coolant loss from expansion tank

[kendog91]

i am new to this forum.

i have noticing coolant loss from my expansion tank but am unable to find a leak anywhere even with pressure testing. i also had a sniffer test done so not the head. i was told by a local mechanic could be the EGR cooler. anyone know what sort of price i would be looking at? or if i could do it myself (nothing of a mechanic but i tinker)

 

[steveq]

There are a few locations where the OM651 engine can develop coolant leaks.
The coolant pump and the fuel filter housing are the two that I know of.

I suggest that you check the coolant pump first -- it is quite easy to check.
To check it you need to remove the large plastic air intake pipe that goes from the front of the car to the air filter housing and then the plastic cover from the front of the engine.

Locate the coolant pump on the front of the engine -- it is towards the left hand side with coolant pipes going to it.

With the engine cold, start the car and watch the centre of the pump for a leak (look at it but keep your fingers in your pockets). It is important to do this with the engine cold as the coolant will evaporate pretty well instantly when the engine is hot so it is hard to detect.

If it is leaking you will need to replace the pump.
It is a reasonably straightforward DIY job but hoseclip pliers will make it easier.

If the leak is at the fuel filter housing it is harder to detect. Maybe putting a dye into the coolant would help you see a leak.
To replace it, there is more dismantling to be done so it is a bigger job.

Best of luck.

 

[kendog91]

thank you steve for getting back to me , im curious though would a pressure test show if it was the coolant pump?

 

[steveq]

Hi Ken,

I don't know but I suspect not.
The coolant is under pressure when the engine is running and the leak wasn't visible.

Like you, I was losing coolant and couldn't identify where it was going.
I suspected the pump but couldn't see any signs of a leak. Only when I looked when the engine was running but cold, I could see a constant drip out of the centre of the pump.
It only leaked when it was spinning and even then it was a small leak (well it was at engine tickover speed). Still it was enough to need a top up every few days.

Since I replaced the pump I haven't had to top up the coolant (> 6 months ago).

 

[markjay]

The expansion bottle cap has a pressure release valve, and a seal.

If the cap is leaking, the possible causes are:

- Leaking seal

- Engine overheating causing the pressure release valve to release coolant

- The pressure release valve if faulty ans releases the pressure too early, or it may have a crack

If opting for a new cap and seal, make sure you match the MB part number, because different caps willbhave different pressure release values.

Another option is a crack in the expansion bottle - they can become brittle over time.

 

[MercedesDriver]

The pump on my OM651 was leaking exactly as yours, only when cold. Second one started to leak after 6 months and changed again. Third so far is good. Couple of months after that oil cooler started to leak, changed that too just to have a leak from fuel filter housing after that. Again visit to my Indy. Those parts have a life of ~100k or 10years so I’ll be safe till 2029 The largest bill was for oil cooler as it is a many work hours job, the car was with indy for 2 days.

 

[MercedesDriver]

Oh and I had changed the expansion tank cap. Pretty strait forward DIY and was the cheapest

 

[markjay]

I think I misread the OP's post to say that he was certain that the leak was 'from the expansion tank' hence my post.

I now think that the OP was saying that he can see the level going down in the expansion tank but he does not know where in the system the coolant is escaping from.

If so, then definitely the water pump and heat exchanger (aka oil cooler) are two possible candidates, as is the head gasket. Other than that it could be from any coolant pipe anywhere... if the leak is on top of a hot exhaust part, the leaking coolant will simply evaporate (there will be some staining, but it might be in an area that is not easily visible).

 

[Tonygw]

I had the same leak on my pump, visible when cold, very common. The genuine mercedes one is an updated metal body pump rather than the original plastic type.
A pressure test wouldnt detect the leak because Mercedes seem to have invented the worlds most complicated coolant system where the engine is cooled on demand and the pump is activated only under certain conditions.

Have a look here. Its gob smacking... LOL

Mercedes-Benz OM 651 Service Manual (Page 38 of 58) | ManualsLib

www.manualslib.com

@MercedesDriver, just out of interest how much did they charge you inclusive to change the fuel filter housing when you had your leak there?

Cheers

 

[MercedesDriver]

@MercedesDriver, just out of interest how much did they charge you inclusive to change the fuel filter housing when you had your leak there?

Cheers

It was £280 with a part

 

[kendog91]

sorry for the late reply, the culprit was the water pump housing, the housing which the pump attaches to was cracked at the back, i have now replaced both pump and housing and fingers crossed no leaks as of yet.

 

[MrGreedy]

Sounds like one to watch out for. My mate is seriously hankering after an OM651 W212 around the 2010/2011 vintage, so I'll remember this in case he comes to me with this issue!

 

[kendog91]

i originally went to a garage with this problem while the car was under warranty, they advised it could be the EGR cooler, i inspected the EGR cooler and while i was at it i inspected the fuel filter housing, both of which was working as they should, i thought how could a garage miss this kind of damage. now the car is out of warranty i have had to fork the bill out myself