Coolant Leak
Check soon — before it becomes overheating
A slow coolant leak is a countdown to overheating. You have time to diagnose it properly, but not to ignore it. Watch the level closely, carry water, and don't plan road trips until it's fixed.
What it means
Coolant (antifreeze) is the usually green, orange, pink, or blue fluid with a distinctly sweet smell. Finding it under the car, smelling it after a drive, or repeatedly topping up the overflow tank all mean the sealed cooling system is losing fluid — and the only question is from where, and how fast.
Leaks divide into external (hoses, radiator, water pump, heater core connections — you can find the wet spot) and internal (head gasket letting coolant into the combustion chamber or oil — no puddle, but the level drops). External leaks are usually cheap; internal ones are the serious ones.
Likely causes
| Cause | Likelihood | Severity |
|---|---|---|
| Aged radiator or heater hose / clamp Rubber hardens and cracks with age. Check hose ends and clamps first. | Most common | Moderate |
| Radiator leak (seams, tanks, or stone damage) Plastic end-tanks on modern radiators are a common failure point as they age. | Common | Moderate |
| Water pump weep or seal failure A drip from the pump's weep hole is the pump telling you its seal is going. | Common | Moderate |
| Expansion tank or cap failure Cracked plastic tanks and worn cap seals are cheap fixes that mimic bigger leaks. | Common | Minor |
| Heater core leak Coolant smell inside the cabin, fogged windscreen, or damp passenger-side carpet. | Less common | Moderate |
| Head gasket (internal leak) Coolant disappears with no external trace; may come with white exhaust smoke or milky oil. | Rare | Serious |
DIY checks before the shop
- Put clean cardboard under the engine overnight — the drip location and colour narrow the source enormously.
- With the engine cold, check hose ends, clamps, the radiator seams, and the water pump area for crusty white/green residue (dried coolant).
- Smell the cabin with the heater on: sweet smell inside points at the heater core.
- Track the level in the overflow tank over days — how fast it drops tells you how urgent this is.
- If no external trace can be found and the level keeps dropping, ask a shop for a cooling system pressure test and a combustion gas test — that separates external from internal leaks.
What repairs typically cost
| Repair | Typical range |
|---|---|
| Hose or clamp The best-case scenario. | Roughly $50–$250 |
| Expansion tank or radiator cap Often overlooked, genuinely common. | Roughly $20–$150 |
| Radiator replacement Varies by vehicle. | Roughly $300–$900 |
| Water pump More on timing-driven pumps. | Roughly $300–$900+ |
| Heater core Cheap part, brutal labor on many cars — the dashboard often has to come out. | Roughly $500–$1,500+ |
Ranges are broad US-typical estimates for common vehicles at independent shops. Luxury brands, dealers, and regional labor rates can land well outside them — treat these as orientation, not quotes.
Rev knows your specific vehicle and service history — describe the symptom and get advice for your car, not a generic answer.
Frequently asked questions
Can I drive with a slow coolant leak?
Carefully and locally, while monitoring the level and temperature gauge — yes, most people do while awaiting repair. The risk isn't the leak itself; it's running low without noticing and overheating.
Do stop-leak additives work?
Sometimes, temporarily, on small leaks — and they can also clog heater cores and radiators. Reasonable as an emergency measure on an old car; not a substitute for fixing the leak.
Why do I smell coolant but see no puddle?
Small leaks can evaporate off hot engine parts before reaching the ground, and heater core leaks stay inside the cabin. Residue and smell are evidence even without a puddle.
