BMW Z4 M40i G29

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.

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.

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.

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.

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.

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.

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.

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.