What’s the straight scoop on used Vernatherm valves (thermostatic oil temperature control valves)? How about used mechanical engine-driven fuel pumps?
You will often see used thermostatic (Vernatherm) oil temperature control valves, and used mechanical engine-driven fuel pumps, for sale on eBay. I just wanted to share a cautionary note on purchasing either of these items as used parts, even if they come with an appropriate documentation of their history that might make them legal for use on a certified plane.
THERMOSTATIC OIL TEMPERATURE CONTROL VALVES (VERNATHERM VALVE)
Used thermostatic valves that I have tested have very seldom met the Lycoming-specified test for initial and full-open temperature response, extension travel, retraction travel, and alignment. Even if they are represented as “passing the test”, unless the seller is very trustworthy and really understands the complete test requirements, it is highly likely that not all the characteristics will be fully examined. Even if the seller offers a warranty, it is a lot of work to have to change out a weak temp control valve. Your odds are not very good, of getting a good used Vernatherm valve.
DIAPHRAGM-TYPE ENGINE-DRIVEN FUEL PUMPS
The diaphragm-style mechanical fuel pumps (sometimes called “Bendix pumps”)come in a low-pressure (carbureted) model, and a high-pressure (fuel injected) model. Both contain two rubber-impregnated diaphragms. These diaphragms have a relatively long shelf life, as long as they are new and uninstalled. Once they are exposed to fuel (pumping diaphragm) and oil (crankcase seal diaphragm), some form of chemical change tends to occur in the diaphragm material. As long as they remain immersed in fuel, they will remain good until mechanical wear (from flexing) actually creates a leak in one or both diaphragms. In most cases, the first indication of a diaphragm failure will be fuel coming out of the “tell-tale tube”, which is connected to the air space between the pumping diaphragm and the crankcase seal diaphragm.
The problem arises when these pumps are removed from service, and the diaphragm dries out. If they are later placed back in service, their service life is often very short; and it is often quite a chore to replace them again on an installed engine. You can try to preserve them by filling them with fuel and capping the ports, and by spraying the engine side of the seal diaphragm with oil (like LPS2), if the pump will only be off the engine for a short time. I would recommend against buying used pumps of this style, unless it will be used only as a core exchange for a rebuilt pump.