Brief super high fuel pressure

[Jeff R]

I dragged my airplane out today for a short flight this morning, running through my normal engine start checklist, and was surprised by what my boost pump did.....

I have the Airflow boost pump that I got from Van's for my IO-320. My normal start routine is full throttle and mixture, run the boost pump for a few seconds (normally get about 37 psi), boost pump off, pull out the throttle and mixture, then crank the ignition until the engine catches, then slowly add mixture and throttle as needed.

Today, things didn't sound right with the boost pump and a glance at the Dynon showed 99 psi (full scale) of fuel pressure! Then, silence. The 5 A fuse for the boost pump had blown.

I called Don at Airflow to discuss some possible things. One thing he mentioned was that I use a larger fuse (he recommended 10A). Before removing the cowling to troubleshoot, I replaced the fuse (7.5 A) and tried a normal startup again and, this time, everything was normal. I flew for a bit and cycled the boost pump on and off several times and all remained well. Pressure was normal at around 37 psi.

Temps were in the upper 60's and I only use av gas.

Any idea why I would have seen such a high pressure, and why it went away?

 

[airguy]

The AFP pump has a high-pressure relief valve, if that sticks closed or is blocked the pressure will spike. That would surely cause the motor to pull a lot more current and pop your fuse/breaker.

 

[erich weaver]

It was my understanding that the AFP pump was physically incapable of such pressures. Maybe Don can confirm this?

Also, the 37 lbs "normal" pressure seems high to me; i typically see around 26 lbs I think.

erich

 

[Jeff R]

Yes, Don said that was higher than the pump should be capable of doing. And, you are correct, my normal fuel pressure is 27 psi, not 37 psi.