Careful visual inspections and old-fashioned persistence are the keys to pinpointing elusive evaporator leaks, air conditioning specialists said. Misdiagnosis is particularly costly because replacing an evaporator usually is a very labor-intensive task. Diagnosing evaporator leaks can be frustrating since the component is hidden inside a housing-usually below or inside the dashboard.
But although the evaporator core is out of sight, the evaporator housing drain hole usually is not. It allows condensed water to run off harmlessly.
Where there's smoke, there's fire, as the saying goes. And where there's refrigerant oil, there's a refrigerant leak. Put your finger into the drain hole or shine a light into it. An oil residue confirms the evaporator is leaking.
Note that if leaves or other debris clog this drain, condensation will collect inside the evaporator housing, causing a foul or musty odor.
When the evaporator leak is fresh, the telltale oil residue may not have accumulated yet at the evaporator housing drain hole.
However, because refrigerant is heavier than air, the best place to detect it is in the bottom of the evaporator case. Therefore, if room allows, place the tip of your leak detector probe into the hole.
On some vehicles, removing a blower motor resistor assembly or other part gives you access into the top of the evaporator housing. Or, carefully drill a hole just big enough for the leak detector probe. (Seal the hole after performing the leak check.)
Slide the probe into the bottom of the housing and take a reading. Allow the compressor to cycle on and off several times because this may dislodge sludge particles that are temporarily plugging tiny holes in the evaporator core.
If the leak detector doesn't react, shut off the air conditioner and allow the high-side and low-side pressures to equalize. Whereas normal evaporator pressure is less than 40-45 psi with the system running, it rises to about 100 psi after the system is shut off. This pressure increase may be all it takes to reveal a tiny pinhole in the evaporator core.