Plastic coolant expansion tank cracks and dumps coolant
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What causes "Plastic coolant expansion tank cracks and dumps coolant" on the Mazda Miata NC (2006-2015 US model years), and how is it fixed?
The tank is pressurised and part of the cooling system. Fix: Inspect the tank at every service and replace it if discoloured or crusted, with a revised Mazda tank (club note gives LFG1-15-350) or an aluminum one.
Symptoms
White crust or a sweet coolant smell around the tank, a discoloured tank, falling coolant, then sudden overheating. The temperature gauge reads flat over a wide range, so it may not move until the engine is already too hot.
Cause
The tank is pressurised and part of the cooling system. With heat and age the plastic, especially the neck that holds the cap, goes brittle and cracks; one forum owner found a cracked lip on a 2014 car at about 28,800 miles.
Fix (summary)
Inspect the tank at every service and replace it if discoloured or crusted, with a revised Mazda tank (club note gives LFG1-15-350) or an aluminum one. It is held by three nuts. A failed tank can ruin the engine, which is why many NCs get 2.5 swaps.
Related parts
- Moroso aluminum coolant expansion tank (NC)
- Mazda MZR / Ford Duratec 2.5 L inline-four (L5-VE family, 2009+ donor engines from Mazda3, Mazda6, Tribute, Ford Escape, Fusion and others)
Related build entries
- NC first steps: recall, coolant tank and drains
- Replace the NC plastic coolant expansion tank
- NC 2.5 swap: MZR / Duratec 2.5 into an NC Miata
Other NC problems
- Recall 06V-103: driver airbag horn-assembly bolt (early 2006 cars)
- Plugged soft-top drains put water in the footwells (2006-2009 TSB)
- PRHT crunches or is hard to open (2007 TSB 09-038/07)
- PRHT windshield header and roof rattle (2007-2008 TSB 09-035/08)
- PRHT front roof panel contacts the deck when open (2007 TSB 09-035/07)
- PRHT slow, stuck or rattling after an aftermarket roof module
More for this car
- All Mazda Miata NC (2006-2015 US model years) fitment answers · NC common problems
- Newsletter archive: past issues with every new and corrected entry
Sources
- NC tech note: coolant expansion tank failures and stuck thermostat — MX-5 Club of Victoria (club tech article) (editorial; read 2026-10-26; full page read)
- NC coolant tank replacement (Sept. 2020 tech article) — Midstate Miata Club (club tech article) (editorial; read 2026-10-26; full page read)
- Another Cracked Coolant Expansion Tank (MX-5 Miata Forum) — Wayback capture 2020-11-11 — Miata.net forum, via the Internet Archive (forum thread; read 2026-10-26; full page read)Owner reports; gauge flat spot. Forum, tier 1 on its own.
- Easy Replacement for More Miata Displacement (NC 2.5-liter swap) — Grassroots Motorsports (editorial; read 2026-10-26; full page read)
- Moroso aluminum coolant expansion tank for NC — Flyin' Miata (parts vendor) (vendor article; read 2026-10-26; full page read)
- NC MX-5 (2006-2015) Buying Guide — Miatafy (enthusiast guide site) (editorial; read 2026-10-26; full page read)
Summaries are in our own words. Not repair instructions; consult the factory service manual.
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