250CDI 106k miles has a radiator expansion tank full of oil. Two weeks ago it definitely didn't as I checked levels before a trip. I found it because on starting it showed a coolant level warning for the first time, and in looking at it, it was pure oil. So if the low coolant level warning was as a result of the oil then it had only happened the last trip.

My first thought was oil cooler, so got it towed to the garage, who thought likewise. They started disassembling the parts and found that actually theres no water in the engine oil, but more puzzlingly no oil in the water tubes around the oil cooler. Their conclusion is that it isn't the oil cooler.

I went to have a look at the car, and they pointed out that the oil cooler looks pretty new (I've only had the car a year). So their view right now is that the oil cooler doesn't look like the culprit as there's no oil in the water pipes around it, and theres no water in the oil.

The line leading to the top of the expansion tank (Bleed line I think according to the drawing attached labelled '250') was full of oil when they popped it off, but the other end where it connects to the top of the rad had some oil residue but not a huge amount. However as the oil level was above this it could have drained back down there, rather than coming in through there. So we don't yet know where the oil has come from.

Looking at the oil in the expansion tank, they say it could be gearbox oil, but I don't think I agree. The oil is jet black, no emulsion, and free running. Even allowing for ATF oil darkening with age I'd be surprised to see it jet black and looking healthy.

Their thought is that it could be the gearbox oil cooler in the rad that has split, but because of the jet black colour I am unconvinced and would appreciate your thoughts.

So we have paused the work while we try to work out what's up. If it isn't the oil cooler, what else could it be? Options:

Head gasket: Kind of think no, because there is a LOT of oil in expansion tank. It had filled the tank to the screwcap. And there was zero emulsion / milkiness so I sort of think that it would be if it was that. Also no smoke of any colour, ran fine driving it onto recovery truck.

Gearbox oil Cooler in Rad: I don't think so, because it was clean and black (it was changed a couple of months back) but is there gearbox oil that appears like that? Gearbox Oil was replaced at a autobox service 6 months ago. Could it still be ATF?

Cracked block: Is it possible for a block to crack to allow oil to flow into water jacket, but not water into oil?

Are there any other potential causes? At the moment the garage have stopped work because they don't see that it can be oil cooler if the pipes around it aren't so the car isn't fully stripped back to get to it, but it's still a couple of hours to get it back together to allow it to run.

Any ideas on how we can confirm gearbox oil for contamination? As a sealed for life unit, there's apparently no sump drain or accessible filler so we can't access the actual gearbox fluid to see it's state. Is there any way to down the blanked off filler pipe? If the oil in expansion tank is black can it really be ATF?

Is there any way you can think of that the investigation can be narrowed down to identify the cause of this? I'd be really grateful if you had any tips. The garage doesn't know and started off suggesting the solution was a top end rebuild, followed by suggesting a new gearbox. So I think they are in the dark on this too.

Any help you can give VERY gratefully received.

 

[markjay]

Welcome.

A compression test can help identify a blown head gasket, because they often fail at the narrowest point, and this will manifest itself as low compression in two adjacent cylinders.

A pressure gauge placed on the cooling system will show if the cooling system maintains the pressure once the engine has been switched off. If it does not, then there's a leak, possibly into the cylinder head.

Squeeze the top radiator hose, then feel the pressure inside. Remove the radiator cap to release the pressure (with the engine switched off), then put the cap back, start the engine and then rev it up. If you can feel the pressure building-up quickly, then the head gasket is blown, because the pressure should build up slowly while the engine is warming-up.

If you take the car to an MOT testing station and ask them to lower an exhaust gas analyser into the expansion bottle (but above the coolant level), then the device will detect any exhaust gasses in the cooling system.

There are also other ways to check this:

https://www.amazon.co.uk/Block-Tester-Bt-500-Combustion-Leak/dp/B06VVBSFTF/

75500 Combustion Leak Detector

Quickly Pinpoints Combustion Leaks.

www.lislecorp.com

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[jjc2020]

Thanks Markjay and great to be here - I just wish under better circumstances

The garage did say they had done a compression test and identified that the cylinder head gasket was fine (or at least the combustion chamber elements were).

The coolant system I think definitely has a leak as it is full of oil meaning there is a breach in the system somewhere and so I am not sure the system would be expected to hold pressure now. Its also pretty much disassembled making further testing hard.

One point I am really struggling to believe, is that the oil could be gearbox oil. It is jet black and looks clean and healthy as the engine oil was recently changed. From what I understand the gearbox oil is green or green / blue but not black. A simple dipstick would tell me what the colour of the oil in the gearbox is, but as the unit is 'sealed for life' they tell us there is no dipstick. However there actually is a dipstick tube for the ATF just to the left of the engine oil but it is blanked off. As I discovered from my AI of choice (Pi) there actually is a ATF dipstick available part number 722589012100 but on calling Mercedes dealers they refused to sell it as its a 'workshop tool', however it is available online.

Anyone any experience of using the ATF dipstick on OM651 / 722.6? I've ordered the Febi Bilstein part match and will let you know if it works. If it does it will allow me to confirm colour of gearbox oil, and whether it's contaminated.

What do you think? Could jet black fresh oil really be gearbox? I think no.

To summarise, at this point:

- I don't think it's gearbox oil because of oil colour (so not gearbox oil cooler),
- The head gasket around the combustion chambers is fine according to the compression test but it could be a leak in the gasket between oil and water ways.
- It could be a catastrophic break in the block / head

However it could also just be the oil cooler. The garage said they didn't think it was because the waterways around it weren't contaminated, but as there is a lot of oil in the coolant it would be circulating all around the engine so it would be very odd for any waterways to be spared and clean. Because the car had new oil and filter just 2 weeks ago and definitely no oil in coolant then, it's done almost no miles since maybe it just hasn't gummed up all the waterways yet?

The choice I have to make now is whether to get the garage to press on with getting to the oil cooler or not. If it turns out not to be the oil cooler, or the gearbox oil cooler, then everything else needs the cylinder head off to diagnose I believe? But looking at it, all the stuff that needs to get to the oil cooler needs to come off anyway if getting to head gasket.

So the only thing I need to confirm before getting the garage to crack on is whether the oil in the expansion tank is gearbox or engine.

Does this make sense?

Thank you!

 

[StMarks]

What do you think?

Fwiw I think you've come up with a real "head scratcher" jjc2020.
I can't think of much that I could suggest that you haven't already covered.

- I don't think it's gearbox oil because of oil colour (so not gearbox oil cooler),

Fwiw I can confirm that EP gearbox oil does emulsify to a cream colour when mixed with water, because I had precisely that problem with an old dumper truck we used for mucking out our stables.
Also, although "EP" oils are gearbox oils, afik most conventional gearboxes do not run pressurised at all, so presumably if there was a heat exchanger in a gearbox, the pressurised coolant would leave the cooling system to a far greater extent than the gearbox lubricant would enter the coolant ?

- The head gasket around the combustion chambers is fine according to the compression test but it could be a leak in the gasket between oil and water ways.
- It could be a catastrophic break in the block / head

I personally don't see how a crack in (the block or cylinder head) between the lubrication & coolant channels, would manifest itself any different to a cylinder head failure across the same channels.: A cylinder head can fail in either/both of those aspects, and still retain complete integrity in respect of the combustion pressure.

So the only thing I need to confirm before getting the garage to crack on is whether the oil in the expansion tank is gearbox or engine.

Hmm...

Fwiw I do recall being told of "very old coolant breaking down into an oily mess" that was apparently resolved simply by renewing the coolant. Iirc there was also some mention of a cooling system sealant additive having previously been used, & that this may have contributed to the symptom N.B. That information is hearsay, so I cannot in anyway validate it.

Good luck anyway, and if you do get to the bottom of it, please let us know.

 

[jjc2020]

Thanks for the great response I really appreciate it.

Fwiw I can confirm that EP gearbox oil does emulsify to a cream colour when mixed with water,

That's good to know. Interestingly, there is no sign of any emulsion. When I say the expansion tank is full of oil, I mean actual engine oil type stuff, of the thick-ish and black variety. Not any creamy emulsion at all. This is is what is leading me to think that it only occurred a few miles before the 'check coolant' warning on startup. My best guess at this point is that the oil cooler split, the oil floated to the top and got 'skimmed off' into the highest part - the expansion tank. And then the engine got switched off as we only do short journeys. That's why there isn't much in the waterways. This may be utter nonsense but is my best guess for now.

I personally don't see how a crack in (the block or cylinder head) between the lubrication & coolant channels, would manifest itself any different to a cylinder head failure across the same channels.

That makes sense. So the garages check of compression proves the combustion chambers are ok, but doesn't say anything about leaky gasket between oil and water, or cracked block / head between oil and water. Now the thing is mostly dismantled, is there any way to check water jacket for leaks? I guess its possible to reattach water hose into block and out of block, and pressure test that, but this would also leak out through oil cooler so doesn't narrow down where the leak is. Only way to tell is to eyeball the head gasket, if the oil cooler comes up and is all fine.

Fwiw I do recall being told of "very old coolant breaking down into an oily mess" that was apparently resolved simply by renewing the coolant.

In this case, coolant is definitely not old as I checked the level and topped it up two weeks ago. Good to know tho.

We'll double check it's not gearbox oil, assuming it still doesn't seem like it is then crack on with oil cooler and hope it's that. Failing that, it's head off time.

Thanks!

 

[DSM10000]

The transmission is certainly not sealed for life.

Mercedes have recommended change intervals for the fluid and filter. These have changed over the years but it is still required.

 

[alabbasi]

Have you had the car for sale? A common scam is potential buyers putting oil in the expansion tank in order to low ball the seller.