Most oil leaks we see on these cars are seals. A valve cover gasket goes hard, a filter housing O-ring takes a set, and oil starts finding its way out of a joint that used to hold. This one was not that, and the difference decided how the job was done.
How it arrived
The GLA came in on a flatbed. The owner had found oil under the car, looked underneath, saw it actively dripping, and decided not to drive it. That was the right decision and it is worth saying so plainly, because the alternative gets expensive fast.
Finding where the oil was actually coming from
Oil runs downhill, which is what makes leaks on a transversely mounted four-cylinder awkward. By the time the car is on the lift, the underside is wet from the sump rail backwards and the oil has travelled a long way from wherever it left the engine. The wettest part of the car is almost never the part that failed.
So the first job was not diagnosis, it was cleaning. The engine was degreased and dried, the car run up to temperature, and then inspected again to see where fresh oil reappeared. On the M270 there is a short list of usual suspects — the valve cover gasket, the oil filter housing seals, the turbo oil feed line, and the oil cooler — and cleaning the engine down is what tells them apart.
The oil came back at the oil filter housing, low on the front of the block. That narrowed it to two things: the seals between the housing and the block, or the cooler bolted to the housing itself.
What we found
The assembly came off the engine and the answer was visible on the bench. The oil cooler was cracked. Not a hardened gasket, not a seal that had taken a set — a split in the cooler itself, with oil staining tracked right down across the plate stack from where it had been escaping.
One thing worth recording: on this car the oil and the coolant had not mixed. The crack was letting oil out to atmosphere rather than across into the coolant side. The oil on the dipstick was oil, the coolant in the tank was coolant, and there was no mayonnaise under the oil cap. That is a better outcome than it could have been, and it is the first thing we check on any cracked cooler, because a cooler that cross-leaks turns a leak repair into a much larger conversation.
The decision: complete housing, not a reseal
There is a cheaper version of this repair available, and it is the wrong one. A shop can fit new seals to the housing, put the same cooler back on, and hand the car over — the engine is clean, the leak is not visible for a week or two, and the invoice is small.
But a crack is not a sealing problem. New O-rings do not close a split in a heat exchanger, and the part that cracked once under heat cycling and pressure is not going to behave differently with fresh seals around it. So the cooler was replaced — and it was replaced as part of a complete genuine Mercedes-Benz oil filter housing assembly, cooler and seals together.
Replacing the whole assembly rather than the cooler alone is deliberate too. The housing is plastic, it lives directly on a hot block, and it has been through every heat cycle the cracked cooler has. Bolting a new cooler to a housing of that age is a job you do once and then do again. The labour to get in there is the expensive part of this repair; doing it twice is what actually costs the customer money. This is the same reasoning we set out in our guide on aftermarket vs OEM parts — the part price is rarely the number that matters.
Coolant flush and oil service
Because the cooler sits in the coolant circuit, the coolant had to come out to do the job at all. Once it is out, putting the old coolant back in through a brand new heat exchanger makes no sense, so the system was flushed and refilled with the correct Mercedes-approved coolant rather than topped back up. Mercedes coolant specification is not a detail to improvise on, and we cover why in why European cars need a specialist.
The engine oil and filter were renewed at the same time, with the correct specification oil for the engine. The car had been losing oil and running low, the filter housing was off the engine anyway, and there is no sense pushing the old oil back through a new cooler. Our engine oil change guide covers what a proper oil service on a European engine involves.
After reassembly the engine was run up, the coolant bled properly, the system brought to temperature and pressure, and the whole area inspected again dry and clean. No weep at the housing, no drop on the floor.
What to take from this
If you own a GLA, CLA, A-Class or any of the other cars running the M270 family of engines and you find oil on your driveway, the question to ask is not "how much for a gasket." It is "what exactly is leaking, and did you clean it down to find out?" The answers to those two questions are the difference between a repair and a delay.
And if the answer comes back as the oil cooler, ask whether the cooler alone is going on or whether the housing is being renewed with it. The labour is the same either way. Only one of them is finished.
