MajorCommonFluid Leaks

BMW 4 Series: B48 oil cooler gasket leak

B48 oil cooler gasket leak is a common fluid leaks fault on the BMW 4 Series on the 2016 onward cars, and we rate it major. Putting it right costs $600 to $1,300 at New Zealand independent workshop rates. Below is what goes wrong, the symptoms to notice on a test drive, where to look, and what to ask the seller before you hand over any money. It is one of 20 documented faults on the BMW 4 Series.

Vehicle: BMW 4 Series
Engine: B48
Years: 2016 onward
Fuel: petrol
Gearbox: auto
System: Fluid Leaks

This page covers the BMW 4 Series as a model. Some faults only affect particular engines or build years — enter a plate below to narrow the list to one car.

All BMW 4 Series faults20 documented faults on this model

What you are looking at

No photograph available

We do not hold a verified photograph for this fault yet. We would rather show none than show a picture of something else.

What goes wrong

The B48 2.0T engine uses a thermoplastic oil-to-coolant heat exchanger mounted on the block. The integrated gasket and O-ring seals degrade with thermal cycling, causing engine oil to weep into coolant or externally at the cooler housing. Documented across 3 Series, 4 Series and X3/X4 with B48. Common after 80,000 km. The B48 replaced the N20 in the 420i/430i from the 2016 LCI onwards and continues into the G22 generation.

What you would notice

Oil in coolant reservoir (brown or milky tinge on cap underside)
External oil seepage around oil cooler housing area below the oil filter
Coolant consumption without visible external leak
Gradual oil level drop with no obvious drips on ground

How to check it

Where to look. Oil cooler housing on right-hand side of B48 engine block, adjacent to the oil filter housing

Remove engine undertray and inspect oil cooler housing on the right-hand side of the block for oil staining.
Check coolant reservoir cap underside for oil contamination.
Inspect oil filter housing gasket simultaneously as it fails in the same area.

What it costs to put right

$600 to $1,300 at New Zealand independent workshop rates. That is for this job on its own, not for the car.

An estimate, in NZD, for the repair as normally carried out. What a specific car costs depends on which part has failed and what it damaged on the way — treat this as the floor for a negotiation, not a quote.

Ask the seller

“Has the oil cooler gasket or housing ever been replaced?”
“Have you noticed oil in the coolant or unexplained coolant consumption?”
“Has the car had any coolant-related work done?”

Now check the BMW 4 Series you are actually looking at

Everything above is true of the model, and it is free. What it does not tell you is whether THIS car has been looked after: its odometer history and whether the reading has ever gone backwards, whether there is still money owing on it, whether it has been reported stolen, how many owners it has had and what it is really worth today. That comes from the official records for that plate. Enter the plate for the free details, or get the full history and a market valuation from $13.49.

Not sure what you would be paying for? See what a full KickTyres check includes.

Where this comes from

The failure described above is documented in published sources — owner forums, technical service bulletins, recall notices and workshop write-ups — held in our fault database. The inspection instructions and the cost estimate are KickTyres’ own interpretation of them, and it is a starting point for an inspection rather than a manufacturer statement.

BMW 4 Series buyer questions

Is b48 oil cooler gasket leak serious on a BMW 4 Series?

We rate it major and common on the BMW 4 Series. The B48 2.0T engine uses a thermoplastic oil-to-coolant heat exchanger mounted on the block. The integrated gasket and O-ring seals degrade with thermal cycling, causing engine oil to weep into coolant or externally at the cooler housing. Documented across 3 Series, 4 Series and X3/X4 with B48. Common after 80,000 km. The B48 replaced the N20 in the 420i/430i from the 2016 LCI onwards and continues into the G22 generation.

How much does it cost to fix on a BMW 4 Series?

$600 to $1,300 at New Zealand independent workshop rates. That figure is for this job on its own. A car showing the symptoms on this page is usually worth either walking away from or negotiating down by at least the repair figure.

Which BMW 4 Series years are affected?

Our research records this fault on the 2016 onward BMW 4 Series. Build dates move around, especially on imported cars, so treat the range as the years to look hardest at rather than a guarantee either way. The official records tell you the exact build and first registration date for the car in front of you.

How do I check for it before I buy?

Remove engine undertray and inspect oil cooler housing on the right-hand side of the block for oil staining. Check coolant reservoir cap underside for oil contamination. Inspect oil filter housing gasket simultaneously as it fails in the same area. Look here: Oil cooler housing on right-hand side of B48 engine block, adjacent to the oil filter housing