← Mazda Miata NB (1999-2005 US model years) problems
Oil leaks: valve cover gasket, rear cam plug and front seals
Q: What causes "Oil leaks: valve cover gasket, rear cam plug and front seals" on the Mazda Miata NB (1999-2005 US model years), and how is it fixed?
A: Mazda Motorsports' leak checklist for 1999-2005 cars lists the valve cover gasket, PCV grommet and oil-cap seal, the cam and front crank seals behind the timing covers, the dipstick-tube O-ring, the rear main seal, the oil pan, and on NBs specifically the round cam seal plug at the back of the head (the NB has no NA-style CAS O-ring there). Fix: Degrease the engine, run it and trace fresh oil from the highest point down.
Symptoms
Oil film on the back or sides of the valve cover, oil running down the back of the head toward the bellhousing, drips under the front of the engine, oil in the spark plug wells, or a burning-oil smell after a drive.
Cause
Mazda Motorsports' leak checklist for 1999-2005 cars lists the valve cover gasket, PCV grommet and oil-cap seal, the cam and front crank seals behind the timing covers, the dipstick-tube O-ring, the rear main seal, the oil pan, and on NBs specifically the round cam seal plug at the back of the head (the NB has no NA-style CAS O-ring there). It also warns that oil from the top of the engine running down the back is easily mistaken for a rear main seal leak. Owner reports on the cam plug differ: one mechanic who has done many NB timing-belt jobs says NBs rarely leak there, while another owner says three of his NB engines did and that it is often misdiagnosed as a valve cover leak.
Fix (summary)
Degrease the engine, run it and trace fresh oil from the highest point down. The cam plug is cheap; owners usually pull the valve cover and rear cam cap to fit a new plug (some reuse the old one with sealant, letting it cure before reassembly) and do the valve cover gasket at the same time if it is hard. Cam and front crank seals are best replaced with the timing belt. Consult the factory service manual for sealant points and torque.
Other NB problems
- NB1 ignition coil packs wear out and can damage the cat
- Thrust-bearing failure in some 1999-2000 cars
- Clutch chatter in 2001 and some 2002 cars
- Plastic radiator end tanks go brittle; airflow blockage
- Corroded coolant mixing-manifold elbow (NB1)
- Crank and cam position sensor problems
More for this car
- All Mazda Miata NB (1999-2005 US model years) fitment answers · NB common problems
- Guide: Miata NA/NB hardtop fitment guide: NA vs NB, OEM vs aftermarket, latches and seals
- Newsletter archive: past issues with every new and corrected entry
Sources
- Engineering Tip #5: Miata Engine Oil Leaks Guide — Mazda Motorsports (Mazda's racing parts and support arm, 2020) (editorial; read 2026-10-22; full page read)
- Stop leaking cam plug 1999 — Miata Turbo Forum (2019) (forum thread; read 2026-10-22; full page read)
- NB CAS Cap-Blank Leak Fix — 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.
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