Fuel Pressure - need help

First off, N397RV is a now a flying airplane in MN (RV7A). Phase I flight completed. Upon flying off my time and working the systems, the one item that has been bothering me is my fuel pressure. The aircraft is equipped with a Dynon engine monitor and the power plant is an O360 carborated. The fuel system is standard plumbing per Vans with electric fuel pump and engine driven fuel pump. The nature of the problem is that the fuel pressure is erratic. Sometimes it will settle down to 1.9 psi without the electric fuel pump on. Once turning on the electric fuel pump it will raise the pressure up above 6 psi (correct me if I'm wrong, is it true that the needle and seat in the carb shouldn't see more the 5 psi or could cause some damage??)

Also, I noticed that my fuel flow will jump from a set cruise when the pressure is that high from 8.5 gal/hr to 12 gal/hr without changing the engine settings. Upon cleaning the belly of the aircraft, I noticed that I have a blue streamer on the belly. Is it that I am pushing the fuel overboard by pushing too much psi fuel pressure?

Please share any thoughts or ideas as to what I should due to tackle this problem and what should I be concerned about?

Thanks, Butch

If you have a blue stain on the belly, you need to investigate. That shouldn't happen.

1.9 PSI of fuel pressure isn't a problem. The engine is happy at any pressure above 0.5 PSI.

Regarding the low and transitory fuel pressure readings, do a search on "low fuel pressure" and you'll find that this is a common problem. Many (most?) of us have concluded that it is an issue with the fuel pressure senders not dealing well with rapid changes in altitude, causing a problem with the *indication* not the actual pressure.

On the change in flow rate with and without the electric pump, that doesn't make sense. Does your mixture setting change? Can you lean farther with the fuel pump "on". That sounds like something else you need to get to the bottom of, but I don't have any suggestions.

Kyle, I believe that my fuel flow reads higher with the electric fuel pump on. O-360 carbureted.

Read other threads on topic before losing your mind

Follow Kyle's advice and read the other threads on this topic. I have a carbed O360 and have the same symptoms you have. It drove me nuts! Replaced the fuel pump, checked the carb, checked the sender, annoyed my friends, etc. Then I read the related threads on this site and discovered that many people see erratic fuel pressure readings. There seems to be some dispute as to exactly why this is the case, but I have finally come to accept that this is just one of the special features of this airplane. Three hundred hours without a hiccup.

Ed

butch said:

First off, N397RV is a now a flying airplane in MN (RV7A). Phase I flight completed. Upon flying off my time and working the systems, the one item that has been bothering me is my fuel pressure. The aircraft is equipped with a Dynon engine monitor and the power plant is an O360 carborated. The fuel system is standard plumbing per Vans with electric fuel pump and engine driven fuel pump. The nature of the problem is that the fuel pressure is erratic. Sometimes it will settle down to 1.9 psi without the electric fuel pump on. Once turning on the electric fuel pump it will raise the pressure up above 6 psi (correct me if I'm wrong, is it true that the needle and seat in the carb shouldn't see more the 5 psi or could cause some damage??)

Also, I noticed that my fuel flow will jump from a set cruise when the pressure is that high from 8.5 gal/hr to 12 gal/hr without changing the engine settings. Upon cleaning the belly of the aircraft, I noticed that I have a blue streamer on the belly. Is it that I am pushing the fuel overboard by pushing too much psi fuel pressure?

Please share any thoughts or ideas as to what I should due to tackle this problem and what should I be concerned about?

Thanks, Butch

Click to expand...

Butch,
Some things to think about.
I heard the best explanation on the fluctuating fuel pressure the other day in another post. Seems normally, we see lower pressures while climbing and higher pressures while descending. The engine driven pump in our low wing type fuel system is sucking fuel in the climb....I think the term would be high "head pressure". conversely, gravity is pushing the fuel to the pump in the downhill phase of flight. So some fluctuation + and - in this area is normal.

Low pressure reading:
some of us RV flyers have experienced low pressure, as in .5 or below as reading in this range are hard to verify because of the quality of the gauges and sender. I've heard and it makes sense that any fuel pressure is enough to fill the carb bowl, but officially the engine manufacturer says .5 psi.
It doesn't sound like you have the low pressure problem as you said 1.9psi.
Lots of guys hear of this low pressure phenomenon and just accept it as normal. I had it and did as well for a period of time until I realized when that was happening I didn't have a working fuel pressure gauge to warn me of a actual problem arising, so I kept searching my setup and finally replaced my
sender/gauge with a higher quality one that works! so don't give up.

High pressure reading:
It would make sense that at some pressure the fuel would overpower the needle valve in the carb bowl, but I have never heard what that number is.
I can't believe it would be as low as 6psi, my guess would be somewhere over 10psi. I see 6psi quite often in my system with no problems at all, that I'm aware of.

Fuel flow: I don't have FF on my plane so I can't speak to erratic readings.

Blue streaks:
never good.
Keep looking!
is it coming from the gascolator area?
Fuel Pump overflow?
Primer line leak?

Good Luck

Bob Martin said:

Blue streaks:
never good.
Keep looking!
is it coming from the gascolator area?
Fuel Pump overflow?
Primer line leak?

Click to expand...

Overflow from hot and full fuel tanks? (which is normal)

Fuel Pressure on Carb 0320 and Dynon180

I see excatly same thing as butch EXCEPT no blue stain on belly . I would investigate that. the 0320 seems to be happy with indicated pressure down to 1.5 at times and I did move the upper limit alarm on fuel pressure up to 7 to avoid the high end beep at times. I also see a change in flow with the fuel pump on , about the same range as mentioned above. Now have 45 hours TTAF

Might be worth considering a second FP gauge to check the readings. Awhile back I needed to check my FP readings. I consider that my FP gauge is reasonably accurate but I did have a couple of unexplained high FP readings. http://www.vansairforce.com/community/showthread.php?t=43579
I got a cheap mechanical FP gauge from the local auto store and ran a 1/8" soft copper tube from the spare port in the VA-168 Transducer Manifold on the firewall. The thin tube was easily inserted through an existing wire bundle to get it through the firewall. Once through the firewall, I then flared the cockpit end of the tube to fit the gauge. So I had two gauges showing FP at the same time.
There is no guarantee that the mechanical gauge was more accurate, but I did "calibrate" it against the permanent gauge so it would tell me if the variations in FP on my permanent gauge were real. I flew for about 17 hours with the two gauges but did not have another high FP event to be able to see if it was real and have since removed the mechanical gauge. The gauge is a "Pricol" brand and although cheap, seems to be of good quality.

Fin
9A

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Fuel pressure/Fuel flow

A follow up after flying two cross country legs. Before leaving on this leg and after cleaning up the aircraft, I filled the gas tanks and upon pulling the plane out I visualed around the vent lines, bottom of airframe. Slight moisture at vent exit. With using a clean paper towel I could get a little bit of residue. I believe that some of the stain which was very little is from fuel expanding pushing out the vent.

Going back to high fuel pressure, I'm still concerned about over pressuring the needle and seat. Where would the excessive fuel go? Consumed through the motor with low temps causing the erratic fuel flow?

Other than metering the amount of fuel that I'm consuming, and watching temps, how accurate are my fuel flow readings? can I trust them?

PS: the airshow in Phillips was excellent. Sad day, one 1 RV showed up other than mine.

Butch

Fuel Pressure...

I was having questions about my fuel pressure on the EIS4000, it would generally run between 4-6 lbs., but sometimes spike to say 10. Lycoming says it should be between 4-8 lbs. Finally put a guage on to check when the electric boost was running only, it shows about 4.5 (that's good) EIS was within a .5 lb. With the engine running it would show about 6-7 lbs. That's okay too. I found you had to use a guage with a range of 0-16lbs. to really get a good read. Using wider range guages are sometimes not that accurate on the low side. Talked with several AP's I trusted they said spikes are not uncommon depending on operation and really not a problem unless it stayed way up.

Your results may vary.

Butch, I have a Rocky Mountain engine monitor. The fuel flow readings do fluctuate. To determine the "real" fuel flow I have to mentally average over a few indicated reading. I am assuming that it is close since the fuel used is good when I fill up.

Other than that, whether the factual instantaneous fuel flow is 7.5 gph or 7.8 gph really is not an issue to me.

The fuel remaining and fuel duration (mentally) averaged over a few numbers are far more important.

Same situation - check vents

Butch:

I assume you are using avgas. I use both mogas and avgas.

I have the D100 D180 combo and an O-360. Fuel pressure in my 9A seems to ALWAYS increase when the electric pump is on. I takeoff with the elec pump on and when I am 3000 agl I turn it off.

I turn it on again briefly when I switch tanks.

If the fuel in the wing is super warm (Tucson in the summer), the pressure will sometimes go below 1.0 so I turn the pump back on to boost it. After the fuel in the tank cools, the low pressure symptom does not occur.

The problem seems worse when I use mogas even though it's ethanol free.

My vents have never streaked fuel onto the belly no matter how full the tanks are. Once airborne, the vents should pressurize the tanks so I am at a loss why there's fuel coming out of your vents.

Pls check the screen over the vents. I assume your vents face forward. Is the screen blocking the hole in the tubing precluding good tank pressurization? If so, you might want to remove the screens temporarily and assess if there is any difference in pressure.

You are almost

certainly seeing the beginnings of Vapour lock Barry..As you say it happens in the Summer..worse with Mogas (I think ethanol actually reduces the vapour pressure I.e it acts a bit more like 100LL thn pure mogas) and the problem goes away when you run the boost pump.

Frank

fuel and fuel vents

frankh said:

certainly seeing the beginnings of Vapour lock Barry..As you say it happens in the Summer..worse with Mogas (I think ethanol actually reduces the vapour pressure I.e it acts a bit more like 100LL thn pure mogas) and the problem goes away when you run the boost pump.

Frank

Click to expand...

Thank you Frank. I mix 1 part avgas to 2 parts mogas in the right wing for takeoffs & landings, just as a precaution but at this point, I cannot detect any performance difference between mogas and avgas. We check for water and volatility with every load of fuel we buy. Volatility increases by a huge factor when 1 part avgas is added to 2 or 3 parts mogas.

A very bright local tech counselor suggested the removal of the screens over the vents and since I did that, all of the mogas fuel anomalies seem to have disappeared. He correctly deduced that there was inadequate pressure being applied to the fuel in the tank, perhaps due to increased vapors from mogas. He also concluded correctly that after 15 minutes in the air, the fuel in the tanks would cool even after sitting in the Tucson sun and any remaining vapor locking would decrease.

The one time I mistakenly got fuel with ethanol was a real eye opener. The plane took off OK, but ran rough all the way down and eventually stalled on the runway. The remainder of that %$#@* fuel went straight into my wife's mini van and out of the airplane.

While I enjoy the savings per gallon, about $1.55, I wouldn't use mogas if there was any safety issue. FWIW, my egt sensors were virtually clean after 150 hours.

Thanks again for your concern. I appreciate constructive comments.

Interesting

MrNomad said:

Thank you Frank. I mix 1 part avgas to 2 parts mogas in the right wing for takeoffs & landings, just as a precaution but at this point, I cannot detect any performance difference between mogas and avgas. We check for water and volatility with every load of fuel we buy. Volatility increases by a huge factor when 1 part avgas is added to 2 or 3 parts mogas.

A very bright local tech counselor suggested the removal of the screens over the vents and since I did that, all of the mogas fuel anomalies seem to have disappeared. He correctly deduced that there was inadequate pressure being applied to the fuel in the tank, perhaps due to increased vapors from mogas. He also concluded correctly that after 15 minutes in the air, the fuel in the tanks would cool even after sitting in the Tucson sun and any remaining vapor locking would decrease.

The one time I mistakenly got fuel with ethanol was a real eye opener. The plane took off OK, but ran rough all the way down and eventually stalled on the runway. The remainder of that %$#@* fuel went straight into my wife's mini van and out of the airplane.

While I enjoy the savings per gallon, about $1.55, I wouldn't use mogas if there was any safety issue. FWIW, my egt sensors were virtually clean after 150 hours.

Thanks again for your concern. I appreciate constructive comments.

Click to expand...

And without trying to sound like I am on a crusade here, but the root cause of the problem is the mechanical fuel pump..Essentially you are proving the mechanical fuel pump really just doesn't work very well.

Interesting why the engine ran rough on the ethanol blend..It runs perfectly in mine..But thats with the elecric fuel pumps in the wingroots and a high pressure FI system...One day i wil test the vapour pressure of E10 compared with straight mogas...But I thought it was supposed to be a little better.

Really if your are seeing the problem go away just by removing the fuel screens then for my money thats way too close to the edge..That will only work at that temperature..if the ambient airtemp goes up a few more degrees the problem will come back.

Personally with the standard fuel system in really hot weather I would stick with 100LL..Vapour lock can be lethal if it truely develops on take off. You have to remember the "increased vapours" is the result of the fuel trying to boil. This is not to be messed with.

Frank

Fuel Pressure

I have experienced increased fuel flow on both my RVs with the boost pump on. I don't get fuel overflow and if I fly with the boost pump on for a long period then the totalizer reads too high. I think this is caused by the fact that the flowscan sensor is between the electric pump and the carb. The electric pump uses a diaphragm which increases the pressure and then relaxes. The pressure forces fuel past the sensor and into the mechanical pump, when it relaxes then the fuel flows in reverse which does not register on the sensor. Just my thinking.