Fuel pressure fluctuates

Hello Listers,

I have recently noticed that the fuel pressure on my Airflow Performance injected O-360-A1A fluctuates between 20psi and 27psi when the fuel level is down to less than 1/2 tanks. With full tanks the pressure stays pretty constant at 26psi - 27psi. The problem seems to be exacerbated when running low fuel flows (7.5 - 7.8 gph) LOP.

When I turn on the electric fuel pump the pressure comes back to a rock steady 28psi.

OAT was below 10C when it has occurred so I don't think is vapor or boiling point issue.

I am thinking that the lower static head pressure when the fuel level is down is the primary culprit, but I'm also worried that the mechanical fuel pump may be going bad. Even though the engine & pump have less than 150 hrs since new.

This may have always been happening and I only recently noticed it, but I don't think so. I keep a pretty close eye on the engine monitor during every flight.

It would ease my mind if others with a similar setup have seen the same pressure fluctuations under similar conditions. Otherwise I may have to replace the pump.

Thanks in advance,

Ben Beaird
-6A
Plymouth, WI

Fuel Pumps

I have the same problem but only at altitude. When I cruise at 11500 to 13000 DA, I get the same fluctuations. Is his a pump getting weak? What is the typical life of the engine mechanical pumps on FI engines?

fuel pressures

Mechanical fuel pumps tend to fluctuate depending on temps, altitude, and fuel quantity. Pressures of 20-28 lbs is well within normal range.(27 lbs. isn't necessarily better than 20 lbs). As long as it does'nt drop below 17 lbs. your engine doesn't know the difference.

If it only happens at altitude, I would strongly suspect a temporary cavitation on the suction side of the pump as it tries to draw fuel in on the start of its stroke. If this is the case, you should see the effect magnified by higher fuel temps and higher altitudes. It would also be increased by a higher vapor pressure fuel, such as mogas instead of 100LL, and mogas with ethanol content for sure. What are you burning?

Let your plane sit in the sun on a warm day till about 2pm or so to heat the fuel in the tanks and then do a max performance climb up to about 15-17k and see if the reading drops further.

Looking like this is normal

I have seen it go down to about 19 psi at Altitude. It has never been a problem in lower altitudes. I never burn anything except 100LL. It does appear to be a vapor lock type problem that is promoted by the lower ambient pressures of high altitude. I am sure heat sinking the fuel would make it worse. Turning on the boost pump solves the problem right away.

I have since read that some certified FI aircraft require the boost pump above certain altitudes. I assume that is because of what I am seeing. So it appears that the pump may be fine and this is the nature of the beast at altitude.

Living in Florida, I also keep one layer of Womens nylon stockings stretched across the fuel vent openings with zip ties. This keeps the many creepy crawlers we have out of the holes and I don't have to remember to take them off. This most likely reduces incoming ram pressure and may make my system more susceptible to the vapor problem. I will try some operations without the bug stoppers in place and see if I notice a difference. Thanks for the replies, it is good to know others have seen this same problem at altitude.

The Cessna Turbo 210 was one of those planes that experienced variation in pressure and even some mild engine surging under the right conditions of altitude and temperature. Solution was to turn on the boost pump. I never found it necessary to run the boost for more than a few minutes.
Terry

Do you get any actual engine noise changes when the fuel pressure drops?

I had the same problem when I first started flying my TMX IO-360. First thing I did was ensure the gauge was accurate (EIS 4000). I hooked up a Vans analog gauge and got the same indications. Then I upgraded to the "industrial quality" fuel pressure sender from Grand Rapids. Its a nice stainless steel piece of gold. I have never had a fuel pressure fluctuation since.

So in short make sure its not just the sender but be cautious about the root of the cause.

Steve
RV7A
Flying