BMW's modern roadster pairs the acclaimed B58 turbocharged inline-six with a ZF 8-speed automatic, joined by a six-speed manual in European production from March 2024, in a lightweight rear-drive convertible platform co-developed with the Toyota Supra. Built at Magna Steyr in Austria, the European Z4 M40i is rated at 250 kW (340 hp), with an LCI facelift arriving in late 2022. The 382 hp figure often quoted for this car is the United States specification. Production ends March 2026.· more· less
Robust B58 engine, proven reliableStrong residual values for segment
Coolant system plastic parts degradeConvertible 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.
Coolant system plastic degradation (OFHG, vent hose, expansion tank)
up to €1,5001 of 8
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. The BMW bulletin usually cited for this, SIB 11 10 25, covers the four-cylinder G29 variants rather than the B58, so it is not evidence of a defect on this car. 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.
PCV diaphragm failure (crankcase ventilation)
up to €1,2002 of 8
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's 15-year/240,000 km PCV warranty extension is limited to specified US-market vehicles operated and repaired in the United States or Puerto Rico, does not list the G29, and has no European equivalent. This is one of the most commonly reported B58 issues but is straightforward to diagnose.
Valve cover gasket oil leak
up to €1,0003 of 8
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.
Steering rack knock
up to €2,5004 of 8
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.
Convertible roof mechanism issues
up to €2,0005 of 8
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 fault in the electric drive components can reach 1,500-2,000 euros, though this is uncommon. The G29's roof pillars are electrically driven and not hydraulically actuated. Regular roof operation and keeping drains clear helps prevent most issues.
Coolant pump and thermostat wear
up to €1,2006 of 8
The belt-driven coolant pump or its thermostat can fail, risking overheating · more· less
The B58's primary coolant pump is mechanically belt-driven rather than an electric unit, and it is usually renewed together with the thermostat because the coolant has to come out either way. A worn pump normally announces itself with coolant weeping at the pump body or a bearing whine, while a stuck thermostat shows up as slow warm-up or a temperature gauge that climbs on a hard drive. Most replacements fall between 100,000 and 160,000 km. 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.
Harman Kardon amplifier cutout
up to €1,4007 of 8
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.
12V battery drain
up to €5008 of 8
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.
Service history
Complete BMW dealer or specialist records essential. Verify oil changes every 15,000 km or annually maximum with BMW LL-01 or LL-17 FE+ approved 0W-20 oil.
Tires
Check tread depth, age (date codes), and uneven wear. Staggered setup: 255/35R19 front, 275/35R19 rear. Rear tires wear faster on RWD.
Cold start
Start the engine completely cold. Listen for unusual rattling, whistling, or rough idle in the first 30 seconds.
Test drive
Minimum 30 minutes including motorway speeds. Check for smooth power delivery, consistent boost, and listen for steering rack knock over bumps.
Specific for this vehicle
Inspect engine bay for coolant and oil leaks
Look around the oil filter housing, coolant vent hose, and valve cover edges for residue. Low coolant without visible puddles indicates a slow internal leak.
Listen for PCV whistling
With the engine running at idle, listen for a high-pitched whistling from the valve cover area. This indicates a torn PCV diaphragm. Also check for blue or white exhaust smoke.
Test convertible roof operation fully
Open and close the roof multiple times. Listen for grinding or hesitation. Verify windows sequence correctly. Check for sloshing sounds indicating blocked drains.
+ 4 more checksShow less
Listen for steering rack knock over bumps
Drive slowly over speed bumps at an angle. A metallic knocking from the front indicates the thrust piece is worn. Usually fixable cheaply, but verify before purchase.
Check transmission fluid service records
The ZF 8HP should have fluid and filter changed every 60,000-80,000 km despite being labelled lifetime fill. Dark or metallic-smelling fluid indicates neglect.
Verify the starter fire recalls
The starter is covered by a pair of campaigns: water ingress on cars built 21 August 2018 to 1 September 2020, and a separate 2026 European action for later cars. Verify with BMW using the VIN, as an affected car can catch fire while parked.
Check for signs of tuning or modification
The Z4 M40i is a popular tuning platform. Look for aftermarket intakes, intercoolers, or charge pipes. Request an ECU flash count check via BMW diagnostic software.
Recalls and Technical Service Bulletins
Water ingress at the starter causing corrosion and fire risk, even when parked (G29 built 21 August 2018 - 1 September 2020)Critical - verify completed
Starter motor fire risk, separate 2026 European campaign covering later G29 productionCritical - verify completed
Steering track rod can fail by vibration fatigue under exceptional bending load (G29 built 27 August 2018 - 1 August 2019)Critical - verify completed
Headlamp control unit failure causing loss of headlights (cars built 17 - 25 July 2019)Verify completed
Fuel tank leakage or weld separation with fire risk (type G4Z built 15 March 2019 - 17 September 2020)Critical - verify completed
The starter fire risk campaigns are the most important, as an affected Z4 can catch fire even when parked, and BMW advises parking outside until the work is done. There are two of them, an earlier water ingress action and a separate 2026 European campaign for later cars. The brake vacuum pump, headlamp adjustment plug, transmission weld and Digital Key window actions often listed for the Z4 are North American campaigns whose European applicability is not established. Contact BMW with the VIN to verify what applies.
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 market only)Not applicable in Europe
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 warranty extension applies only to specified US-market vehicles operated and repaired in the United States or Puerto Rico, and does not list the G29 at all.
Sources: Research draws on model-specific owner forums and owner communities, recall information from official authorities, German automotive press, and multiple workshop cost references cross-checked against each other.
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.
