So I just bought a 2009 ML550 with 132000 miles that looked in great shape with good service record and I was going over it once I got home (forgot to check coolant before i bought it... i know, my bad) and noticed a small amount of oil in the coolant tank. The oil in not low and no coolant in oil as far as I can tell.

What should I be checking and what could be at fault?

From past experience with other vehicles I would assume head gasket, oil cooler, water pump... but not sure what is the usually suspect on these.

Thanks

 

Check for signs of oil seeping out from the oil cooler (rectangular thing next to the oil filter). The gasket is prone to leaking and could lead to mixing of oil and coolant.

 

Does this oil cooler typically fail also? Or just the gasket?

What's the proper coolant for this vehicle?

 

I've never heard of the cooler failing, just the gasket. It is made out of rubber, gets hard over time and starts leaking. Failure shows up as oil leakage below the cooler. The cooler has two plates that are held together with I think 6 bolts, and there is a gasket that sits in a channel between the plates to keep the coolant and oil separate. The gasket is cheap but it is a a little bit of work to replace because the power steering reservoir has to come off to get to it.

 

Proper coolant is G48 formula.

That's blue.

If you still have gold G05 formula coolant, it's time to replace it with G48.


OIl cooler seals ... https://blog.fcpeuro.com/the-definit...d-m273-engines

Coolant ...https://blog.fcpeuro.com/how-to-pick...r-car#Mercedes

 

Oh... When did that happen? I run G05 in all of my vehicles...

 

So do you typically replace both gaskets? or just the one between the oil cooler and the oil filter housing? (other one appears to be between oil filter housing and engine).

 

You still can, but you can't buy gold from Mercedes any more My 2010 CL came with blue

From the FCP Euro link above ...Over the last three decades or so, Mercedes has used three different coolant variants in their gasoline and diesel-powered cars. The oldest is Q1030002, a Yellow or Gold-colored G05 HOAT formula that is no longer available through Mercedes. It may still be available in small quantities through manufacturers like Zerex, but Mercedes recommends you drain and refill with a more modern variant.

Two current versions of Mercedes coolant are available:

G48 Coolant (HOAT)

• Model Years: 2014 and earlier
• Mercedes Spec: 325.0/326.0
• Internal Code: Q1030004
• Color: Blue
• Compatible/Mixable With: G05 (Q1030002, Yellow)
• Negative Effects: While technically compatible with the older Yellow coolant, the early formula is very old, and any car using it should be drained, flushed, and replaced with the modern Q1030004 fluid

 

The gasket that attaches the finned cooler to oil filter assembly is what fails so change that. When I did mine, I removed the whole assemly and changed both as I figured "do it once." That takes longer as youll need to remove the thermostat and one of the idler pullies to get it off.

Beware of the different length bolts when reattaching the assembly.