Porsche 928, Known Issues and Common Problems
Overview
The Porsche 928 is a sophisticated, complex car. An all-aluminium V8, rear transaxle, electronic fuel injection, power everything, vacuum-operated climate control, and the innovative Weissach rear axle all add up to a machine that demands attentive maintenance. When properly cared for, the 928 is remarkably reliable and will cover enormous distances. The M28 V8 is fundamentally strong, the transaxle is durable, and the body resists corrosion well thanks to its aluminium panels.
The problems arise from neglect and age. These cars are now 30 to nearly 50 years old, and many have been through periods of deferred maintenance by owners who underestimated the cost of keeping a complex GT car in proper order. The 928 community has decades of collective knowledge on these issues, and none of them are unsolvable, but they are expensive if you let them stack up.
1. Timing Belt Failure
Severity: Critical. Engine destruction if it fails.
What happens: The timing belt snaps or jumps teeth, causing the valves to collide with the pistons. The result is bent valves at minimum, and often cracked pistons, damaged cam journals, and scored cylinder walls. The engine is destroyed. This is not an exaggeration, it is an interference engine and belt failure is catastrophic.
Why it happens: The 928’s DOHC V8 (all models from the 928 S onwards) uses a toothed rubber timing belt to drive the four camshafts. Rubber degrades over time, losing elasticity and developing micro-cracks. Heat cycling in the engine bay accelerates degradation. The tensioner rollers also wear, and a failed tensioner can cause the belt to jump teeth even if the belt itself is still intact.
Symptoms: There are no reliable warning symptoms. A timing belt that looks fine can fail without warning. The only prevention is proactive replacement on schedule.
Fix: Replace the timing belt, all tensioner and idler rollers, and the water pump (which is driven by the timing belt) every 4-5 years or 60,000 km, whichever comes first. This is the single most important maintenance item on any DOHC 928. Cost: $3,000-5,000 AUD at a specialist, including parts and labour. If the belt has already failed, you’re looking at an engine rebuild: $15,000-30,000+.
2. Water Pump Failure
Severity: High. Can cause overheating and head gasket damage.
What happens: The water pump seizes or its impeller fails, causing loss of coolant circulation. The engine overheats, potentially warping the aluminium cylinder heads or blowing the head gaskets.
Why it happens: The water pump is driven by the timing belt and spins at high speed whenever the engine is running. The bearing and seal wear over time. On the 928, a seized water pump can also take out the timing belt, compounding the problem catastrophically.
Symptoms: Coolant temperature rising above normal, coolant leak from the pump weep hole (visible at the front of the engine), squealing or grinding noise from the timing belt area, or sudden catastrophic overheating.
Fix: Replace the water pump with every timing belt service. This is standard practice and adds minimal cost to the belt job since the belt must come off anyway. Water pump alone: $300-600 for the part. If the pump has already seized and taken the belt with it, see item 1 above.
3. Vacuum Hose Deterioration
Severity: Moderate to high, depending on which systems are affected.
What happens: Multiple systems stop working or work erratically. The climate control blows only hot or only cold. The cruise control stops functioning. The headlight washers don’t activate. The idle speed fluctuates. The car may run rich or lean.
Why it happens: The 928 uses an extensive network of vacuum hoses to operate the climate control system, cruise control, headlight washers, and various engine management functions. There are literally hundreds of small rubber hoses throughout the car, and after 30-40+ years, every single one of them is brittle, cracked, or collapsed. Vacuum leaks are cumulative, each one individually might not cause a noticeable problem, but collectively they degrade the operation of every vacuum-dependent system.
Symptoms: Hissing sounds from the dashboard or engine bay, climate control stuck on one setting, erratic idle, cruise control inoperative, poor fuel economy, engine management warning lights.
Fix: The only proper fix is a complete vacuum hose replacement, every hose in the car. This is a time-consuming job (8-15 hours of labour) but the parts are cheap, it’s just silicone hose cut to length. Total cost: $1,500-3,000 AUD at a specialist, or a dedicated weekend if you’re doing it yourself with the factory workshop manual. Many 928 owners consider this a one-time restoration job that transforms the car’s drivability.
4. LH Fuel Injection Module Failure (1987+)
Severity: High. Car may not start or run poorly.
What happens: The engine runs rough, idles erratically, hesitates under acceleration, stalls at traffic lights, or refuses to start altogether. Cold starting is particularly poor. Fuel consumption increases significantly.
Why it happens: From 1987, the 928 S4 and later models use Bosch LH-Jetronic electronic fuel injection. The LH control module (ECU) is mounted in the engine bay, exposed to extreme heat cycling. The internal components, particularly the solder joints and capacitors, degrade from thermal stress. The air mass sensor (hot-wire type) also fails, sending incorrect airflow data to the ECU.
Symptoms: Hard starting (especially cold), rough or hunting idle, flat spots under acceleration, black smoke from the exhaust (running rich), poor fuel economy, intermittent stalling.
Fix: The LH module can be sent to a specialist for rebuilding ($800-1,500 AUD) or replaced with a refurbished unit. The air mass sensor should be tested and replaced if faulty ($400-800). Some owners relocate the LH module to a cooler location (inside the cabin or boot) to prevent future heat damage. This is a worthwhile modification if you’re doing the repair anyway.
5. PSD (Viscous Limited-Slip Differential) Failure
Severity: Moderate to high. Affects handling and safety.
What happens: The rear differential locks up partially or completely, causing the car to feel unstable in tight turns, produce tyre scrubbing noises, and exhibit unpredictable rear-end behaviour. In severe cases, the car will resist turning and the rear tyres will chirp on tight corners.
Why it happens: The PSD (Porsche Slip Differential), fitted to S4 and later models, uses a viscous coupling to provide limited-slip function. The silicone fluid inside the coupling degrades over time and can solidify, causing the unit to lock progressively tighter until it effectively becomes a locked differential. Heat and age are the primary causes.
Symptoms: Tyre scrubbing in tight turns (car parks, U-turns), rear end feeling nervous or unpredictable, uneven rear tyre wear, binding sensation when manoeuvring at low speed.
Fix: The PSD unit can be rebuilt by a specialist ($2,000-3,000 AUD) or replaced with a rebuilt unit ($2,500-4,000). Some owners opt to replace the PSD with a conventional open differential or a Quaife ATB (automatic torque-biasing) differential for more predictable behaviour and lower long-term maintenance.
6. Torque Tube Bearing Wear
Severity: Moderate. Worsens progressively.
What happens: A vibration or rumble develops, typically most noticeable between 80 and 120 km/h. The vibration may come and go depending on load, temperature, and speed. In advanced stages, the vibration becomes a persistent drone.
Why it happens: The torque tube is a rigid aluminium tube connecting the front engine to the rear transaxle. Inside it, the propeller shaft spins on a centre support bearing. This bearing wears over time from the constant rotational loads and is not a serviceable item, it’s replaced when worn.
Symptoms: Vibration or humming at highway speeds (80-120 km/h), vibration that changes with load (accelerating vs coasting), occasionally a clunk on gearshift.
Fix: Torque tube bearing replacement requires separating the engine from the transaxle, which is a major job. The bearing itself is inexpensive, but the labour is substantial. Cost: $3,000-5,000 AUD at a specialist. Most owners combine this with clutch replacement (manual cars) or rear main seal replacement since the torque tube is already out.
7. Oil Leaks
Severity: Moderate. Common on all 928s with age.
What happens: Oil appears on the engine, on the ground under the car, or both. Common leak locations include the cam covers, the oil cooler lines, the rear main seal, and the valve cover gaskets.
Why it happens: Gaskets and seals harden and shrink with decades of heat cycling. The aluminium engine and its steel fittings expand and contract at different rates, gradually working gaskets loose. The oil cooler lines (which run from the engine to the front-mounted oil cooler) corrode externally and develop leaks at their fittings.
Symptoms: Oil spots on the garage floor, oil residue on the engine block, burning oil smell from the engine bay, low oil level between services.
Fix: Cam cover gaskets are straightforward: $300-600 including labour. Oil cooler line replacement: $500-1,200 depending on condition and access. Rear main seal: $3,000-5,000 because it requires torque tube removal. A car that’s leaking from multiple locations benefits from a comprehensive reseal during a major service.
8. Coolant Pipe Corrosion
Severity: High. Can cause sudden coolant loss and overheating.
What happens: The underbody coolant pipes develop corrosion, pinhole leaks, or outright failure, causing coolant loss. If the leak is sudden and significant, the engine can overheat before the driver notices.
Why it happens: The 928’s coolant system includes steel pipes that run along the underside of the car, connecting the front-mounted radiator to the rear-mounted engine. These pipes are exposed to road spray, salt (less of an issue in Australia than in Europe or the US), and general environmental corrosion. After 30-40 years, even in a dry climate, external corrosion eventually penetrates the pipe wall.
Symptoms: Coolant level dropping between services, coolant puddles under the car (particularly towards the middle), visible corrosion or staining on the underbody pipes, sudden temperature spike while driving.
Fix: The corroded pipes must be replaced. This is a labour-intensive job because the pipes run the length of the car and require access from underneath. Stainless steel replacement pipes are available from aftermarket suppliers and are the permanent solution. Cost: $2,000-4,000 AUD fitted, including stainless lines. This is a once-and-done fix with stainless.
9. Power Window Regulator Failure
Severity: Low to moderate. An annoyance rather than a safety issue.
What happens: One or more power windows stop working, move slowly, make grinding noises, or drop into the door.
Why it happens: The window regulator mechanisms are cable-operated with electric motors. The cables stretch and fray over time, the plastic guides crack, and the motors wear out. The 928’s frameless windows (no window frame on the door) put additional stress on the regulators because the window must seal tightly against the roof line.
Symptoms: Slow window operation, grinding or clicking noises, window dropping into the door, window not sealing properly when closed (wind noise, water leaks).
Fix: Replacement regulators are available from Porsche and aftermarket suppliers. The job requires removing the door card and inner panel. Cost: $400-800 per window including parts and labour. Many 928 owners replace all four at once for peace of mind.
10. Dashboard Cracking
Severity: Cosmetic, but expensive to fix properly.
What happens: The dashboard develops cracks, typically starting at the defroster vents and spreading across the top surface. In severe cases, the entire dashboard surface is cracked and crazed.
Why it happens: The 928’s dashboard is made from a foam-backed vinyl or leather material that is highly susceptible to UV degradation. Australian sun is particularly brutal. The dashboard sits directly under the windscreen, exposed to intense UV radiation for the car’s entire life. The material dries out, shrinks, and cracks.
Symptoms: Visible cracks on the dashboard surface, particularly around the defroster vents and the instrument binnacle. The surface becomes sticky in heat or brittle and flaking.
Fix: A dashboard cover or cap is the cheap option ($200-500) but looks cheap. Professional dashboard recovery (stripping and re-covering in leather or vinyl) costs $2,000-4,000 AUD. A good-condition used dashboard is $1,500-3,000 if you can find one. Some owners fit a Porsche Classic replacement if available for their model year. Prevention is better than cure: always use a windscreen sun shade.
11. Wiring Harness Degradation
Severity: Moderate to high. Causes cascading electrical issues.
What happens: Multiple electrical systems develop intermittent faults. Lights flicker, instruments behave erratically, the engine management system throws spurious faults, and components stop working seemingly at random.
Why it happens: The 928’s wiring harness uses insulation that degrades with heat and age. The engine bay section is worst affected because of constant heat cycling. The insulation becomes brittle and cracks, exposing copper conductors that then short to ground or to adjacent wires. Connector pins corrode, and the pin tension weakens, causing intermittent connections. The 928 has a complex electrical system with numerous relays, and each corroded connection adds resistance that affects downstream components.
Symptoms: Intermittent electrical faults, blown fuses, flickering lights, erratic instrument readings, components that work sometimes but not always, burning smell from behind the dashboard or engine bay.
Fix: A partial harness repair (engine bay section) can be done by a specialist auto electrician for $2,000-4,000 AUD. A complete harness refurbishment (removing the harness, replacing all degraded insulation, cleaning and re-pinning connectors) is $4,000-8,000 but is the definitive fix. Some specialists offer new-manufacture replacement harnesses for specific 928 models. This is a job best left to someone with 928 experience, the wiring routing is complex and mistakes can be costly.
12. Clutch Replacement Expense (Manual Cars)
Severity: Moderate (it’s a wear item, but the cost is significant).
What happens: The clutch begins to slip under hard acceleration, the pedal feel changes (either too light or too heavy), and eventually the clutch can no longer transmit the engine’s torque to the transmission. The car becomes undriveable.
Why it happens: The clutch is a wear item. On the 928, the clutch disc, pressure plate, and throwout bearing are located at the rear of the car, between the torque tube and the transaxle. Normal wear eventually requires replacement, typically every 80,000-120,000 km depending on driving style.
Symptoms: Clutch slip under hard acceleration (engine revs rise but speed doesn’t increase proportionally), high or inconsistent bite point, difficulty engaging gears, shuddering on takeoff.
Fix: Clutch replacement on the 928 requires separating the torque tube from the engine and transaxle, which is a significant labour exercise. While the torque tube is out, it makes sense to replace the torque tube bearing, rear main seal, and inspect the flywheel. A clutch kit (disc, pressure plate, throwout bearing) is $800-1,500 for parts. Total cost including labour and associated items: $4,000-7,000 AUD. Always replace the torque tube bearing and rear main seal while you’re in there, paying the labour twice is painful.
13. Alternator and Charging System Issues
Severity: Moderate. Can leave you stranded.
What happens: The battery light illuminates on the dashboard, the battery goes flat, electrical systems begin to behave erratically as voltage drops, and eventually the car dies and won’t restart.
Why it happens: The 928’s alternator works hard. It powers a substantial electrical load: fuel injection, ignition, power windows, climate control, headlights, and numerous other systems. The alternator’s internal voltage regulator, brushes, and bearings wear over time. The charging system wiring and connections also degrade (see item 11), adding resistance that reduces charging efficiency. Heat from the engine bay accelerates wear on the alternator’s internal components.
Symptoms: Battery warning light on the dashboard, dim headlights, slow cranking, battery going flat overnight, voltage gauge reading below 13.5V at idle.
Fix: An alternator rebuild (new brushes, regulator, bearings, and diodes) costs $400-800 AUD and is the cost-effective option. A new aftermarket alternator is $600-1,000. Ensure the charging system wiring and ground straps are clean and tight, a poor ground connection is often the real culprit. Total fix including diagnostics and wiring repair: $800-1,500. Carry a portable jump starter in the boot as insurance.
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