BMW 3 Series: B48 cylinder head coolant vent line coupling breaks, sudden coolant loss (SIB 17 01 21, 320i/330i)
B48 cylinder head coolant vent line coupling breaks, sudden coolant loss (SIB 17 01 21, 320i/330i) is a common cooling fault on the BMW 3 Series on the 2015–2019 cars, and we rate it major. Putting it right costs $200 to $600 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 88 documented faults on the BMW 3 Series.
- Vehicle
- BMW 3 Series
- Engine
- B48
- Years
- 2015–2019
- Fuel
- petrol
- System
- Cooling
What you are looking at
Example of the part — not this vehicleA six-cylinder diesel head off the engine, valve-gear side up. Cracks run between the valve seats on the face beneath.
What goes wrong
On the four-cylinder B46/B48 turbo engine in the F30/F31/F34 320i/330i, a thin plastic coolant vent line runs from the cylinder head to the expansion tank. BMW found that its quick-disconnect coupling cannot cope with engine heat over the life of the part and can snap off at the cylinder head, dumping coolant and risking overheating if the car is driven on. BMW issued service action SIB 17 01 21 (revised April 2022) to replace the line on F22, F23, F30, F31, F32, F33, F34 and F36 cars built from 24 April 2015 to 24 September 2019. That bulletin is a BMW North America document, so a NZ or AU car may never have had the new line fitted; the part itself is cheap, the risk is overheating damage if the coupling lets go on the road.
What you would notice
- Low coolant warning, sometimes suddenly
- Coolant smell or steam from the engine bay
- Coolant residue around the top of the engine near the expansion tank
- Temperature warning or reduced power
How to check it
Where to look. Top of the engine: plastic coolant vent line between the cylinder head and the coolant expansion tank
With the engine cold, check the coolant expansion tank level against the marks. Use a phone torch to follow the thin plastic hose running from the top of the cylinder head to the expansion tank and look at its joint on the engine for cracks, weeping or white or pink crusty residue. Smell for coolant after the test drive and watch the temperature reading. Ask whether the cylinder head coolant vent line has been replaced, and ask a BMW dealer to check the VIN for open service actions including SIB 17 01 21. If a workshop inspection is possible: have the cooling system pressure-tested and the vent line coupling inspected at the cylinder head.
What it costs to put right
$200 to $600 at New Zealand independent workshop rates. That is for this job on its own, not for the car.
Ask the seller
- “Has the cylinder head coolant vent line been replaced under BMW service action SIB 17 01 21?”
- “Has the car ever shown a low coolant warning or overheated?”
- “How often do you top up the coolant?”
Now check the BMW 3 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.
Where this comes from
- BMW SIB 17 01 21 "Service Action: Replace the Coolant Vent Line on Cylinder Head" (Rev. 1, 2022-04-05; NHTSA file MC-10212788), F2x/F3x B46/B48 built 24 Apr 2015 to 24 Sep 2019, part 17 12 9 845 173
- F30post.com low coolant thread: dealer confirmed the coolant vent line service action (SIB 17 01 21) as the cause
BMW 3 Series buyer questions
Is b48 cylinder head coolant vent line coupling breaks, sudden coolant loss (sib 17 01 21, 320i/330i) serious on a BMW 3 Series?
We rate it major and common on the BMW 3 Series. On the four-cylinder B46/B48 turbo engine in the F30/F31/F34 320i/330i, a thin plastic coolant vent line runs from the cylinder head to the expansion tank. BMW found that its quick-disconnect coupling cannot cope with engine heat over the life of the part and can snap off at the cylinder head, dumping coolant and risking overheating if the car is driven on. BMW issued service action SIB 17 01 21 (revised April 2022) to replace the line on F22, F23, F30, F31, F32, F33, F34 and F36 cars built from 24 April 2015 to 24 September 2019. That bulletin is a BMW North America document, so a NZ or AU car may never have had the new line fitted; the part itself is cheap, the risk is overheating damage if the coupling lets go on the road.
How much does it cost to fix on a BMW 3 Series?
$200 to $600 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 3 Series years are affected?
Our research records this fault on the 2015–2019 BMW 3 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?
With the engine cold, check the coolant expansion tank level against the marks. Use a phone torch to follow the thin plastic hose running from the top of the cylinder head to the expansion tank and look at its joint on the engine for cracks, weeping or white or pink crusty residue. Smell for coolant after the test drive and watch the temperature reading. Ask whether the cylinder head coolant vent line has been replaced, and ask a BMW dealer to check the VIN for open service actions including SIB 17 01 21. If a workshop inspection is possible: have the cooling system pressure-tested and the vent line coupling inspected at the cylinder head. Look here: Top of the engine: plastic coolant vent line between the cylinder head and the coolant expansion tank