1991 Audi 90 Problems: 5 Known Issues

On the 1990-1995 Audi 90, the 7A 2.3L 20-valve inline-five and the AAH 2.8L V6 used in the Audi 90 are generally treated as interference designs, while owners dispute whether the NG 10-valve inline-five is. If the rubber timing belt snaps, stretches, or jumps teeth, the pistons strike the open valves and bend them, turning a routine service item into a major engine repair. The belt, tensioner, and water pump are wear items; the belt can crack or stretch with age and the tensioner pulley weakens over time, allowing incorrect cam timing, misfires, or sudden engine stoppage. Owners are repeatedly warned that the AAH 12-valve V6 in particular is an interference engine where a broken belt means bent valves. Because these cars are now 30+ years old, belts often fail from age rather than mileage.

On the 1990-1991 Audi 90, the inline-five 80/90 commonly develops oil leaks at the back of the cylinder head. The rear cam seal is essentially an O-ring on the silver dust cap at the rear of the camshaft; it hardens and cracks with age and weeps oil. Combined with an aged valve cover gasket, oil seeps down the back and sides of the engine, where it can mimic a rear main seal or oil pan leak and make diagnosis confusing. These are age-related rubber seal failures typical of 30-year-old engines and are a frequent source of the oily film and burning smell owners report.

On the 1990-1995 Audi 90, the Audi 90 quattro uses a central Pentosin hydraulic system that powers both the power steering and the brake servo. A nitrogen-charged pressure accumulator (universally nicknamed the "bomb") stores energy to provide brake boost if the pump or engine quits. The nitrogen gradually diffuses through the internal diaphragm, so almost all accumulators degrade after roughly seven years of service. As the charge bleeds down, the brake warning light stays lit for several seconds after start-up and, with the engine off, the pedal loses its power assist after fewer than about ten pumps. A weak or failed bomb makes the brake pedal hard and unboosted, sharply increasing stopping distances. The shared hydraulic pump and servo can also leak, and using ATF instead of Pentosin ruins the seals and sticks the accumulator check valves.

On the 1990-1995 Audi 90, the plastic clips that attach the window glass to the regulator break, causing the window to drop into the door. The cable-driven regulator mechanism also frays over time. Very common failure on all four doors.

On the 1990-1995 Audi 90, the VDO instrument cluster in the 80/90 uses small plastic gears in the speedometer/odometer assembly that strip teeth or break with age, a well-documented failure across the B3 and B4 chassis. The odometer typically stops counting while the rest of the cluster keeps working. On B4 cars the vehicle-speed signal passes through the cluster to the engine ECU, so a dead speedometer from a sensor or cluster-connector fault can also set the check-engine light; that is a separate fault from the odometer gear. The original pot-metal/plastic gears are the weak point.