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High Fuel Pressure

Sam I Am

Member
Happy New Year VAFs! I have a recurring issue with my fuel pressure in my 2003 9A. I replaced a Lycoming low pressure mechanical fuel pump a year ago due to high fuel pressure issues. The pump rides on a Lycoming O-320 E2D. The old pump had been in place since 1999 and had 650 hours. At the same time I put on a rebuilt MS carb (rebuilt by MS) that eventually was rejetted because of lean fuel flows and high CHTs when the rebuilt carb was in place (seems to be a common condition based on other threads). All has been great since Feb 2019 and 200 hours of flight. The last few flights I have noted fuel pressure readings that are above 8 psi on the ESI 4000 especially on climb out but will start on the ground. I typically have the electric fuel pump on for climbout and have shut it off to only find the pressure drop a couple of tenths. Once I get to cruise elevation, it settles back to normal of 1.5 to 3 psi. The plane is hangared and the engine kept warm with an EZ Heat pad on the oil pan. It is hangared in Amarillo and Topeka for work. The oil temp is a nice 110F at start up and with blankets the whole engine compartment is warm.

Reading other threads on the forum, I have confirmed that the ESI readings agree with a dial guage that I put in its place. The crankcase ventilation tube was removed and checked for blockage for which there was no blockage. I cleaned the gascolator bowl which there was very little sediment. The carb bowl was drained and no sediment was found. I did not find a screen in the carb fuel inlet side. I blew into the vent tubes of the fuel tanks and found no resistance. After the diagnostics on Friday, I flew it around the pattern twice and the pressure never read above 6 psi. Problem solved....I was hoping.

Yesterday I took the bride to our favorite lunch spot and the high pressure readings were back. I climbed to 4,500 ft (900 AGL) for a slow 20min sight seeing flight at 2100 rpm. The readings would spike to as high as 11.5 psi. The really high readings were only spikes and would mostly stay in the 8 to 9 psi range. What I realized yesterday was there was a difference as to the fuel tank I was using. The high psi occurs when I use the right tank. The right tank I keep 100LL avgas for climbout and landing. When I switched to the left tank that contains 87 mogas, the readings would drop to normal, 1.5 to 3 psi. I switched tanks several times on our return to Amarillo and the readings would take several seconds to change as I assume the fuel in the system was being used and the other fuel replaced it. The engine does not seem to be affected with the pressure changes but worry that the carb will be affected in time. It now makes sense that I would have high pressure readings on the ground too as I place the fuel selector to avgas for landing then the following take off until I reach cruising altitude.

When I bought the plane in the summer of 2017, there were 2 fuel leaks in the cabin. An A&P replaced some fuel line and fittings and talked me into replacing the Vans fuel selector valve with an Andair. To this day, there is a distinct fuel smell of fuel in the cabin when I first open the canopy. Some have suggested that the floor insulation had been saturated with fuel when they changed the lines and fuel selector and would be there unless I replaced. I have taken the cabin apart and cannot find fuel leaks now. The A&P used the analogy of buying a used car that was smoked in...you never get the smell out. Since I'm not a builder and was not provided the plans when I purchased, I'm not sure how the vent tubes run from the tank to the underside of the fuselage. Could the right tank vent be open in the cabin creating the issue of smell and high fuel pressure???

Please provide any thoughts. I'm hoping someone will know a solution that doesn't mean replacing the darn pump again as it took a lot of tongue twisting to get in place last year! Thanks in advance for your consideration.
 
I would check for vent blockage on the tank that is causing the issue. I would expect air leaks in the supply line on the subject tank would cause low pressure. Also, my experience is that fuel fully evaporates and does not leave a lingering smell once evaporated. Not highly experienced, but what I have seen/smelled. Pour a couple ounces of avgas in a plastic bowl with a paper towel and let it evaportate and see for yourself. The towel will hamper evaporation and it may take a few days for the smell to disappear. This will simulate gas in your insulation.

Larry
 
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Thanks Larry

Larry,

I have confirmed by blowing into the air vents that there is not resistance to air. Is there a better way to confirm? It's been a few weeks but have overboarded fuel through the vents from both sides when I filled tanks to top and left out in the sun. Great idea on the towel test, I will give it a go. Thanks, Sam
 
Fuel pump questions

Sam, Thanks for the information on the plans. I will order today!

It does seem implausible that I would have another failure. I flew it again yesterday afternoon and the fuel pressure went high on both tanks and fuel. I had readings as high as 11.5 psi yesterday.

Can anyone tell me how the pressure is regulated by the pump? Without a return line, does the fuel "leak" back on the source line when adequate pressure is reached? Is there something that can be adjusted with the pump?

Thanks, Sam
 
Can anyone tell me how the pressure is regulated by the pump? Without a return line, does the fuel "leak" back on the source line when adequate pressure is reached? Is there something that can be adjusted with the pump?

Thanks, Sam

I ran into this problem many years (decades?) ago with a Grumman Tiger and recall a few things about it. To answer your question, pump output pressure from the Lycoming pump on your engine is established by springs inside the pump. A pushrod is driven by a cam on the left accessory case drive gear and operates a lever on the pump. That action does not pump any fuel by itself but instead compresses springs that do pump fuel when the cam releases the pushrod. There is no adjustment on the spring tension, or at least not way back then.

HOWEVER, a subtlety may be involved here. Because the bottom of the pump is open to the engine case, case pressure actually helps the springs push fuel out of the pump and forward to the carburetor. So any restriction in the engine vent line will increase case pressure, thereby augmenting fuel pressure, as crazy as it sounds. Keep in mind that a restriction can come from delamination well inside a vent hose and not be visible or detected by blowing through it. And that restriction may be intermittent depending on how it sits in the line.

This does not explain the left/right/mogas/avgas dilemma but since you recently had the problem with either tank selected, I am very suspicious of the vent line. I would suggest simply replacing whatever vent hoses you have in the installation. Any such restriction would likely be in one of them and not in an aluminum tube that is part of the vent system.
 
Crankcase ventilation tube

Bill, Thanks for the detailed information on the pump. We did pull the crankcase vent tube last week which is aluminum down to the exhaust and connected to the crankcase by a 4" black hose. All was good there and no evidence of delamination of the rubber. I have ordered a replacement pump as I believe I have ran all the other trails.

Thanks to the brain trust!

Sam
 
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