1990-2005 Mazda Miata Problems: 6 Known Issues

A well-documented manufacturing defect in 1999-2000 NB1 Miatas: the #4 main cap block support was machined with an excessively deep thrust bearing recess, and the factory installed standard-thickness thrust bearings regardless of clearance. The undersized bearing allows the crankshaft to walk fore-and-aft under clutch pedal load. Over time the crank machines into the block, causing the failure mode owners describe as 'pulsing clutch pedal that progresses to stalling when the pedal is depressed at idle.' Most affected engines show symptoms by 40,000-80,000 miles. If ignored, the result is catastrophic engine failure requiring a replacement short block.

Early 1990-1991 Miatas (VIN 209446 and below) were built with a smaller-diameter crankshaft nose (short-nose crank, or SNC) and a 4-slot crank pulley. The design is intolerant of assembly errors and high accessory belt loads: incorrect key installation, improper pulley bolt torque, or aging can allow the crank pulley to wobble on the keyway. The keyway in the crank nose progressively wears, throwing crank-to-cam timing (and with it ignition timing) off, and ultimately can destroy the crankshaft. The large-nose crank (LNC) introduced mid-1991 (VIN 209447 and later) eliminated this design defect. Owners of affected cars are urged to verify pulley bolt torque to 80-87 ft-lbs and inspect for visible wobble at idle.

Two related oil leaks plague essentially every B6 and BP Miata as it ages. (1) The crank angle sensor (CAS, also called the cam angle sensor) at the rear of the cylinder head uses a small O-ring that hardens with age and seeps oil down the back of the block. Because the leak runs down behind the engine, owners often misdiagnose it as a rear main seal failure. (2) The valve cover gasket and its half-moon end seals are a high-mileage failure point — oil weeps onto the exhaust manifold (causing smoke and burning smell) and into the spark plug tubes (fouling plugs and shorting plug wires). Both leaks are inexpensive to repair if caught before they soak the starter, alternator, or motor mounts.

Both NA and NB Miatas are notorious for hidden structural rust, most commonly at the rear ends of the rocker panels and (especially on NBs) the front frame rails where they meet the floor pan. Water enters via the wheel-arch pinch welds and clogged convertible-top/trunk drain channels and rots the rockers from the inside out. The damage typically goes unnoticed until paint bubbles or a foot goes through the floor. Cars that have only seen winter weather once or twice are still affected; even garaged cars in humid climates show rocker rust. Because the rocker is a structural box section, rust here compromises chassis rigidity and crash safety.

The clutch slave cylinder on both NA and NB Miatas is a known wear item — internal seals deteriorate with age and brake fluid contamination, allowing hydraulic fluid to leak past the piston seal. Because the slave's rubber boot can hold a surprising amount of fluid before any external drip is visible, owners often experience a gradually softening clutch pedal long before they spot a leak. Eventually the pedal goes to the floor and the car cannot be shifted into gear (or out of gear once moving). The clutch master cylinder has its own small fluid reservoir (separate from the brake reservoir on NA and NB cars), so a slow slave leak shows up as a dropping clutch-reservoir level.

Original 1990-1997 NA Miatas were fitted with a vinyl convertible top featuring a zippered, clear plastic rear window. The plastic yellows, crazes, and cracks with UV exposure, and folding the top with the window zipped in causes creases that progress to permanent fractures. Additionally, the zipper teeth (and especially the slider) wear out, leaving owners unable to zip the rear window closed or seal the cabin properly. By 15-20 years of age virtually all unreplaced original tops have a failed window. NB Miatas (1999+) moved to a heated glass rear window with a non-zipper top, which eliminated this specific issue.