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High fuel pressure

bdserv

Member
I bought a flying RV-6A with an Aerosport 0-360-A1A and an MVP-50 engine monitor.

Lately I have been getting high fuel pressure warnings. The engine will be running along at 5 psi for some time and then there will be one or more excursions to sometimes 12 psi and higher. Then back to 5 psi for awhile and then another excursion. No predictability - can happen at any time and comes and goes during a flight.

During the excursions the engine continues to run just fine. I think I see an increase in fuel flow during the excursions but cannot be sure because my fuel flow readings have never been all that stable.

I assume this engine uses the standard Lycoming low pressure mechanical pump. If so, is it possible for the pump to fail in a manner that would produce higher fuel pressure - as high as 12 psi or more? Doesn't seem like it could but thought I'd ask - seems like maybe a sensor or perhaps a wiring issue?

Also, would the engine be expected to flood with 12 psi or more at the carburetor fuel inlet?

Appreciate any help or comments.

Thanks,
Bruce
 
The engine driven pump uses the internal spring to generate pressure. The engine only resets the spring once the pump delivers fuel. So it cannot develope more pressure than designed.
So I'd suspect you have a gauge issue
Tim Andres
 
Hi, the last posting is dangerously incorrect!
The engine driven fuel pump diaphragm is open to crankcase pressure and if the crankcase vent is restricted the increased pressure in the crankcase is superimposed on the pump diaphragm adding to the spring pressure.
This can result in greatly increased fuel pressure.
It appears to be most common in O-235 Lycomings with the vent line over the crankcase from the front, I have seen one case on the ground of this. Also a friend was flying a Longeze back across Australia when a vent blockage occurred, he noticed the increased fuel pressure but did not realise the cause or significance.
The increased pressure in the crankcase blew the crankshaft front seal out and all the oil and he had to put down on a dry salt lake in the desert many miles from civilisation.
Do not think blowing back through the vent line will prove the vent is ok. In the case of the Easy, he blew back through it at the previous airport after I warned him but the lining in the vent hose was acting as a one way valve which was clear in reverse. Please do not ignore this issue.
 
Hi, the last posting is dangerously incorrect!
The engine driven fuel pump diaphragm is open to crankcase pressure and if the crankcase vent is restricted the increased pressure in the crankcase is superimposed on the pump diaphragm adding to the spring pressure.
his can result in greatly increased fuel pressure.

I'll take my whipping on that one I guess. I can sure see how it could happen, but enough to develop 12psi? I don't know the area of the diaphram but it must be ~6-7 sq in or so, that would mean something like~2 psi crankcase pressure on the diaphram to develop that much fuel pressure right?
It ought to be leaking oil every where by then but certainly needs to be investigated.
Thanks for the correction

Tim Andres
 
I would think engine crankcase pressure with a blocked crankcase vent could be huge, depending on amount of blockage and ring blow by.
It is certainly enough to blow the front seal out of the crankcase as this has occurred on many occasions.
Cheers
 
Do not think blowing back through the vent line will prove the vent is ok. In the case of the Easy, he blew back through it at the previous airport after I warned him but the lining in the vent hose was acting as a one way valve which was clear in reverse. Please do not ignore this issue.

Thanks for the heads up on this. I wonder if you could do a quick check by blowing into the line to check one direction, and then blowing into the dipstick tube to check for blockage in the normal direction?

I have had the "one way valve" situation happen in hoses in the past - particularly brake hoses.
 
Hi Bruce
For what it's worth, had a similar issue with fuel pressure on my O320 and GRT EIS. They use a VDO sensor with a life of 200-500 hrs. Mine made it to 700+ before it started giving intermittent erratic readings on the high end. Replaced the sensor and all is good. I'm not familiar with the sensors on the MVP50. Please keep in mind that I wasn't smart enough to check the breather issue previously discussed. Since the engine was running fine, I just assumed it was the sensor. Hope this helps and good luck.

Don
6A
 
I had a similar experience with 3 failed vdo fuel pressure senders. When they fail they appear to typically become intermitently high. I was not so lucky with lifespan... I only have 350 total hours on the plane! I finally gave up and installed the more expensive but more reliable sender that grt sells with 3 wires.
 
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I've seen this, too

A friend of mine had high gauge readings on his O-320. Verified it with a master gauge and replaced the pump. All normal after that. Never thought a mechanical pump could fail high, but that one did.

Ed
 
Lots of parts in a mechanical fuel pump and I believe that they can fail high for various reasons. You need to confirm whether it is sensor or the pump. Most needle/seats on carbs won't hold 12 PSI and your bowl will overfill causing a rich condition. A full bowl, due to leaking needle/seat, is enough to cause very rough running during full rich T/O conditions; a Significant safety issue if you don't catch it and lean it.

Larry
 
Had high fuel press problem about a year ago. Eng flooded on TO ran rough, aborted. cleared runway and eng stoped.CLOSE. Pulled carb, every thing checked good, put in new nedle valve and seat and blue floats while there. Same problem next TO. Rerouted fuel lines to by pass fuel pump, ground run eng with elec fuel pump- ran good. Changed fuel pump, no more problem. This was on an 0-360 eng. Still flying. Be safe Gordy
 
Had high fuel press problem about a year ago. Eng flooded on TO ran rough, aborted. cleared runway and eng stoped.CLOSE. Pulled carb, every thing checked good, put in new nedle valve and seat and blue floats while there. Same problem next TO. Rerouted fuel lines to by pass fuel pump, ground run eng with elec fuel pump- ran good. Changed fuel pump, no more problem. This was on an 0-360 eng. Still flying. Be safe Gordy
 
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