The NB-generation Miata (1999-2005) is mechanically solid and simple, but the 1.8L BP engine in early NBs is prone to catastrophic short-block failure due to inadequate oiling at the #4 rod bearing. This platform is otherwise reliable if maintained, but that one failure mode is expensive and common enough to warrant serious scrutiny on any used purchase.
Crankshaft #4 Rod Bearing Failure (Short Block Failure)
Common · high severityTypical onset: 80,000-120,000 mi
Symptoms: metallic knocking or ticking that intensifies with RPM, sudden drop in oil pressure, catastrophic engine failure with metal shavings in oil, oil light flickers at idle when warm
Fix: Requires short block replacement or full engine rebuild with upgraded oil pump and bearings. 12-16 hours labor for short block swap, more if machining needed. Many techs replace with known-good used engine or go aftermarket forged internals if budget allows.
Estimated cost: $3,500-6,500
Transmission Oil Cooler Line Leaks
Common · medium severityTypical onset: 100,000-150,000 mi
Symptoms: transmission fluid pooling under car near radiator, low trans fluid on dipstick, hard or notchy shifts when fluid is low, pink fluid drips
Fix: Steel hardlines from trans to radiator-mounted cooler corrode and crack at bends or fittings. Replace lines, flush cooler if contaminated. 2-3 hours labor. Often done with transmission mount replacement at same time.
Estimated cost: $400-700
Transmission Mount Collapse
Common · low severityTypical onset: 80,000-120,000 mi
Symptoms: excessive driveline clunk on throttle lift or engagement, vibration at idle in gear, notchy shifter feel, visible powertrain movement when revving in neutral
Fix: Rubber mount degrades and allows excessive powertrain movement. Replace mount (OEM or polyurethane upgrade). 1.5-2 hours labor, straightforward job from underneath.
Estimated cost: $250-450
Cam Angle Sensor O-Ring Leak
Occasional · low severityTypical onset: 90,000-140,000 mi
Symptoms: oil seepage at rear of valve cover near firewall, oil smell in cabin with heat on, slow oil consumption, dirty engine bay around sensor area
Fix: Small o-ring at cam angle sensor (rear of head) hardens and leaks. Remove sensor, replace o-ring, reseal. 1 hour labor. Easy access but often overlooked until oil drips onto exhaust.
Estimated cost: $150-250
Rear Main Seal Leak
Occasional · medium severityTypical onset: 120,000-180,000 mi
Symptoms: oil pooling at bellhousing/engine junction, drips from transmission bellhousing, oil consumption without visible top-end leaks, oil-soaked clutch if severe
Fix: Rear main seal weeps or fails, especially if crankshaft has worn surface. Requires transmission removal. 6-8 hours labor. Often done during clutch replacement to save redundant labor.
Estimated cost: $800-1,400
Fuel Filter Clogging (Pre-Cat Substrate Contamination)
Occasional · medium severityTypical onset: not mileage-driven
Symptoms: loss of power under acceleration, stumbling or hesitation at WOT, rough idle or stalling, check engine light with lean codes
Fix: If catalytic converter has disintegrated internally, ceramic substrate particles migrate upstream into fuel tank and clog pump filter/sock. Requires tank drop, pump/filter replacement, and tank cleaning. 5-7 hours labor. Verify cat condition first.
Estimated cost: $600-1,200
Buy one if the short block has been replaced or verified healthy with oil analysis and compression test — otherwise, you're playing bearing roulette. Great platform if that one issue is addressed.
AI-assisted summary drawn from NHTSA recall data, our labor-times database, and platform knowledge. Not a substitute for a pre-purchase inspection on a specific vehicle.