Lear Romec Fuel Pumps

Debris lodged between the valve and seat of the relief valve assembly is the single-most cause of pump failure. The smallest speck of contamination can effectively reduce fuel output pressures to nil. Sediment in the form of rust, scale, lint, sealant, aluminum compounds, have all been known to cause pumps to lose their pressure. It is strongly recommended that fuel lines be flushed prior to installing them to the pump. Furthermore, installation of pump fittings can introduce unwanted Teflon® paste, tape, or material pulled from the fitting or housing itself. Be certain to clean the fittings and pump threads thoroughly before installing the fittings to the pump. Avoid using a vise to hold the pump when installing or repositioning fittings, as this can permanently damage the assembly. If possible, mount the pump to the engine prior to making any fitting changes.

On occasion, fuel-borne contaminants can actually be purged from the relief valve seat. The procedure is simple. First, remove the safety wire that secures the 9/16-in. jamb-nut to the valve housing and turn it one-half turn to loosen it.

Next, using a straight-bladed screwdriver, turn the relief valve adjusting screw counter-clockwise until it no longer turns. The screw has an internal stop that prevents it from falling out. It's important that you count the amount of turns required to fully release tension on the relief valve spring. Now, with the throttle in the wide-open position and the mixture at full rich, engage the boost pump for a few moments. Before attempting to re-start the engine, be certain to return the adjustment screw to its original setting and safety-wire the nut. About 30 percent of the time this effectively flushes the contaminant from the seat. The odds aren't great, but it's worth trying before removing the pump and sending it out to be repaired.

Vapor locking

Low or pulsating fuel flows can also be attributed to vapor locking. Vapor locking is the partial or complete interruption of fuel flow due to the formation of vapor within the fuel system. This phenomenon most commonly occurs after a flight when the engine has become heat soaked and outside air temperatures are warm. But, it is also known to happen in flight and may cause engine roughness or power loss accompanied by fluctuations in fuel flow meter indications.

Excessive heat transfer from the engine to the fuel lines will naturally cause a vaporization of fuel within the line. Lines in close proximity to cylinders or exhaust heat sources are prime candidates for the creation of a vapor lock situation and should always be fire-sleeved. The problem of vapor locking is more pronounced at higher altitude airfields. At higher altitudes, low atmospheric pressure on the fuel results in a lowering of the boiling point of fuel and increases the likelihood of fuel vaporization. Careful consideration should be given to the routing of fuel lines and the radius of turns within the lines. Tight bends or kinks in the fuel lines can cause air to become trapped in the radius of the bend. Furthermore, it also encourages agitation or turbulence in the fuel flowing through the line, again contributing to the formation of vapor. The onset of vapor lock is readily apparent in a plane equipped with both an analogue gauge and a Shadin- or Hoskins-style gauge. The needle on the analogue gauge begins to drop off while the digital flow gauge begins to spool up. The reason for the disparity is that the digital flow is measuring the movement of vapor racing through the line.