Question: My cooling system keeps losing coolant, but
I don't see any leaks. Where is it going?
Answer: You probably have an "internal" coolant
leak inside your engine. The coolant is escaping into the combustion
chamber or crankcase through cracks in the cylinder head or block,
or through a leaky head gasket.
In rare instances, coolant may also leak into the automatic
transmission fluid cooler if one is located inside the radiator.
But usually when automatic transmission fluid leaks into the coolant
it means the line is leaking.
Pressure testing the cooling system is necessary to diagnose
an internal leak. A "cylinder leak-down test" can tell
a mechanic if the coolant leak is in the combustion chamber.
But to pinpoint an internal leak, it is usually necessary to remove
the head(s) from the engine. The head may then be pressure tested
and/or checked for cracks using special equipment.
Minor internal leaks can sometimes be temporarily sealed by
adding a sealer to the cooling system. But large leaks or ones
that do not respond to a sealer will have to be fixed.
If the problem is a cracked head or block, repairs may or
may not be possible depending on the nature of the crack. Cracks
in aluminum can often be repaired by welding while those in cast
iron can be fixed by pinning the damaged area. But some cracks
may be so bad that they are beyond repair or in a location that
makes repair impossible. In such cases, the head or block must
be replaced.
If a leaky head gasket is the culprit, replacing the gasket
may only temporarily cure the problem if the head or block is
warped. The mating surfaces on both the head and block should
be checked for flatness and resurfaced if necessary to restore
flatness for a proper seal.
