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Fuel Pump Expectations

Cth6

Well Known Member
Working on troubleshooting an issue on a carbed O-320 using a Facet 40108 and want to make sure my logic is sound.

1. Engine off, FP on, should see 4.5 to 6 psi on gauge
2. Engine running, FP off should see .5 to around 6 psi on gauge
3. Engine running, Turning FP on, I should see pressure below 4.5 jump up to at least 4.5psi. If the pressure is already above 4.5 I may not see any movement or a very slight increase.
 
Doesn't sound right. The boost pump sounds like it has a flow issue. Assuming a "regular" application, the boost pump should maintain ~ constant pressure across it's flow range. This range should exceed the max flow of the engine. Typically, the BP pressure setting is slightly above (~0.5 psi typ) that of the mechanical pump so that the pilot gets some secondary indication that the pump is operating when energized. Repeat your measurements under different power settings/fuel flows and see where the pressure starts to tail off. Let us know.
 
You said with engine on and boost pump off you'd expect to see 0.5-6 lbs of pressure. I think the engine driven fuel pump specs are 4-6psi. The lycoming manual does state the **minimum** fuel pressure as 0.5 psi.
 
Point 3 is a bit vague...
For what it’s worth, my ship, carbed O-360, sports a 15 year old engine pump... delivering 2-3psi. This is augmented, for the critical phases of flight, by the elec Facet, and brings the pressure up to between 5 and 6psi.
 
Thanks for all of the input.

I am not seeing a significant change in my boost pressure with the electric pump running.

The facet is making the noise you expect for a properly operating pump.
No blockage on the tank vents.
Good stream coming out to gascolator when only the facet is running. (I have not done a flow test to see if I am close to the 25-30 Gal an hour rate).

My next step is to put a mechanical gauge on the fuel manifold to compare to the Vans gauge to see if it is a sender issue.

If that doesn’t work, going to do a flow test just before the carb.

Before I just start swapping the pump or the sender I would like to narrow it down. I know these pumps can get clogged and freeze up, but I have not seen one operate but not put out much pressure when they are flowing.
 
Cth6.... I'm struggling with the exact same issues at this time. I've checked everything on the fuel supply side including tank venting, fuel line obstruction, fuel selector valve problem. Even fuel flow. I was getting 24 gal hr so I put in a new Facet pump. Checked the screen at the carb inlet as well as the gascolator. Replaced the fuel pump sending unit and I'm still seeing numbers too low for my comfort. The Aircraft has been flying for nine years so I know what "normal" was. Typically I saw nothing less than 4 1/2 psi. About two months ago I started seeing half that. Flying along with boost pump off I saw readings as low as 1.5. I did remove the sender and screwed in a $20 auto parts store fuel pressure gauge. With engine off and boost on it's reading 7 psi which make me feel better. (Same set up with the dynon coming through the fuel pressure sensor reads a low normal of 4.5). I also checked wiring and grounds all the way to the DSUB connector into the back of the Dynon. My solution is to install a Mechanical fuel pressure gauge and the stuff for that is suppose to arrive next week. I don't think I have an engine driven fuel pump issue either. I'm anxious to get the new gauge in the panel and see what I get in flight.
I'll report back.
 
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Cth6.... I'm struggling with the exact same issues at this time. I've checked everything on the fuel supply side including tank venting, fuel line obstruction, fuel selector valve problem. Even fuel flow. I was getting 24 gal hr so I put in a new Facet pump. Checked the screen at the carb inlet as well as the gascolator. Replaced the fuel pump sending unit and I'm still seeing numbers too low for my comfort. The Aircraft has been flying for nine years so I know what "normal" was. Typically I saw nothing less than 4 1/2 psi. About two months ago I started seeing half that. Flying along with boost pump off I saw readings as low as 1.5. I did remove the sender and screwed in a $20 auto parts store fuel pressure gauge. With engine off and boost on it's reading 7 psi which make me feel better. (Same set up with the dynon coming through the fuel pressure sensor reads a low normal of 4.5). I also checked wiring and grounds all the way to the DSUB connector into the back of the Dynon. My solution is to install a Mechanical fuel pressure gauge and the stuff for that is suppose to arrive next week. I don't think I have an engine driven fuel pump issue either. I'm anxious to get the new gauge in the panel and see what I get in flight.
I'll report back.

That pretty much sounds like my situation. I do have an engine driven pump that holds 3-4 PSI in normal flight conditions. In certain configurations I can see .5 - 1 PSI on the gauge. It was like this well before I purchased it, but still doesn’t make me feel 100% until I get to the root cause. Good news is there is an extra tap next to the current sender so I can test against it
 
OP says working on an issue on carb 0320… what’s the issue? Your numbers look perfectly fine. Are the numbers as was day one engine start, all new, likely not. Today, I see nothing wrong with your fuel pressure readings>.5 and <7. Go fly.
 
I’ll state again. The boost pump should provide almost constant pressure regardless of flow demand. The associated relief valve should be set ~ 0.5 psi (whatever is noticeable on your FP gage). Maybe just the relief valve setting is off but it sounds like both pumps are a bit tired; possibly on their way out.
 
I’ll state again. The boost pump should provide almost constant pressure regardless of flow demand. The associated relief valve should be set ~ 0.5 psi (whatever is noticeable on your FP gage). Maybe just the relief valve setting is off but it sounds like both pumps are a bit tired; possibly on their way out.

Freemasm, where is the relief valve setting? Not familiar with that.
 
Depends on the component. Some are adjustable and obvious. Others, not so much. Commonly, the BP used with a once through system (simplex pump/no fuel return) has an internal relief that ports excess flow back to the pump inlet. Some of the automotive pumps that have been adapted are combined with external relief valve in an assembly. I can’t say anything definitively so look for the model number of your pump and do a web search. Don’t be surprised if there’s an adjustment screw (or similar) at the pump outlet. It will actuate/tension a relief valve spring. That force balance sets/maintains pressure for the varying flow. Post a picture. Someone here will have the relevant experience. If it won’t hold a ~constraint pressure, you have some decisions to make. Let us know.
 
If you think you;re getting good flow, then the electric pump is probably working fine. There is a long history in the carbed RV’s of folks seeing what seem to be fairly low fuel pressures, but very rarely does anyone report problems with their engine running. Personally, I thin kit has a lot to do with where your transducer is mounted relative to the pump - you loose head pressure if its mounted up high and the pump is down low. But I’ve never experimented with moving it.

If you’re concerned, then by all means do a flow test - its not hard to disconnect at the carb inlet, and time how long it takes to pump a half gallon, or a quart- whatever you want to do to convince yourself you have a good sample size versus time, then do the math. If you have good flown the pump is fine, regardless of the instrumentation.

Paul
 
I have been having a fuel issue as well. We have 35 hours on our O360 in an RV7, and as soon as the weather turned hot our fuel pump started cavitation before engine start up during a hot start. We would normally turn the fuel pump on before engine start to pressurize the system and then start the engine. After flying for 2 hours and then landing to refuel, the engine sat for 20 minutes. Then, trying to restart the engine is when the pump would sound like it is not grabbing any fuel (high pitch sound versus a low pitch sound). We do not have a check valve anywhere and are wondering if we need to add one. We have a line from the fuel selector to the facet pump, then through the firewall to the Andair gascolator and then to the engine driven pump. If anyone has ideas, we are open to them.
 
I have been having a fuel issue as well. We have 35 hours on our O360 in an RV7, and as soon as the weather turned hot our fuel pump started cavitation before engine start up during a hot start. We would normally turn the fuel pump on before engine start to pressurize the system and then start the engine. After flying for 2 hours and then landing to refuel, the engine sat for 20 minutes. Then, trying to restart the engine is when the pump would sound like it is not grabbing any fuel (high pitch sound versus a low pitch sound). We do not have a check valve anywhere and are wondering if we need to add one. We have a line from the fuel selector to the facet pump, then through the firewall to the Andair gascolator and then to the engine driven pump. If anyone has ideas, we are open to them.

Without knowing, you’d vapor pressures associated with car gas. To have it vaporize before the BP implies losses (really dirty pre-filter) or unrealistic heat problems though probably a combo. Give us some more details (fuel type, ambient, etc. I’d be cautious running the BP dry/without prime. They can destroy themselves quickly or at least lose some life. Let us know.
 
We are running 100 LL in an O360 A1A. We started flying in late may when temperatures were in the 70’s but they quickly shot up in the mid and upper 90’s. When it first happened, I took the gascolator off to clean it and found that there was little to no fuel in it. There was a little metal in it but nothing significant. I blew air through the vent tubes and they were clear. I have wrapped some heat shielding around the gascolator in case it was getting residual heat from the exhaust as well. When we took it out a couple of days ago in the morning when temperatures were low and the pump cavitated again and never seemed to grab any fuel. However, when we started the engine, it cranked twice and started right up. We are flying out of Denver and an airport elevation of 5500 feet, all while trying to break the engine in. So we take off at full power and leave the throttle there because of the high density altitude. The fuel system consists of a direct line from the tank to the selector and the the line from the selector to the boost pump. From there it goes through the firewall and into the gascolator, then the engine driven pump and then off to the fuel pressure sensor and the carburetor. Some ideas we have are to bring everything into the cabin and put in a check valve and filter between the selector and the pump, but then everything is inside and cleaning the filter means fuel and fumes. Maybe even just putting in a check valve. We are wondering if anyone else has had problems with fuel siphoning back into the tanks, or at least past the boost pump since it is a tail wheel? Any suggestions on what a possible solution may be would be helpful. Our next steps are to start replacing things in hopes that we fix the problem. Thanks.
 
The situation you describe would make sense for the main pump to lose prime, not the boost pump; assuming I'm following your description.

It still doesn't make sense to me why the BP is losing prime. The conditions you describe are well within the properties of 100LL so evaporation isn’t the first suspect. Is the pump uncharacteristically mounted above the fuel level? Check all of your lines/connections before the BP for leaks and BP housing for that matter. Maybe air getting in through a loose connection or crack? Almost all of such pumps require a flooded suction (prime) to get going. I'm out of ideas.
 
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The BP is higher than the tanks, but that is expected with the tail wheel. For the first time we are getting a smell of fuel in cockpit so there may be a small leak somewhere so I will check that first. We have been out of ideas as well and were about to start replacing every thing. Thank you for your input.
 
To close this out, it turned out that the sender was sending erroneous data. Replaced and pressures are as expected.
 
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