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Fluctuating/erratic fuel pressures

I've always had these fluctuations with my IO 540

In the summer when it is warm, the fuel is warm and the engine temps are higher I got this a lot for the last 1400 hours. The worst was when I was doing a very slow low powered climb on a hot day LOP. I was in no hurry and wanted to save fuel. The fuel pressure was fluctuating more than usual and my engine was running bad. I thought my mechanical fuel pump was going out. I turned around to go back and as soon as I started to go down hill at the same power setting the engine smoothed right out. More air over the the engine and fuel lines. I turned around and used a higher power setting to continue my trip. Engine ran great. After that I put a cooling shroud over the fuel pump and that helped some. My fuel system is direct from the tanks to the engine, no return back to the tank.

I attribute this to vapor in the system and it has never been a performance issue except as I described above. Use of the electric fuel would have taken care of the rough running engine but I didn't use it that one time. Now if I suspect vapor in the line I use it.

Steve
 
I've been getting the same kind of fuel pressure fluctuations on my IO-320 since it got warm this summer, along with mildly problematic hot starts. I notice it on taxiing, mostly taxiing in after landing, not at all once airborne. The engine runs fine, mainly the gauge performance is affected, dipping down into the red every now and then. The engine driven pump only has about 60 hours on it.

At my most recent oil change, I had my A&P wrap the fuel line with heat tape, and installed an additional exhaust heat shield where the rest of the exhaust gets a little close to the Red Cube (suggestion on a thread here on VAF). I also took the opportunity to have him install new upgraded (Kavlico) fuel and oil pressure sensors per the AFS/Garmin/Dynon service bulletin (was overdue for that). I've only flown it once since then and saw no issue, but not really a fair test since the ambient temps were relatively cool. I've had the engine driven pump go TU before, this isn't acting like that, but I'm pretty sensitive to it.
 
same thing here. fluctuating fuel pressure and stable when switching ON the boost pump. reason is the mechanical fuel pressure sensor (most use VDO type in RV's) has a resistive element that wears out at the spot where the fuel pressure lingers most of the time. turning on the boost pump moves the sliding contact to an area on the resistive element that isn't worn out. this is just a generic thing with potentiometers which see a lot of vibration, temperature changes, humidity etc.

in case you are using a mechanical fuel pressure sensor, you shouldn't rule out a sensor issue. replacing with a new sensor or changing to solid state technology should cure the problem.

before going into the hassle of replacing fuel pump etc. i would consider that.
 
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Replaced the sender and it fixed the problem! This is an interesting explanation!
same thing here. fluctuating fuel pressure and stable when switching ON the boost pump. reason is the mechanical fuel pressure sensor (most use VDO type in RV's) has a resistive element that wears out at the spot where the fuel pressure lingers most of the time. turning on the boost pump moves the sliding contact to an area on the resistive element that isn't worn out. this is just a generic thing with potentiometers which see a lot of vibration, temperature changes, humidity etc.

in case you are using a mechanical fuel pressure sensor, you shouldn't rule out a sensor issue. replacing with a new sensor or changing to solid state technology should cure the problem.

before going into the hassle of replacing fuel pump etc. i would consider that.
 
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