another low fuel pressure warning

I installed the new fuel pump (P/N 893110, S/N 13/1939) in my RV-12 S/N 120241 on 5 Nov. 2013, Tach time 209.9, to comply with Rotax SB 912-0630L. Tach time is now 268.7. Everything has been fine until this morning when I got a low fuel pressure warning on takeoff (17 gallons of fuel on board). Post landing check showed that the electric pump was running and providing 3.1 psi with engine off. With engine running at 2500 rpm pressure was in mid-4 range. Advancing throttle to WOT dropped pressure back to low 3 values, returning to 2500 brought pressure up. I have consistently been seeing fuel pressure 4.5-4.9 range since installing the new pump.

This no evidence of leakage, no fuel stains, no drips, no fuel at pump drain, and no fuel on carb drip trays. Indications seem to me that the new mechanical pump is failing.

Open for suggestions-- if problem re-occurs I'll check wiring to sensor but that has not been a problem before.

Thanks

Wayne 120241 N143WM

Before you spend a ton of time looking where the issue might be you need to put a mechanical gauge inline. 10 psi gauge. It's easy to do and it will help diagnose whether it's really the pump or a sending unit. It can help diagnose to see if it's a poor ground connection.

Wayne, Download the data for the flight in question from your Dynon. It's a big spreadsheet but you can compare your fuel pressure with fuel flow and other indices at the exact time of the low FP occurrence. This may give you some idea whether there was truly a drop in FP or an electronic hiccup. If there are other flights on the download, take a look at takeoff FP readings for suspicious drops which may not have quite triggered the alarm.

When you test with the inline gauge Roger recommends, compare that Dynon download with the subject flight and your observations of the second gauge.

Jim
#264
flying 280 hours

Not so easy

So far the problem has only been observed during takeoff. Kinda hard to plumb a gauge into the cockpit through the firewall and panel. I'm going to see what happens today when I pull the electric fuel pump fuse. I'm very definitely NOT computer orientated and the few times I've tried to download Dynon data I've gotten a HUGE mass of numbers of which I could make no sense at all. I'm better with a slide rule!

Wayne

Most of the owners I have dealt with having this particular type of issue (loss of power at full power take off) has been because of fuel starvation due to debris. You most likely won't be able to duplicate it on the ground because the carb bowl has a depression in it that for debris to sit in and you aren't pulling enough fuel long enough to cause the debris particle to get trapped in the upward movement of the fuel through the main jet. The fuel in the bowl on the ground is sitting still and on take off it is bouncing around in the bowl getting that debris up off the bottom and moving right into the fuel flow stream.
Have you removed the carb bowls and looked in them? This would absolutely be my first check. It can also be debris at the banjo bolts. If you have no debris in the bowls then remove each banjo bolt with a fuel line and look inside the bolt openings.

Anytime you have removed a fuel hose on and off a fitting and then experience any power loss such as yours the first place to check is the bowls for debris. You just changed the fuel pump and had to work with the hose so this should be #1 on the suspension list. I have seen many an issue at hose change from debris from poor sterile installation techniques. (wrong cutting tools, wrong type of clamp, wrong hose, over tightening of clamps)
After a hose change this seems to happen around 5-20 hrs.

Here is an easy way to check the bowls. It is shown on a CT, but the general idea still works on an RV12.

https://www.youtube.com/watch?v=XBT-yQ_X3N8


I hope this helps you trouble shoot and get back in the air.

In a fuel system, the greater the fuel flow, the greater the pressure drop. Any obstructions to fuel flow will increase that affect. I recommend checking the gascolator screen and performing the flow test as described on page G4-1 of the RV-12 PRODUCTION ACCEPTANCE PROCEDURES (PAP). Your fuel pressure sender seems to be working fine because it indicates pressure changes with RPM.
As for the Dynon data, I open the downloaded file with MS Excel which shows the data in columns labeled oil pres, oil temp, fuel pres, volts, rpm, fuel flow, gals rem, fuel lvl 1, egt 1, egt 2, cht 1, cht 2, hobbs, airspeed, altitude, vert spd aoa, oat and etc. I check the data after most flights to look for any parameters that are out of spec.
Joe Gores

The fuel pressure sender used in the RV-12 is notoriously inaccurate. We expect a digital display to be accurate to within a decimal point. But the display gets its info from the sender which is analog. I do not put much faith in the fuel pressure display. I mainly look for a change in pressure from electric pump only before engine start to an increase in pressure after engine start with both pumps running. I do not pay much attention to the actual numbers as long as the engine runs fine.
Joe Gores

" I do not pay much attention to the actual numbers as long as the engine runs fine.
Joe Gores"

I think Joe is right. Look for trends and not absolutes.



When Rotax (Kodiak) tested the first batches of Corona pumps they built a very expensive test stand with some high dollar gauges. I have seen it. They did find some pumps with too high a pressure due to a very tight diaphragms in some of the first batches of pumps. (That's was fixed a while ago) .
What they didn't intend to find is how inaccurate many of the fuel pressure senders are. They then tested a number of senders and the pressures were all over the board. I now only use the senders both oil and fuel as general indications and look for trends and not absolutes. Because many of these have been bad I carry 2-3 spares in the shop at all times.

If you have a bad indication it can be debris, vapor lock, bad sender, bad ground or bad pump.

If just swapping the sender, inspecting the bowls or tightening the grounds doesn't turn something up that fixes the problem then a mechanical gauge inline almost become a must or you may spends weeks looking for the problem.

If the engine doesn't cough and sputter with an extremely low pressure for an extended run time then think more along the electrical side.

Your issue is only at full power during climb. That still says debris to me.

All our comments here all have merit. You'll have to make a list and go down through it for a diagnoses, but do the easy simple cost effective things first. Make you list starting with the number one cause and work down to the last possible item it could be.

I have found all these years that solutions on our engine tend to be simple answers, but most want to start out with the most complicated and least plausible.

Low fuel px, more info

I too have experienced the same thing....low pressure alert at T/O power.
Not predictable. Happens with auto gas and 100LL. Most of the time it's in the initial climb, but has happened otherwise.
Mechanical pump replace, boost pump replaced, same event. Carbs are clean and well tuned.
Only thing I haven't looked at (yet) is the xducer. I'll have a look at that next (and grounding). Mine also regains pressure when the RPM is decreased a bit. Over about 2-3 minutes, it recovers back to normal pressure.
One thought is airflow across the fuel overflow tube causing some sort of px differential at the pump. I know, far out.....but trying to brainstorm all the possible influence.
Will advise if I find something useful.

Didn't lose power

Only indication was low fuel pressure alarm-- then went to that screen and saw 2.1 psi indicated. As stated yesterday, never got back above 4.

4 takeoffs and landings today with 2.3 hours flying and everything normal. Will post pressures with & without boost pump tomorrow but all indications today are fine. Mechanical or electrical glitch? will check grounds to sender but didn't see anything yesterday. Almost like mechanical pump didn't want to go flying (pretty windy) and took the day off.

Wayne

Air or a small vapor lock area can cause this, but it shouldn't be long term.

If you haven't upgraded to the Kavlico sensor, suggest you do it. That solved my very similar problem. Be sure to install the recommended noise capacitor while you're at

Go here and next two entries to see my story.

FWIW, yesterday's ground test readings

RPM Pressure both Pressure mechanical only

1800 4.8 - 4.9 4.3-4.6

2500 4.7 - 4.8 3.9 - 4.1

4000 4.8 ` 3.0 - 3.1

WOT (4950) 3.7 3.1


Wayne

Those pressures are in the normal range, but John may have a very valid point about the sender.
This looks like more a sender issue. One last thing, If you have a poor or weak ground the pressure can very like this.Not necessarily the main ground, but the ground directly affecting this particular sender.

BigJohn said:

If you haven't upgraded to the Kavlico sensor, suggest you do it. That solved my very similar problem. Be sure to install the recommended noise capacitor while you're at

Go here and next two entries to see my story.

Click to expand...

John, how did you install the capacitor? In series along one of the two wires or parallel across the wires coming from the sensor? Any pics would be appreciated. Thanks!

Jetguy said:

John, how did you install the capacitor? In series along one of the two wires or parallel across the wires coming from the sensor? Any pics would be appreciated. Thanks!

Click to expand...

The capacitor goes in parallel (across) the sensor. I can't remember exactly how I wired it physically, and now it's buried inside some shrink wrap. I checked and I didn't get any pictures before I covered it up. My bad.

The method of the connections is not really important, as long as it is physically strong.

Hope this helps.

Years ago I used to be into Heathkit electronics. The electrolytic capacitors have a polarity to the connections. I connected one backwards and it went off like a firecracker.

I've been going through a bunch of old threads about low fuel pressure on takeoff, as I have been seeing this issue lately. So far, I have checked the carb bowls (clean), checked the top banjo bolt (clean), flushed the fuel pressure sensor line, re-crimped the fuel pressure sender connections, removed the fuel pressure sender environmental seal. Just last month I did the annual and the gascolator screen was clean. Haven't checked it since.

Open to other suggestions, but my planned next step is to blow out the jets in the carbs. Only problem is: I have no idea how to do that. Can anyone point me to a good step-by-step guide to accomplish this?

Thanks!

You might be trying to fix a problem that does not exist. The fuel pressure senders are notoriously inaccurate.

Hi Joe,

I understand that's a possibility, but since it is repeatable under the same conditions (take off only), doesn't that seem unlikely?

The greater the fuel flow, the greater the pressure drop. At takeoff power, the fuel flow is maximum; therefore pressure is lowest. Whatever the fuel pressure indication, if the engine runs fine, there is not a problem unless low pressure is verified too low with a known accurate gauge. Just for reference, high wing airplanes that do not have fuel pumps depend on head pressure. If my math is correct, 36" equals about 1 psi.
I am not saying to ignore a potential problem, but the fuel pressure should be verified before trying to fix the fuel system.