Cooling System Diagnosis

Cooling System Diagnosis

Fix leaks & prevent boil-overs yourself

An overheating car can make anyone hot under the collar. The old cliché "heat kills" especially applies to engines: high temperatures make hoses and gaskets brittle, can cause cylinder heads to distort, and create other unhappy problems. Granted, some heat is needed during start-up to burn off condensation that accumulates while the vehicle isn't running (which explains thermostats, EGR valves, and such). But once the vehicle is warmed up, excessive heat becomes the engine's archrival.

Cooling systems operate at a specific pressure. This raises the coolant's boiling point. When the system's intended pressure is exceeded, the spring in the radiator cap is designed to retract, allowing coolant to pass into the overflow "puke" tank and pressure to be relieved. When too much coolant bypasses into the overflow tank, boil-over occurs.

The old cliché "heat kills" especially applies to engines.


The cooling system is largely an out-of-sight, out-of-mind scenario: Most drivers ignore it until the temperature gauge goes into the red, the car overheats or won't start in the winter, or a green puddle forms on the garage floor. But just as tire pressure and oil level should be checked regularly, the cooling system needs periodic attention.

After every oil-dipstick inspection, give the cooling system a glance. Look for cracks or bulges in the heater and radiator hoses, especially near the clamps (see Tip 8). Hose clamps should be snug enough to seal the hose to the metal or plastic nipple, but not so tight that the metal band cuts into the rubber. Also look for signs of chaffing anywhere a hose touches or comes close to a metal or moving part.

Next, eyeball the coolant's color. Depending on the vehicle manufacturer's recommendation, the coolant should be neon green, pink or orange. The photo above shows a truck that had no antifreeze in the system, only water. Rust formed, a leak sprung, and the fan liberally dispersed the iron-oxide-color fluid over the engine compartment and front suspension. Which brings us to the subject of 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.


Many of the companies listed below make products designed to plug cooling-system leaks. These chemicals come in different forms-liquid, pellet, powder and tablet-to fix leaks of varying natures and degrees. (Remember that leaks are symptoms of a problem that should be diagnosed to make sure it isn't major.)

Leak-stoppers flow through the cooling system and accumulate at the exit point or expand to plug the hole. Their particles are miniscule so that they won't plug coolant flow through the radiator and heater core. In emergency situations, black pepper has been known to plug minor leaks because it expands when moist.

Leaks that generally respond to leak-stoppers: radiator and heater-core pinholes (but not cracks), seepage from freeze plugs and gaskets (thermostat, manifold and head), and porosity leaks in the engine block and heads. Leaks that often aren't plugged by stop-leaks: cracks in hoses, bad radiator-cap seals, and seepage at the water-pump's shaft.

Troubleshooting cooling systems doesn't involve quantum physics or any other weird science.
Pressure Testing

Service stations pressure-test cooling systems to pinpoint problems. This can also be done at home with cooling system test kits (which can be rented or borrowed from many auto-parts stores). This setup takes the coolant's temperature to see if the thermostat and electric fan sensor (if equipped) are working properly. It also allows the system to be pressurized through a radiator adapter.

A vacuum/pressure gauge helps narrow down the dilemmas. Vacuum readings reveal potential cracks in a cylinder-head intake port or valve seat. Pressure readings above the system's normal maximum indicate an exhaust-port or valve seat crack. Rising pressure can also indicate a faulty head gasket. (Certain kits have optional radiator-cap adapters to test the cap's vacuum and pressure-relief valves.)

Troubleshooting cooling systems doesn't involve quantum physics or any other weird science. Using the tips here, you can isolate the reasons why your engine is blowing its cool. Then you can decide if you want glycol under your fingernails or would rather pay someone else to get dirty.



Bar's Leaks,



Justice Brothers,






how a cooling ystem works, parts of a cooling system, fixing a cooling system
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