The reservoir drops half an inch every week. There is no puddle on the driveway, no dripping under the car, nothing on the garage floor. The coolant is going somewhere — and 'somewhere' is a short list. Every case of disappearing coolant falls into one of six places, and each leaves its own fingerprint. Finding the fingerprint early is the difference between a clamp and an engine.
The six places coolant goes
- 1Water pump weep hole. The pump seal is designed to leak before the pump fails — a small drain hole on the housing weeps coolant onto the ground or, on a hot block, evaporates before it ever drips. Look for a white or green crust below the pump pulley.
- 2Hoses and clamps that only leak hot. A hose softens under temperature and pressure; the seep happens at operating temperature and dries on the hot surface within minutes. Inspect the seams and clamp seats with a torch after a 20-minute drive, when the residue is still damp.
- 3The heater core. A small leak inside the cabin drips onto the passenger footwell or evaporates into the air. The fingerprint is a sweet, maple-syrup smell inside the car and a windshield that fogs on one side in damp weather.
- 4Head gasket into the combustion chamber. The most expensive entry on the list. Coolant enters a cylinder, leaves with the exhaust as white smoke, and the level drops with no external trace. Persistent sweet-smelling white smoke on startup, bubbles crossing the reservoir with the cap off, or a misfire that fades as the engine warms all point here.
- 5Cross-contamination through an oil cooler or intake manifold. Coolant and oil swap addresses. The fingerprint is 'mayonnaise' — a pale beige emulsion — under the oil filler cap, or transmission fluid turned strawberry-pink slurry on cars with a coolant-cooled automatic gearbox.
- 6A radiator cap that no longer holds pressure. If the cap's spring is weak, the system boils over into the overflow bottle on every hard run and vents it through the overflow tube. The reservoir looks full one day and empty the next, with a dried trail under the overflow hose as the only clue.
| Fingerprint | Most likely location | How to confirm |
|---|---|---|
| Sweet smell in cabin, fogged windshield | Heater core | Check passenger footwell carpet; pressure test with heater on |
| White or green crust at pump pulley | Water pump seal | UV dye in the system, torch check after a drive |
| Sweet white smoke on startup | Head gasket | Block test (combustion leak tester) or CO2 test on the reservoir |
| Mayonnaise under oil cap | Oil cooler / head gasket | Check dipstick for emulsion; compression test |
| Overflow full, then empty overnight | Radiator cap venting | Test cap pressure rating; inspect overflow tube trail |
| Nothing visible anywhere | Hot-side seep or head gasket | Pressure test cold and hot; UV dye is definitive |
What to do first — in order
Start with a cooling-system pressure test, cold and then warm, because some leaks exist only at temperature. If pressure holds, add UV dye and re-check after a drive. Pull the oil dipstick and look under the filler cap for emulsion before anything else — catching cross-contamination early saves the engine. And never open a radiator cap on a hot engine: the system is under pressure and the release scalds. Wait for it to cool, then turn to the first stop to release pressure before removing.
For fleets: track litres, not impressions
Fleets have an advantage passenger owners lack: records. Log top-up volume per vehicle per 1,000 km. A healthy sealed system takes almost nothing between services. A rising trend — even one that looks normal in absolute terms — is the earliest head gasket warning you will ever get, weeks before white smoke appears. Set a threshold, and any vehicle crossing it goes to the workshop instead of finishing the route.
Coolant service intervals for fleets
A top-up log only works against a known service baseline. Read how often coolant should be changed fleet-wide, what to test at each interval, and how distributors structure a service programme around it.