Avstar Aircraft of Washington, Inc.

10415 172nd St. E., Hangar A1
Puyallup, WA  98374
office (253)770-9964
or (253)770-0120
email:  avstarair@att.net

12-12-2012 Ask Mike! Question
Slow increase in registered fuel pressure


I have a 1979 A36TC with the standard TSIO-520UB engine with 410 TSO.  I have an AuRACLE CRM2100 engine monitoring system installed.  I am noticing a very slow increase in registered fuel pressure over the last 2 years at full throttle 36" and 2700 rpm.  The max fuel pressure would hit a maximum 17 to 17.5 psi on takeoff in the beginning but now on occasion I have seen 18.3 to 18.5 and of course this sets off the fuel pressure warning alarm in the AuRACLE system.  These fuel pressures seem high compared to book figures and the slowly increasing maximum is making me wonder.

Any ideas what might be happening?  What should be done?

Your wisdom would be welcome!


Good Morning, Steve;

The first question I ask is this occurring on every take-off or just the first one of the day?  Overboost and subsequent following of fuel flow / pressure (they travel hand-in-hand) is not uncommon for a less than normal operating temperature oil temp.

Second question is concerning the servicing schedule of your system.  CMI/TCM recommends cleaning, inspection and adjustment of the FI system yearly.  Coinciding with annual (typically) I clean screens, air filter servicing / replacement (depending on type), clean nozzles, replace nozzle seals, replace upper deck seals as required, inspect induction and exhaust for defect.  Then, a test gauge is installed to the unmetered portion of the fuel system, between the pump and the metering unit.  This is the gauge that is used for setting the low RPM fuel pressure, and as a reference gauge when setting the high pressure.  Fuel flow (metered side) is set to spec (pressure / flow), if the unmetered side is way out of line, something was missed.  This is pretty well spelled out in CMI/TCM SID97-3E.  The only place I differ with 97-3E is the same place everyone does:  Generally set high fuel flow at redline minimum to assure enough fuel flow for cooling.

The components of the fuel system change due to wear, leaks (fuel, air, and in the case of a TC / TN, exhaust), and debris build up, so a yearly cleaning, check for condition and compensation for wear (adjustment) will generally keep these systems in good operation for the life of the engine.  What you describe may very well be dirty injector nozzles; as dirt builds, the orifices are slightly restricted, so pressure will increase.  Note that on an analog fuel flow gauge, this can show (erroneously) as an increase in fuel flow.

On occasion, a diaphragm will fail in the flow divider (generally on engines that do a lot of sitting), or a pump will fail (more noticed by a drop on low end - engine dying on roll-out).

So, my second question:  When was the last time your engine received this check up?

You didn't mention location, but if you are West Coast, I'd be happy to schedule you in.