BMW's modern roadster pairs the acclaimed B58 turbocharged inline-six with a ZF 8-speed automatic (manual available from 2025) in a lightweight rear-drive convertible platform co-developed with the Toyota Supra. Built at Magna Steyr in Austria, the Z4 M40i offers 340-382 hp depending on model year, with an LCI facelift arriving in late 2022. Production ends March 2026.
Robust B58 engine, proven reliable
Strong residual values for segment
Coolant system plastic parts degrade
Convertible roof mechanism complexity
Buy if: You want a powerful inline-six roadster with modern tech and can commit to proactive coolant system and oil seal maintenance.
Avoid if: You want minimal-maintenance ownership or plan to leave the car sitting for weeks at a time without a battery tender.
Expected Annual Maintenance Costs
Common Problems
Plastic coolant components become brittle and leak, typically between 60,000 and 100,000 km · more· less
The B58 uses several plastic and composite coolant components that degrade under constant thermal stress. The most common failure point is the small-diameter coolant vent hose connecting the oil filter housing to the expansion tank, which becomes extremely brittle from heat cycling. The oil filter housing gasket (OFHG) itself also degrades, allowing coolant to seep onto the engine block. BMW issued SIB 11 10 25 acknowledging that the OFHG seal can deform the insert bushing, creating an external coolant leak. A class action lawsuit was filed in early 2026 alleging that B58 oil filter housings leak prematurely. Replacing the vent hose and OFHG together costs 400-700 euros at an independent specialist, or 800-1,200 euros at a BMW dealer. If the water pump or thermostat housing also needs replacement, total costs reach 1,200-1,500 euros. Upgrading the plastic coolant connector to an aluminium aftermarket version is recommended as a permanent fix.
Crankcase ventilation diaphragm tears, causing oil vapour ingestion and whistling noise · more· less
The positive crankcase ventilation valve uses a rubber diaphragm that can tear or rupture, typically after 60,000-100,000 km. Symptoms include a high-pitched whistling from the engine bay, blue or white exhaust smoke, increased oil consumption, and rough idle. On pre-LCI Z4 M40i models (2019-2022), the PCV diaphragm can be replaced separately using aftermarket repair kits for 100-300 euros including labour. On LCI models (late 2022+), the PCV is integrated into the valve cover, requiring the entire cover to be replaced at 800-1,200 euros. BMW extended the PCV warranty in the US to 15 years/240,000 km, though European coverage should be confirmed with a BMW dealer. This is one of the most commonly reported B58 issues but is straightforward to diagnose.
Rubber gasket hardens from heat cycling, causing oil seepage onto exhaust manifold · more· less
The valve cover gasket on the B58 typically begins to leak between 80,000 and 150,000 km as the rubber degrades from constant heat exposure. Oil seeps along the valve cover edges and can reach the exhaust manifold, creating a burning smell. Some B58 variants use a magnesium valve cover that can warp slightly, creating additional leak paths. On LCI cars with the integrated PCV, the valve cover replacement addresses both issues simultaneously. At an independent specialist, expect 400-700 euros. BMW dealers charge 700-1,000 euros. This repair is often combined with the OFHG service to reduce overlapping labour.
Internal thrust piece wears, causing a knocking sound over bumps and during low-speed turning · more· less
A well-documented issue across BMW's CLAR-platform cars including the G29 Z4. The electric power steering rack develops internal play in the thrust piece, resulting in a knocking noise when driving over bumps at an angle or during low-speed manoeuvres. Forum reports on g29.bimmerpost.com confirm the G29 is affected. BMW released a thrust piece repair kit (approximately 30-50 euros) that can be installed in under an hour, resolving the issue in most cases. An E20 adjustment bolt at the top of the rack can also be tightened as a temporary fix, though care is needed to avoid overtightening. If the rack is severely worn, full replacement costs 1,500-2,500 euros. The repair kit fix works for the majority of affected cars.
Blocked drains, motor hesitation, or window sequencing errors prevent smooth roof operation · more· less
The G29's fabric soft-top mechanism is simpler than the E89's folding hardtop, but still requires maintenance. The most common issue is blocked roof drains in the cavities where the roof folds, causing water to pool and potentially damage electronic components. A sloshing sound when driving is the tell-tale sign. Drain cleaning is straightforward via the inner wheel arch panel. Sensor and microswitch failures can prevent the roof from completing its cycle, and some owners report the window sequencing going out of order. A basic drain clean and adjustment costs 200-400 euros. Motor or sensor replacement runs 500-1,000 euros. A more complex mechanism fault with hydraulic components can reach 1,500-2,000 euros, though this is uncommon. Regular roof operation and keeping drains clear helps prevent most issues.
Electric water pump fails abruptly, causing rapid overheating with no gradual warning · more· less
The B58 uses an electric water pump with a plastic/composite impeller that can crack or deform under thermal stress. Failure is abrupt and the engine can overheat within minutes. Most failures occur between 100,000 and 160,000 km, though some earlier failures around 60,000-80,000 km have been reported. The thermostat is often replaced simultaneously as a preventive measure. OEM pump and thermostat together cost approximately 400-500 euros in parts. With 3-4 hours of labour, total repair runs 600-900 euros at an independent specialist, or 900-1,200 euros at a BMW dealer. Preventive replacement around 120,000 km is recommended by specialists.
Audio amplifier cuts out intermittently, losing bass or all sound until the car is restarted · more· less
A defective batch of Harman Kardon amplifiers affected 2020-2021 Z4s as well as other BMW models (X3, 3 Series) from the same production period. The amplifier randomly drops bass or all audio channels, and the system must be power-cycled by turning the car off and on again. This was identified as a supplier quality issue rather than a Z4-specific design flaw. Under warranty, BMW replaces the amplifier unit for free. Out of warranty, a dealer replacement costs approximately 800-1,400 euros. Third-party amplifier repair services can fix the unit for 200-400 euros.
Battery depletes within 7-14 days if car sits unused, common on all modern BMWs · more· less
The Z4 G29 draws significant parasitic current from its telematics control unit and numerous electronic modules. If the car sits unused for 7-14 days, the AGM battery can drain completely. A faulty Intelligent Battery Sensor can worsen the issue by preventing proper sleep mode. The battery itself needs replacing every 3-5 years at 200-350 euros. A quality trickle charger (CTEK MXS 5.0 recommended) is essential for cars not driven regularly. This is a characteristic of modern BMWs rather than a defect, but particularly relevant for a weekend sports car that may sit for extended periods.
Reliable engine, but cooling system and convertible add complexity
The B58 inline-six is widely regarded as one of BMW's most reliable modern engines, and many examples exceed 200,000 km without major powertrain issues. The ZF 8-speed automatic is similarly robust when its fluid is serviced every 60,000-80,000 km despite BMW calling it a lifetime fill. The main ownership concerns centre on the coolant system's plastic components (OFHG, vent hose, expansion tank), the PCV diaphragm, and the convertible roof mechanism. None of these are catastrophic, but they require proactive monitoring and timely replacement. As a roadster that may sit for weeks between drives, battery management is also important. Overall, the Z4 M40i is among the more reliable sports cars in its segment when properly maintained.
Recalls and Technical Service Bulletins
Engine starter relay corrosion causing fire risk, even when parked (2019-2022 models)
Critical - verify completed
Brake booster vacuum pump damage from engine management software (2019-2021 M40i models)
Verify completed
Steering tie rod fracture risk under high temperatures (2019-2020 models)
Verify completed
Headlamp control unit failure causing loss of headlights (2020 models)
Verify completed
Headlamp adjustment plugs reversed at factory (2019-2020 models)
Verify completed
TCU transmission oil leak from improper weld seam (2023 models)
Verify completed
Power window ECU software allows window closure with digital key removed (2022-2023 models)
Verify completed
The starter relay fire risk recall is particularly important, as affected 2019-2022 Z4s can catch fire even when parked. BMW advises parking outside until the recall is completed. Contact BMW with the VIN to verify all recalls have been addressed.
Warranty Status
Factory warranty (3 years from Nov 2016, no km limit)
Expired on 2019-2022 models, may remain on 2023+
PCV extended warranty (US: 15 years / 240,000 km)
Check with BMW dealer for European coverage
Rust perforation warranty (12 years)
Still active on all G29 models
The BMW Z4 G29 benefits from the 2+1 warranty program (3 years from November 2016 onward, no km limit). All 2019-2022 models are outside factory warranty. 2023+ models may still have coverage depending on delivery date. The PCV valve cover has extended warranty coverage in the US, but European availability should be confirmed with a BMW dealer.
This report is for informational purposes only and does not constitute professional advice. Estimates may be inaccurate. Always have a qualified specialist inspect the vehicle before purchase. We accept no liability for decisions made based on this information.