fuel pressure drop

olderthandirt · #1 12-15-2016, 09:51 AM

Yes it has been discussed many times...however a couple questions linger..does the red cube fuel pump have a screen in it?? Does the red cube fuel flow sensor have a screen in it..and are these and gascolator the main issues when looking for source of drop in fuel pressure on climb out?? I pulled fuse to electric pump and not change in pressure..only time pressure changes is on full throttle climb out...other wise it runs about 3.9 psi...never a drop in engine performance...thanks for help

Mich48041 · #2 12-15-2016, 11:23 AM

How low does the pressure drop? How do you know the fuel pressure displayed is accurate? Most likely it is not. The pressure dropped will always increase with increased flow. The engine driven fuel pump has a built-in screen. If fuel pressure is inadequate, the engine pump is a prime suspect.

olderthandirt · #3 12-16-2016, 07:38 AM

the pressure drops from 3.9 psi to 2.2 psi on climb out then settles back to 3.9 in cruise ...pulling fuse for electric pump made no change, so would guess the engine pump is good?

Mich48041 · #4 12-16-2016, 11:35 AM

According to the Rotax operating manual, the fuel pressure range is 2.2 to 5.8 PSI. So your pressure is within that range. I would measure the fuel pressure with an accurate and calibrated gauge. There might not be a problem.
The accuracy of your fuel pressure sender can be checked using 10 feet of clear plastic fuel line. Connect one end of the clear tube to the sender. Fill the tube with gasoline. Suspend the other end of the tube from the hangar ceiling so that the level of gasoline is 9 feet 4.5 inches above the sender. The indicated fuel pressure should be 3 PSI, plus or minus 0.1 (if my math is correct).
Or a 6 foot 11 inch water column could be used instead of gasoline at the risk of contaminating the aircraft fuel system with water, or the water freezing.

ben barron · #5 12-16-2016, 05:19 PM

After a long bout with the same issue i just replaced my rotax fuel pump and seemingly solved the problem. From the latest 1.5 psi at roll up the new pump delivered a low of 3.5 4,5 to 5 once out of initial lift off) on test flight today and no alarm. I will be sending Lockwood/Rotax a warranty claim along with the replaced pump in hope of getting my $175 back...

TS Flightlines · #6 12-16-2016, 06:38 PM

Following this thread and others of the same subject---it just seems to me that there are alot of supposedly bad or failing Rotax pumps. Seems changing the pumps solves the issue for a while then it returns. Or we overlooking something else? I know this discussion has been going on for as long as the RV12 has been flying, so I wonder what the deal is. I dont hear about similar issues with other Rotax equipped aircraft.
Tom

RFSchaller · #7 12-16-2016, 07:50 PM

Gee, wasn't it nice to fly airplanes that had no fuel pressure indication? You just flew about without a care and turned on the boost pump when the engine faltered!

bruceflys · #8 12-17-2016, 06:10 AM

Yep, ignorance is bliss. When I built my SeaRey several years ago, I did not see the advantage of fuel pressure or flow indicators for a carbureted engine. 400 hours later I have never had a power irregularity from its 912ULS Rotax.

My wife's RV-12 has had a few fuel low pressure warnings, usually on rotation and initial climb. Other RV-12s on the field have experienced the same. A Czech Sport owner fussed with this problem for a year, including multiple trips to Lockwood for repairs, without improvement.

None of these short-duration low pressure warnings has been accompanied by power fluctuations. We collectively decided just to live with them.

TS Flightlines · #9 12-17-2016, 06:25 AM

I need to backtrack on my statement from earlier in this post. After doing some research, it does seem that Rotax has a pressure issue in some instances where during climbout the pressure can drop to an indication that sets off an alarm.
I still find it hard to believe that Rotax has that many bad fuel pumps. But, if the pump isnt supplied with sufficient fuel to begin with, then yes it cant develop enough working pressure to stay in the 'safe' zone.

So a couple of questions for thought here:
1) how much fuel pressure can the carbs take before you overload the needle and seats and cause a dumping situation? Maybe Roger Lee can answer that one.
2) On another forum there was a post about using a different boost pump that apparently solved that builders issue. So---is the Facet pump that Vans is using the same pump as in other carbureted models of Vans planes? And is that pump still wired to run all the time?
3) On these planes that have this issue, has anyone plumbed in 2 mechanical gauges---one just after the boost pump, and one just upstream from the fuel pressure sender? That would check the system and all the tube routings, shutoff valve, flow trransducer, gascolator, etc, and give a good indication of the pressure drop between the 2 gauges.
4) Since it appears that this is a problem with the Rotax 912's, maybe we can figure out how to solve it.

Other ideas??

Tom

rv7charlie · #10 12-17-2016, 07:06 AM

I know less than nothing about the 912, so take this with a grain of mogas.

That symptom has been fairly common on carb'd Lycs. In some cases, indicated pressure actually drops to zero, but the engine continues to run normally, even in an extended (many minutes) climb. My memory of the details is a bit foggy, but it's basically something to do with the pressure sensor not being vented properly to the atmosphere, and during climb, the vent pressure doesn't keep up with atmospheric, causing erroneous (low) pressure readings. When altitude is stable, the vent pressure slowly equalizes and pressure readings return to normal.

FWIW....

Charlie