Other than a drop in oil pressure, one of the most attention-getting engine issues might be a significant drop in fuel pressure. A reader with a Lycoming IO-540-powered Rockwell has been chasing this unsettling issue where the fuel pressure drops to the lower red line above 6000 feet and lasts up to five seconds.
There is also a slight drop in RPM and fuel flow and this will repeat itself a couple of times and then go away on its own. If he turns on the electric pump (a logical response), the drop in pressure tends to go away until the pump is switched off. He tried different fuel selector configurations—left tank, both and right tank only—with the same results.
The owner said the problem has occurred regularly since he has owned the plane and if it doesn’t happen during the first hour of  light, it won’t occur. The engine was new when he bought the plane, the electric fuel pump has been rebuilt and the gascolator is new. All fluid-carrying hoses were replaced along with the new engine. A&P/IA Mike Berry weighs in:
I can see why some mechanics might suggest shotgunning the engine-driven fuel pump. The inlet to the pump acts as a check valve and, if leaking, will provide erratic fuel pressures and diminished flow to the engine. The aux pump will override this defect and allow for normal operation. However, because this is an intermittent problem, it’s either a fuel supply versus fuel flow demand problem, or simply fuel vaporization due to localized heating in the engine compartment during climbout.
The first condition can result in low fuel pump pressures and slightly altered fuel flows. The fuel tanks provide a large volume of fuel at a very low pressure, usually no more than 2 to 3 PSI when in a static condition. When the engine is operating, the engine-driven fuel pump pulls fuel from the tanks through the selector valve, gascolator and aux fuel pump—which drops the head pressure to a -1 PSI, or so. This negative pressure will apply a slight vacuum to all fuel components located upstream of the engine.
As a result, a gascolator, or any other fuel system component, even though it does not leak externally, could be allowing air to be pulled into the fuel on its way to the engine. This air will pass through the engine-driven fuel pump—with a subsequent drop in pressure—on to the servo, and then to the engine. When the aux pump is activated, a higher head pressure is applied to the engine-driven pump, thereby diminishing the pressure loss. Capping lines before and after the fuel system components and applying a vacuum with a handheld vacuum gun like a Mighty-Vac (not electric) will isolate the offending component. As a matter of clinical review, check all fuel tank vents for obstructions or collapsed lines.
However, if the problem is vapor lock, it usually results in rough running, hesitation or stumbling during climb. Check that all fuel lines are fire-sleeved and that the routing of all fuel supply lines does not bring them close to exhaust or cylinder components. Also, reroute lines that have a high loop or bend in them; these could trap vapor at the high spot of the bend and restrict fuel flow. If your aircraft is equipped with a fuel-flow transducer (used with engine monitors and fuel totalizers), cover the lines and the transducer with fire sleeve or reroute them to some location away from the bottom of the cylinders. The best first step is to check the factory recommended way of routing lines. Over time, lines get rerouted with no concern for heat, but rather ease of installation.
Controls rigging
An owner of a Cessna retrac asked how to check the aircraft’s rigging because it isn’t making book speeds, and he’s confident the engine is making power. Larry Anglisano weighs in on the basics any owner can try:
Pick a calm day (turbulence will make this more difficult) where you can fly VFR without interruption. If possible, ask a mechanic to come along. Fill the fuel tanks and empty the baggage compartments. Use caution, as some aircraft are out of their balance envelope in this configuration, so you might have to compensate. Remember that you are looking to achieve lateral balance in the air. If you nearly always have passengers, then you will want to proceed to check your rigging based on a balanced lateral load (which means taking an equally weighted passenger along). If you fly solo most times, check the rigging based on the unbalanced load of your butt in the left seat.
Once airborne, accelerate to the normal cruising speed in level flight. Feel free to use the elevator trim, but leave the rudder or aileron trim tabs neutral. Come off the rudder pedals and roll the wings level using aileron (providing a wing was low) referencing the attitude instrument. With the heavy wing leveled and the skid ball centered (without your input), note the rudder trim indicator’s position and mark the exact position. Masking tape works. Note whether the heavy wing is still heavy. Chances are it’s not, or at least not as bad as it was. If it still takes some aileron input to hold one wing up, adjust the aileron trim to compensate.
Note the aileron trim indicator’s setting the same way you did for the rudder trim. On a side note, if your instructor never clued you in, this is really the correct way to trim an aircraft for level flight. Some refer to it as the tail-first method. Trim the elevator to hold proper pitch, then the rudder to get the ball centered and finally the ailerons to get the wings level. This is easier in aircraft with cockpit-adjustable trim tabs or no movable tabs. If the tabs are put into a neutral position and you still find that constant control pressure is required to keep the ball centered and the wings level, the airplane’s rigging demands more scrutiny.
Its inability to fly straight and level without having one or more trim tabs deflected means it is losing airspeed to trim drag.
