J

jjhoneck

Well Known Member
Ever since we installed our GRT Horizon HXr, we've been getting a low pressure warning. Often the display actually shows a NEGATIVE fuel pressure, which -- since the engine continues to run, happily -- is obviously bogus.

Then, when I turn on the electric fuel pump, I often get a warning for "fuel pressure too high". Again, nothing changes in the engine.

Meanwhile, the fuel flow indicator seems correct. It will blip up a bit with the electric pump on, and blips down a bit when you turn it off. It behaves logically with throttle changes.

The Horizon gathers engine data from/through our old EIS 4000. I don't recall this happening when we had only the EIS, but, frankly, the data presentation on that instruments was so poor, who would know?

I've tried changing the settings in the Horizon, to no avail, so I'm assuming that I either have a problem with the sensor, or I'm going to have to change something in the set up of the EIS.

Anyone seen this? Any advice in what/how to change in the settings?
 
jjhoneck said:
Ever since we installed our GRT Horizon HXr, we've been getting a low pressure warning. Often the display actually shows a NEGATIVE fuel pressure, which -- since the engine continues to run, happily -- is obviously bogus.

Then, when I turn on the electric fuel pump, I often get a warning for "fuel pressure too high". Again, nothing changes in the engine.

Meanwhile, the fuel flow indicator seems correct. It will blip up a bit with the electric pump on, and blips down a bit when you turn it off. It behaves logically with throttle changes.

The Horizon gathers engine data from/through our old EIS 4000. I don't recall this happening when we had only the EIS, but, frankly, the data presentation on that instruments was so poor, who would know?

I've tried changing the settings in the Horizon, to no avail, so I'm assuming that I either have a problem with the sensor, or I'm going to have to change something in the set up of the EIS.

Anyone seen this? Any advice in what/how to change in the settings?
Click to expand...

This topic has been discussed every few months for the last dozen years (or when someone buys a built RV....whichever occurs first...). ;)

Use 'Search' and look for "low fuel pressure" and similar terms to find a bunch of threads on this topic.

Bottom line....the sensors in many of our planes are not consistently accurate at low fuel flows due to their inherent design. Lycoming specs call for at least 0.5 psi with a carbed engine. You can expect fuel flow indications to go up with the electric pump running even though the engine is not using more fuel. It's that inherent design thing again, only this time for the flow sensor....also in the archives.

Sorry I can't help with the GRT settings, no personal experience there but I'm sure someone will chime it who can provide that info.
 
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Jay,

Do you have a carburetor?

Which settings have you tried to change in the HXr? If you don't remember the warning popping up before you installed your HXr, either the sender is going bad or your warning thresholds were set up differently. If your EIS is blind mounted, all the warning threshold limits should be set to zero, and set all your limits in the HXr instead. More info here: http://grtavionics.com/HXr%20Engine%20Instrumentation%20Supplement.pdf

As for the EIS-- yeah it's old technology, but it is quite simple to read, it's just a number. If you don't know what you're looking at, press the Display button for labels. If you don't see what you need, press NEXT until you do. If it was as bad as you say it is, there wouldn't be 23,000 units built and flying over the past 23 years. Greg chose simple and robust over a pretty face. ;)
 
I have an AFS5600, carbureted O360 -- same thing -- consistently low, sometimes high, sometimes negative, now and then showing where it's "supposed" to be. Before I upgraded to the 5600 I had a different sensor and mechanical gauge -- same thing. Irritating, seems like there would be better sensors out there by now. In the meantime, fuel pressure indication is essentially useless, makes me wonder why we even bother with fuel pressure & indicators in these things.
 
Fuel vent vacuum

Probably not exactly related to your issue - but on the topic of negative fuel pressure:

When I was investigating causes of the aforementioned "normal" fuel pressure fluctuations I attached an old airspeed indicator to one of the fuel vent bulkhead fittings that point straight down from the belly of our RV's. Yes, I had to disconnect the aluminum fuel vent line from the fitting so I could attach the ASI to the fitting (and I got a bit of fuel smell in the cockpit on the test flight with the fuel tank vent open into the cockpit - no smoking)

Basically I used the standard belly fuel vent fitting as a "pitot tube" and I found that I had to hook it to the "static" side of the ASI to get a reading which means it was drawing a vacuum. How much? At 140 IAS the fuel vent was reading 80 so that would be the VACUUM equivalent of the PSI created in the pitot tube at 80 MPH - I don't know how much PSI that is, but it pretty low, like a fraction of 1 pound. I changed the fitting to 45 degree fitting pointing forward (yes that could cause a bug to get rammed into it and plug it, etc etc but FYI these are the pressure readings) The 45 degree fitting gave about 80 MPH positive pressure at an airspeed of 140 MPH IAS and adding a small tube bent another 45 degrees forward which resulted in the fuel vent basically pointing straight forward like a real pitot tube would caused the two ASI's to read almost exactly the same. I expected the one being behind the prop blast might show higher, but it really didn't in cruise - maybe on takeoff it wold have indicated higher than the actual ASI but I didn't look at it then.

Did any of this effect my fuel pressure reading/fluctuation? Not really. When I hooked one fuel tank to a forward facing fuel vent and the other tank to the standard straight down vent I got maybe 1/4 PSI higher fuel PSI on the forward facing vent, but the tendency for fuel pressure fluctuations remained about the same. I have replaced both my fuel vents with the bulkhead fittings facing 45 degrees forward so they are at least "positive pressure" and not a vacuum, and they have given no problems in the last 50 hours. I've seen some F-1 Rockets with vents like this also.

I eventually eliminated my fuel pressure fluctuations by other means, but thought some of you may find the above info interesting and maybe even expected. It is pretty much as I expected because hi velocity air flowing over an orifice is how the old pump bug sprayers pulled the chemical out of the can and introduced it into the airstream as a mist.
 
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