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Indicating low fuel pressure - RV-6A O-320

Will Bentley

Active Member
Folks,

At our airport here in NW Arkansas there are two of us that have 6A's that are curious about why we are seeing (at cruise flight) fuel pressures indicating anywhere from "0" psi to about 1.5 psi. When we activate the boost pump pressure seems to come up to 5psi or more.

My POH says .5 to 1.0 psi is normal at cruise. Have we a problem with our fuel pumps or is this typical? Neither of us built our 6A's...

Thanks,
Bill
 
I had that also and fixed it once and for all...

I have a 6 with an 0-320 and found my fuel pressure dropping often during a climb up through 6k etc. I replaced lines, fuel pump, added a cooling shroud and verified I had not leaks in the system. I was determined to fix the problem. I left one thing to the end which cost me dearly because I was told that the pressure sensors 'never go'. I was told that the either work or do not... Well thats BS.

After replacing the pump, lines etc. I still had the problem.

I replaced the pressure sensor 60hrs ago and its been rock solid since then. I would advise doing this and verifying you have not getting leaks around the sensor etc. You should have rock solid pressure after this assuming your pump is fine.

I can't explain why the secondary pump brings it up even to this day.

Hugh.
 
Hugh,

Thanks for your input. Certainly seems reasonable. Will see what else
comes in as a response to our delima, but I'll bet you are right on.

Thanks
Bill
 
Folks,

At our airport here in NW Arkansas there are two of us that have 6A's that are curious about why we are seeing (at cruise flight) fuel pressures indicating anywhere from "0" psi to about 1.5 psi. When we activate the boost pump pressure seems to come up to 5psi or more.

My POH says .5 to 1.0 psi is normal at cruise. Have we a problem with our fuel pumps or is this typical? Neither of us built our 6A's...

Thanks,
Bill

The low fuel pressure indication in carbed RV's has been discussed heatedly since.....well....probably since the first RV-6's in the mid-80's. This forum has seen several threads devoted to this issue, here are a couple the search feature brought up:

http://www.vansairforce.com/community/showthread.php?t=31788&highlight=low+fuel+pressure

http://www.vansairforce.com/community/showthread.php?t=19988&highlight=low+fuel+pressure

I'm not going to tell you that there is no problem with your fuel system, but read the referenced threads and others you can search then decide how you want to approach this matter.
 
Last edited:
Sam,

As far as I'm concerned (with my particular situation) you are right on the money! I have seen this "0" fuel pressure, with no engine problem, for
about two years. That, to me, indicates something other than a pumping problem. Reading the archives you made available to us absolutely confirms that I shouldn't be losing an engine because of this. Unquestionably, it is mysterious, but I sure get that warm & fuzzy feeling knowing this has been an ongoing RV situation for years. I'll do as I have been doing....I'll watch it closely but not panic. If things begin to add up that a mechanical issue is at hand I'll handle it accordingly. But, from what I've been seeing, it just doesn't seem to be an issue that requires any major mechanical modifications.

And, Sam, I'm sure I can speak for many of us, I've appreciated your input on many, many issues throughout the years. You've been spot-on regarding a variety of aircraft subjects. I've just sort of hidden in the shadows and enjoyed the benefits of reading a number of your suggestions. This, in no way, takes away from the tremendous amount of information we have all gleened from all the other "above average" meticulous builders and gifted aeronatical experts out there helping the "average" RV drivers keep their aircraft safe to fly another day... Thanks for being there for us.

Bill (Will) Bentley
 
low fuel pressure readings.

I too had the low fuel pressure readings on my 0-360.
I read the threads and lived with it for awhile but then figured I should keep trying to find it.
It was a bad fuel sender.
Some are better quality than others, I suspect.
Once it was replaced, I got normal pressure readings.
Keep looking for the problem!!
What good is a pressure indicator if you can't trust it to tell you when you are having a problem.
I can't tell you how good it feels to see normal fuel pressure reading in my scan while flying. Keep after it!
 
Intermittent High Pressure Reading

Hey everyone, We've been searching the databases for info that may help us figure out our little fuel pressure issue. Any input would be much appreciated.

We run a 6 with a carberated 360. We use the Grand Rapids EIS 4000 engine monitor and have been getting an intermittent HIGH fuel pressure reading. We have the warn limits in the EIS set for low of 5 psi and High of 8. In discussing the issue with Grand Rapids Tech, they said it may be a failing fuel pressure sending unit so we ordered a new one. We switched that out but still have the same problem. Any suggestions?

Anyone else use different warn limit settings in thier EIS?

If it isn't a sending unit problem and the fuel pressure is actually intermittently higher, any ideas what may be causing it?

Anyone with similar setup mind sharing thier average fuel pressures and settings?

Any ideas would be greatly appreciated.
 
RVONE,

I have been seeing a similar issue in my 9A. Different engine (IO-360) and electronics (Dynon). My fuel pressure generally ran around 24 psi (normal for injected) but has been creeping up of late, and now is at 27. It also is reading much noisier than in the past and occasionally spikes to 33-35 (35 is the supposed upper limit for my engine). Boost pump seems to smooth out the fluctuations but I haven't experimented extensively. I posted a thread here a couple weeks ago and it was suggested perhaps air in the fuel pressure line and harmonics being set up. Haven't had a chance to do anything about it yet, but I plan to add a flow restrictor in the line (rivet with a tiny hole in it) and see if that helps. Maybe its just something about Reno....

greg
 
Thanks for input

Greg,

I appreciate the ideas, I'll have to look into them. Air in the fuel pressure line is an interesting theory.

It could just be Reno.....

Thanks,

Chris
 
Not just Reno

I've noticed the same thing but haven't yet investigated. I seem to get a high spike every now and again without the fuel pump on. Thinking about it I have never tried to bleed the air out of the line to the firewall transducer. I'll try that first.

Superior IO360 Dynon DEK180

Peter
 
I get the spikes when it is bumpy. IO-360. I think it is the sensor only likes the smooth upper air and then it is rock solid.
 
Webb,

I just went up twice today. Morning flight, smooth with maybe a few bumps and the pressure is still spiking occasionally. Landed, changed the oil and a bunch of other stuff including taking off the fuel pressure line from the gauge. I had intended on adding a small rivet as an orifice restrictor, but they (rivets) are all at home, so I just closed it back up. Went flying this afternoon, similar conditions mostly smooth with a few bumps. Now the FP gauge is rock solid at 22 psi. Go figure! I have no idea what I did that caused this.... We'll see if it stays solid.

greg
 
I have a IO-360-M1B and GRT EIS 4000 and see my fuel pressure goes up to 50 PSI when the fuel pump is on for about a second or two and then comes back down. Otherwise it says steady at about 27 PSI without the pump. I am not sure what is going on and if this is an issue or just the fuel pressure gauge, thinking of changing it which is pretty new.
 
What about fuel flow readings?

Not being sure that my skin is as thick as Sam B's, I throw in a few observations anyway.

First off, in a former life at GM-Pontiac, we learned that vapor pressure of fuel increases with temp, altitude, and octane. In the mid 80's, we had vapor lock issues particularly with the Firebird 5.0L, carb and later FI, that would actually cause the car to run rough at altitude on a hot day, eg Tahoe/Denver in the Summer, or be difficult to restart at all. On the carb model, could actually smell fuel vapor if it had been sitting for a few minutes shutoff, hi alt/temp. Fuel lines would get hot, especially fuel injection lines to injectors, and boil fuel inside, etc.. Precisely metered FI systems and injectors, carbs with throttle body injectors, or float type did not like that, for various reasons.

With airplanes we have all the same conditions present, i.e. even higher octane, altitude, temps. Recipe for vapor lock. Remember that in a pressure reading situation, vapor/air is far more compressible than fuel, so if vapor is acting on the FP sensor, not as much pressure will be sensed, assuming sensor is working as designed. And if the fuel pressure sensor happens to be at the highest point of the system, well there you have it, especially if you've ever tried to keep the air bled out of that high up oil pressure line/sensor, avoid that few second wait for oil pressure at start up. If you say well how does air get in the closed fuel system prior to the engine pump or sensor, consider how a diver gets the bends fm nitrogen bubbles forming in a closed blood circulatory system. I'll defer here to the chemical engineers and the seasoned SCUBA divers among us.

In climb vs level flight, angle of air entering AND exiting cowling changes, as does flow rate. In fact, the flow pattern in the engine compartment could very well change. This goes to cooling of fuel pumps, sensors, cylinders, anything under cowling. With a C/S prop, prop pitch changes also change air flow.

Since we can't really control altitude or octane, and if this is a vapor lock issue, cooling is left, with one caveat, pressure on the supply side of the system. More pressure on the supply side, especially at higher FF/power settings tends to alleviate vapor lock in many applications, i.e. at the FP sensor. If the engine mechanical pump is demanding more flow, it tends to put more suction on the supply side, better chance of forming vapor bubbles somewhere, i.e. FP sensor. This is alleviated somewhat by an electric (pre) boost pump.

But getting to MY question, for those with fuel flow and fuel pressure sensors co-located, what is the fuel flow doing while the fuel pressure is fluctuating? In my case, this fluctuating reading issue is definitely better after just refueling on a cool day (i.e. fuel farm tank fuel is cooler), and worse in climbs, and especially if doing a lot of pattern work with prop at lower/flatter pitch settings, higher deck angles.

Here it's an O-360 with C/S prop, VM1000 system (early), and with a prior history of a fuel flow sensor swap to no avail. Fuel pressure sensor has never been swapped out to my knowledge. Boost pump off, FP can be anywhere from near zero to 5 psi. Boost pump on, always an increase, max 6 psi. Now fuel flow, that has a mind of its own. Only other comment is that my fuel line takes a 90 degree turn a little sooner after the FF/FP sensor manifold than might be ideal. A no-no at least with my swimming pool water flow sensor, flow disruption, etc..

Thanks to all that risk abuse by giving answers.
 
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