WileEcoyote75

WileEcoyote75

Member
Last Sunday I went up for a short flight, OAT was 18F, Tanks were both half full and the engine is an O-360 with a gascolator. I use the booster pump to prime only and then shut it off immediately after startup. I was on the ground for at least ten minutes for run up and back taxi with no fuel pressure issues. The booster pump was started for takeoff. I was climbing out through 2000 agl with around 5psi of fuel pressure when I shut down the booster pump. The pressure immediately starting dropping off and was down around 0psi within 30 seconds, when I restarted the booster. The pressure climbed back to around 5psi. The engine ran perfect the entire flight. I stayed close to the airport and continued to climb, at around 6500 agl I shut down the booster pump again with the same results. On the third attempt the pressure stayed at around 3.5psi using only the engine driven pump. Will the engine driven pumps start to act up before failure? Bad Pump? Fuel Pressure Sending Unit? Air in the system? Is a Vapor Lock possible when its that cold? Let me know what you guys think.

Chad
 
WileEcoyote75 said:
Will the engine driven pumps start to act up before failure? Bad Pump? Fuel Pressure Sending Unit? Air in the system? Is a Vapor Lock possible when its that cold? Let me know what you guys think.
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Chad,

I had similiar issues on my first flight. Idle RPMs and everything was OK. The problem didn't show itself until WOT. In the case of the RV-10, I found previous incidents that were caused by either a bad pump or vapor lock.

The two of the people I talked with that experienced vapor lock, replaced their pump multiple times before installing a cooling shroud, which resolved the issue.

I probably should have taken a more scientific approach than I did to determine the exact root cause, but I wanted to complete Phase I before the weather started to become problematic. Lycoming replaced the fuel pump under warranty and I installed a cooling shroud at the same time. It was a PITA to uninstall and reinstall, so I didn't want to do it twice. My problem was solved, but unfortunately I don't know if vapor lock was an issue or was it simply a failed pump.

bob
 
I've had the same problem since day one, 1150 hours ago. I have an 0-360, MA4-5 carb with the enrichment kit and Lycoming factory pump. On mine it only happens at WOT, above 4,000 ft, with OAT below 25-30F. I have a Piper cooling shroud and 2" SCAT (with some restriction to cut the airflow) feeding it. As soon as I move the throttle off WOT the pressure recovers right away even with the electric pump off.
 
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I know this is probably not part of your problem, but

Please look at Lycoming SB 548A. I had an odd loss of fuel pressure
from engine driven pump and this might have been it.....not sure....
 
Low *indicated* fuel pressure is a common issue. Use the search function on the site and search on "low fuel pressure" and you will find an abundance of discussion on the subject.
 
O-360 Fuel pump

Chad
SB548A is worded a little odd but:
"models affected are O-235 ; O290 ; O-340 ; (L)O and IO-320,
-360"

I have a O-360 A4M with a LW-15472 pump dated "4900" "must be replaced"

Yes, just a mandatory service bull.......
It may be why I lost fuel pressure and used the aux pump, just saying.
 
I should have made it clear in my earlier post that I am actually experiencing a partial power loss, not just low pressure indication. It is very predictable so I just turn on the boost pump during WOT operation at low OAT's. I built up my engine in 1999-2000 or so, and installed a new Lycoming pump. I'm going to check on that SB right away. Thanks for the heads-up.
 
Fuel pump in question

Don't thank me. I was told about this a few weeks ago on "Doug's" site
after someone else had similar problems. Missed this SB, but will try very hard to not miss another one, and pass it on to others.

"Stay safe" and I mean it !
 
I experience 0 to .5 psi fuel pressure on climb out after electric pump is turned off. When I level off or pull the mixture back, it goes to normal psi. Never experienced loss of power.
 
Same issue

I have the same issue with loss of fuel pressure on climb out with the electric fuel pump off. I have never lost power. My GRT EIS is set for 1 psi increments so it does not show .5 psi. I intend to chnge the resistor so as to get 0.1 psi increments to see if the pressure is still above the .5 psi minimum set by Lycoming. I often wonder if part of the problem is having the fuel pressure sensor mounted so high or if I might have an air pocket at the top of the line. If it ever gets warm here I plan to check these possibilities out. Currently, if I turn the electric fuel pump off in a climb and don't like what I see, I turn it back on until I level off.
 
My suspicion is that this "problem" of low fuel pressure indications during climb is largely due to the use of automotive pressure senders, with very small vent passages to the atmospheric side of the pressure sensing diaphragm. These pressure sensors measure the difference between the fuel pressure and atmospheric pressure. The atmospheric pressure decreases as we climb, so air must escape from the atmospheric side of the pressure sensing diaphragm. If the vent passage is very small, it could take some time for the pressure to equalize between the sender and the atmosphere. The temporary high pressure trapped in the atmospheric side of the sender will give a false low pressure indication.
 
Kevin Horton said:
My suspicion is that this "problem" of low fuel pressure indications during climb is largely due to the use of automotive pressure senders, with very small vent passages to the atmospheric side of the pressure sensing diaphragm. These pressure sensors measure the difference between the fuel pressure and atmospheric pressure. The atmospheric pressure decreases as we climb, so air must escape from the atmospheric side of the pressure sensing diaphragm. If the vent passage is very small, it could take some time for the pressure to equalize between the sender and the atmosphere. The temporary high pressure trapped in the atmospheric side of the sender will give a false low pressure indication.
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Kevin, that sure would explain it. Sounds logical to me. Is there a "fix" for the issue?

My wife points to the FP gauge periodically until I turn the aux pump back on for a few minutes. Pressure goes up. Then she is happy again.:D
 
GLPalinkas said:
Kevin, that sure would explain it. Sounds logical to me. Is there a "fix" for the issue?

My wife points to the FP gauge periodically until I turn the aux pump back on for a few minutes. Pressure goes up. Then she is happy again.:D
Click to expand...

Options for a fix appear limited to:
  1. Replace the automotive sender with one designed for aircraft that climb at 2,000 ft/mn, or more. The aviation sender must also be compatible with the fuel pressure gauge. This sender, if one exists, would likely be very expensive.
  2. Determine how the automotive sender is vented, and modify it to increase the size of the vent. This modification risks damaging or destroying the sender. It also risks introducing debris into the diaphragmn cavity, leading to failure later. The larger vent hole size may also increase the risk of dust or debris getting into the sender, decreasing its reliabilty.
Neither of the above options is palatable to me. I would either explain to your wife that this is simply an indication problem due to the use of an automotive sender, and that the fact that the engine keeps running happily is the best indication that the actual fuel pressure is adequate. Or, simply run the boost pump during climb to keep her happy. Of course the more hours you put on the boost pump, the more likely it is to fail someday.
 
rvsxer said:
I should have made it clear in my earlier post that I am actually experiencing a partial power loss, not just low pressure indication. It is very predictable so I just turn on the boost pump during WOT operation at low OAT's. I built up my engine in 1999-2000 or so, and installed a new Lycoming pump. I'm going to check on that SB right away. Thanks for the heads-up.
Click to expand...

Kevin Horton said:
My suspicion is that this "problem" of low fuel pressure indications during climb is largely due to the use of automotive pressure senders, with very small vent passages to the atmospheric side of the pressure sensing diaphragm. These pressure sensors measure the difference between the fuel pressure and atmospheric pressure. The atmospheric pressure decreases as we climb, so air must escape from the atmospheric side of the pressure sensing diaphragm. If the vent passage is very small, it could take some time for the pressure to equalize between the sender and the atmosphere. The temporary high pressure trapped in the atmospheric side of the sender will give a false low pressure indication.
Click to expand...

The low pressure readings on RVs have been a known issue for many years since electric pressure sensors came into common use. Kevin described the accepted reason why we often see very low indications.

I often saw these readings, even zero at times, on my uMonitor and didn't worry about them since the fan was stilling blowing rigorously. But if there is a drop in power something has happened other than a low indication.

I knew the mechanical pump was failing after a couple of episodes of power drop which could be remedied by turning on the electric pump. The pump had been telegraphing its coming demise for awhile with inconsistent pressure readings other than the common low readings.

Installation of a new Tempest pump cleared up the problem and restored pressure to consistent readings. Don't worry about the low readings if the engine continues to run smoothly (Lycoming specs allow pressure as low as 0.5 psi) but any loss of power means it is time to promptly shop for a new pump.
 
Mechanical fuel pump low pressure

I'm getting to the party late on this since I've only had my plane eight months. I bought it - didn't build it. The builder did a superior job as well as being a superior guy too. RV7, IO360. Fuel lines are shrouded, routed through the coolest part of the cowling and blast air to pump; yet - the mechanical pump does not supply enough pressure to keep the engine running when operating in "hot" conditions. Tried several different pumps. He thinks (and I agree) that's it's just the nature of the diaphragm becoming too pliable when hot. Since it's an experimental - has anyone heard of making a diaphragm out of a less pliable, more heat resistant material? Perhaps a crazy idea, but never hurts to ask.
 
Fuel PSI

Below is something I posted a while back:

FYI- This relates to a low pressure system - not fuel injected.
Also - My fuel pressure gauge is an aviation gauge that has live fuel pressure going to the back of it - no sending unit.



This is not a new topic here on the forum. Perhaps this thread will add to what's already out there.

My RV-4 experienced in flight fuel pressure fluctuations. From reading the forum and talking to Van's techs it seems I'm not alone. The typical occurrence was a drop in fuel pressure to about 1.5 - 2 psi when in a climb with the boost pump off. Activating the boost pump ALWAYS fixed the issue. Occasionally the pressure would drop in level flight. Van's described the occurrence as fairly common, not entirely understood, and maybe nothing to worry about. That is probably reasonable advice. Finally in level flight I experienced a fuel psi near zero, my EGT rose and I experienced engine roughness (from an excessively lean mixture I assume). I wasn't sure the engine would continue running without intervention. Pushing the mixture in probably would have helped, but I just turned on the electric boost pump and it immediately gave me 5 psi and all was fine, except it made me think maybe I needed a SPARE electric boost pump since it seemed an electric boost pump failure could at times lead to a forced landing....

So, I no longer was of the "probably not really a problem" mindset and began to consider the problem again. I have numerous pieces of construction equipment that use the same facet electric fuel pump as on my RV-4. Anytime this pump fails (usually due to electrical problems, circuit breaker, etc. - I've never actually had the pump physically fail) my equipment will not run very well or for very long, even with an engine mounted mechanical fuel pump. I attributed this to internal resistance in the pump, since it has no internal bypass. Incidentally, from blowing into a couple of new and used pumps I found the newer the pump, the higher the internal resistance. I did not "quantify" this resistance with any gauge, but just what it seemed from blowing through them.

I installed a check valve (Andair valve from Vans) parallel to my electric pump to provide a fuel bypass so fuel did not have to flow through the pump when the pump was turned off. Schematically this is very simple. In actuality it was a bit tricky due to the limited space (especially the proximity of the pump to the fuel valve) in my RV-4. I used a couple "uncommon" aeroquip aluminum fittings that had to be special ordered and moved the electric pump about 8" aft of its original location, but it all worked out well in the end.

I've only flown about 25 hours since this change, but the common occurrence of fuel pressure falling to 2 psi is now a very rare occurrence of fuel pressure falling from 5 psi down to maybe 4 or 4.5 psi, never lower. At idle with the electric pump off (and the engine driven pump running slowly) I occasionally see a fuel pressure down to 2 psi but never in flight.

I believe the internal resistance of the electric pump is a significant cause of the in flight fuel pressure fluctuations. I am not a fluid engineer, or engineer of any type, so this info is "for what its worth". Perhaps it will be of use to someone...

I would be happy to provide more detail or pics of my assembly.

Pics here: https://picasaweb.google.com/1176086...3490/FuelPSI02
 
fby757 said:
I'm getting to the party late on this since I've only had my plane eight months. I bought it - didn't build it. The builder did a superior job as well as being a superior guy too. RV7, IO360. Fuel lines are shrouded, routed through the coolest part of the cowling and blast air to pump; yet - the mechanical pump does not supply enough pressure to keep the engine running when operating in "hot" conditions. Tried several different pumps. He thinks (and I agree) that's it's just the nature of the diaphragm becoming too pliable when hot. Since it's an experimental - has anyone heard of making a diaphragm out of a less pliable, more heat resistant material? Perhaps a crazy idea, but never hurts to ask.
Click to expand...

Hmmm.... let see now, we have thousands and thousands of fuel injected lyc's flying around with the stock pump for a zillion years with no problem but it doesn't work on yours.

I think I'd re-evaluate your setup and find out what's wrong.
 
Walt - You are correct. I finally got a look at it with a consultant and a correct fuel line routing may take care of the issue. Much better to get that right first. I'm supposed to be in 52F tomorrow, but weather will probably hold me back. Thanks.
 
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