← Mazda Miata NA (1990-1997) problems

Oil leaks from the CAS O-ring and valve cover gasket

Symptoms

Drops of oil under the back of the engine, oil on the firewall or bellhousing, a burning-oil smell, or oil in the spark plug wells.

Cause

The cam angle sensor (CAS) at the back of the valve cover seals with an O-ring that commonly hardens and leaks; the valve cover gasket is the other usual suspect. Oil from a CAS leak can drip onto the heater hose below it and rot it, turning an oil leak into a coolant leak. A DIY guide and forum owners agree these, plus the front main seal, are the classic old-Miata leaks.

Fix (summary)

Degrease the engine, drive, then look for fresh oil to find the source. Both are cheap DIY jobs: mark the CAS position before removing it (one 12mm bolt) so ignition timing stays put, fit a new O-ring, and many owners replace the valve cover gasket at the same time. Check the heater hose under the CAS while you are there. The front main and cam seals are easiest to do with the timing belt.

Other NA problems

• Short-nose crankshaft (1990 and early 1991)
• Rocker and floor rust from clogged drains
• 1990-93 1.6 uses a smaller 6-inch rear differential
• Cooling parts crack with age and heat
• Verify timing belt and plug service history

Sources

• NA Miata Cam Angle Sensor (CAS) O-Ring Replacement — Miata.Repair (DIY guide site) (vendor article; read 2026-10-22; full page read)
• Where does oil leaking from a CAS (cam angle sensor) show up on the engine? — MX-5 Miata Forum (forum thread; read 2026-10-22; search excerpt only)
• Do all old Miatas leak oil? — MX-5 Miata Forum (forum thread; read 2026-10-22; search excerpt only)
• NA Miata Valve Cover Gasket Replacement — Miata.Repair (DIY guide site) (vendor article; read 2026-10-22; search excerpt only)

Summaries are in our own words. Not repair instructions; consult the factory service manual.

Report a correction — spotted an error or have a better source? Email hello@chassisdex.com.