OM651 oil leaks - oil cooler

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pontiac1978

New Member
Joined
May 28, 2014
Messages
2
Location
North yorkshire
Car
CLS 250 CDI Shooting Brake W218 , SLK 320 R170
Just sharing my experience chasing oil cooler leaks. There'd been a slight leak for a year or two (63 plate CLS250 CDI) which became more noticeable after a long run - the air-con condensate was washing the collected oil out of the undertray onto my drive. Seemed to come from below the oil filter housing. A bit of research suggested o rings on the oil filter housing, oil spray control solenoid seal, or oil cooler itself.
I tried replacing the oil filter o rings, but I didn't have an exact size match in my o ring kit. In fact, the leak got worse, so I thought I was on the right track. I ordered a set of genuine seals from Mercedes, but the slight leak was still there. I suspected a hairline crack in the plastic filter housing, so I bought the complete oil cooler/filter unit from Mercedes Stockton (it was in stock, suggesting a common fault perhaps?) It was £130, and had a brand new oil filter element installed, and a full set of o rings, so not too bad I thought. I replaced the filter housing (not the cooler - looks a nightmare) but still no good. Oil still dripping from below the filter housing. Visibility isn't great, so couldn't see an exact source.
I tried the oil spray solenoid valve next. 44mm 6 point socket does the job. There was no evidence of a leak, but others mentioned the o ring/back up ring can leak. Mine looked fine so I used some hydraulic thread sealer on it and put it back.
Still leaking, but no worse than before, which is good.
Only thing left in that area is the oil cooler. I still didn't fancy tackling that, and the source of the oil drip suggested it couldn't be the seals where it mounts to the engine block - too far away - and the casing was dry.
But, there is a seal on the end that's almost invisible on the car. It seems that most people who get this far replace the cooler despite the work involved, and maybe don't see this last seal until its removed. Anyway, by elimination, it was the only thing left to try.
I wasn't sure if it could be done in-situ, so I started removing the water hose elbow from the water pump. The seal seems to be on the end of the cooler bypass control? There's a long rod connected to the cap, so I had to take the water pump off to actually remove it. It pulls out towards the radiator fan. I used the new rod/cap/o ring complete from the new cooler, and it worked, thankfully! 2 days work but worth it in the end.
PS, if you go down this route, first buy the water pump seal, as it will tear when you remove it. And find a used vacuum solenoid for the water pump control on ebay, as the plastic spigot on it breaks when you pull the water pump off. Don't ask me how I know...
On the photo I have indicated the two screw to remove to get the 'cap' off.
1693317506653.png

On this photo, you can see how the leaking 'cap' on the cooler drips oil onto one of the oil filter housing spigots, which makes you think that's what is leaking - you just can't see the actual source because it is behind the filter housing.
1693317745091.png
 
Just sharing my experience chasing oil cooler leaks. There'd been a slight leak for a year or two (63 plate CLS250 CDI) which became more noticeable after a long run - the air-con condensate was washing the collected oil out of the undertray onto my drive. Seemed to come from below the oil filter housing. A bit of research suggested o rings on the oil filter housing, oil spray control solenoid seal, or oil cooler itself.
I tried replacing the oil filter o rings, but I didn't have an exact size match in my o ring kit. In fact, the leak got worse, so I thought I was on the right track. I ordered a set of genuine seals from Mercedes, but the slight leak was still there. I suspected a hairline crack in the plastic filter housing, so I bought the complete oil cooler/filter unit from Mercedes Stockton (it was in stock, suggesting a common fault perhaps?) It was £130, and had a brand new oil filter element installed, and a full set of o rings, so not too bad I thought. I replaced the filter housing (not the cooler - looks a nightmare) but still no good. Oil still dripping from below the filter housing. Visibility isn't great, so couldn't see an exact source.
I tried the oil spray solenoid valve next. 44mm 6 point socket does the job. There was no evidence of a leak, but others mentioned the o ring/back up ring can leak. Mine looked fine so I used some hydraulic thread sealer on it and put it back.
Still leaking, but no worse than before, which is good.
Only thing left in that area is the oil cooler. I still didn't fancy tackling that, and the source of the oil drip suggested it couldn't be the seals where it mounts to the engine block - too far away - and the casing was dry.
But, there is a seal on the end that's almost invisible on the car. It seems that most people who get this far replace the cooler despite the work involved, and maybe don't see this last seal until its removed. Anyway, by elimination, it was the only thing left to try.
I wasn't sure if it could be done in-situ, so I started removing the water hose elbow from the water pump. The seal seems to be on the end of the cooler bypass control? There's a long rod connected to the cap, so I had to take the water pump off to actually remove it. It pulls out towards the radiator fan. I used the new rod/cap/o ring complete from the new cooler, and it worked, thankfully! 2 days work but worth it in the end.
PS, if you go down this route, first buy the water pump seal, as it will tear when you remove it. And find a used vacuum solenoid for the water pump control on ebay, as the plastic spigot on it breaks when you pull the water pump off. Don't ask me how I know...
On the photo I have indicated the two screw to remove to get the 'cap' off.
View attachment 145941

On this photo, you can see how the leaking 'cap' on the cooler drips oil onto one of the oil filter housing spigots, which makes you think that's what is leaking - you just can't see the actual source because it is behind the filter housing.
View attachment 145942
Hello, planning to get this done soon, could you please send a picture or a link of the vaccum solenoid for water pump please.
 
Great write up. I think this is fairly common on this engine, once the car reaches a certain age mileage, My local indy mentioned it at my last service, mine isn't leaking but he said it will eventually.

Out of interest what mileage is on your car? mines at 127K and I've owned it for 5 years (C250CDi S204 registered late 2012)
 
Great write up. I think this is fairly common on this engine, once the car reaches a certain age mileage, My local indy mentioned it at my last service, mine isn't leaking but he said it will eventually.

Out of interest what mileage is on your car? mines at 127K and I've owned it for 5 years (C250CDi S204 registered late 2012)
Mine is a 2011 e220 and got 112k miles and started a small leak. I have ordered the oil cooler now, need to see locate if the leak is from the thermostat or the oil filter housing and get it changed soon. still not sure whats the vaccum solenoid that might brake.
 

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