Cooling System Diagnosis

Leaks
Coolant- or rust-colored puddles under the vehicle indicate a cooling system leak. Begin your inspection at the radiator, checking all hose connections for rusty trails and other signs of coolant-colored moisture. If all appears well, work toward the firewall, checking the thermostat and water-pump housing seals. (Water-pump failure is often foreshadowed by squealing bearings; if you can wiggle the fan's shaft easily, the water pump is on its way out).


Next, check the heater-hose connections, both at the engine and firewall. Moisture on the firewall or wet front-seat carpet/floorboard points to heater problems. Hopefully, a loose hose is the culprit instead of a rotted-out heater core.


If everything checks out up top, jack up the car, secure it on jackstands, and crawl underneath. Look for leaks where the lower radiator hose attaches to the engine and also for any firewall moisture trails below the heater. Finally, check the engine itself. Many engines have drain plugs, which can corrode and allow coolant to seep past their threads. Engine freeze plugs can also deteriorate and allow coolant to ooze from the block.


If the radiator or overflow bottle is constantly low but no leaks are visible, the puke tank might be cracked or coolant could be seeping from a radiator hose, then become atomized by the fan before moisture can accumulate and drip. To check, start the engine and look/feel for mist, being careful to keep body parts away from the fan.


If coolant loss can't be located externally, suspect an internal engine leak. The engine's cylinder heads could be cracked or there may be gasket(s) leaking. Indicators of this are a pervasive antifreeze smell inside the car, whitish exhaust smoke after the vehicle is at normal temperature and milky crud on the dipstick. Blown head gaskets are serious—coolant in the cylinders and oil pan can do expensive internal damage. In extreme cases, the block itself could be cracked, which would obviously account for a leak.