Fuel Pressure fluctuations? problems?

Thanks Sean. This airplane was built by Rocky Mountain Kit Planes. I am the fourth owner and it could have been changed since it was initially built. When I refurbished the fuel system, I just replaced it as it was. All gravity flow with a filter (plastic) in the engine compartment near the firewall. It had a Pierburg pump. It ran perfectly. I used better clamps and tried to improve the parts I thought were not good. The metal filter I have is better positioned away from heat. 
in my test runs that included a near full power run for 10 minutes to simulate a takeoff and climb out, the gauge gradually dropped to about 3 pounds. My question, since this is in spec above 2.2 psi, is if this is a characteristic of the pressure and flow matching carb demand and stabilizing. If so, fine. It’s a simple pump design and should function properly. If my linear pressure drop is not normal, I’d like to know. Changing pumps is cheaper than finding out.

Only a "bush mechanic" so any comments/observations/ advice I may give, should be taken with a rock sized piece of salt:

".....filter (plastic) in the engine compartment near the firewall."

Glad to hear that you have replaced/relocated this - whenever possible fuel filters should not be located within the engine compartment.

The plastic bit is okay IF uses a gauze filter and is clear ie you can check the condition of the filter for trapped contaminants.

"...Pierburg pump"

I understand that the fuel injected Rotax, use at least two electric fuel pumps, so having an electric pump in lieu of the usual mechanical pump (driven off gearbox) is not such a crime BUT I would advise fitting the mechanical and having an electric back up/boost pump. This combination gives proven reliability and redundancy, if the mechanical one fails.

"...used better clamps"

Don't know what you have used however I recomend non serrated fuel injection clamps (not your standard hose clamp) or the older screw/nut type, both of which have smooth edges, an inner protective band & a good 360 degree clamping pressure. A correctly sized/fitted, good quality hose (eg Gates have a great selection) should not require clamping pressure to seal against the spigot - the clamp is more for security than preventing leaks, so should located close to the barb/bulge and only be tightened enough to prevent the hose from slipping past the barb.

"....metal filter I have is better positioned away from heat."

Good thinking but preferer it not in the engine compartment at all and a clear plastic eg Hengst H102WK, H103WK, Baldwin BF7850, BF7863, BF9906

"...if this is a characteristic of the pressure and flow matching carb demand and stabilizing."

My fuel pressure only fluctuates when I shut off the boost pump - momentary drop followed by recovery. So I would not consider a   "linear pressure drop" normal.

As your engine continues without hesitation, I would think the problem may be in your fuel pressure sensor or gauge.

Some fairly recent sensor's are very well sealed (agents moisture) - in some instances the seal has been blamed for not allowing atmospheric pressure to enter thus causing an erroneous (comparative) pressure reading.

If it's an older pressure reading system (Bowden tube) the back of the gauge must have a small opening, again to allow atmospheric pressure to enter.

Another thought - the Pierburg pump is likely to be a Centrifugal type - if its is wearing out (or has partial blockage) it may not be delivering its rated flow/pressure - this could show up as a high start followed by a gradual drop in pressure.

Thanks Sean for the reply. I am a farmer mechanic. This Rans S6 is my fourth airplane. All have had gravity only fuel systems. (The only fuel boost pump I’ve ever run was on a Beechcraft Skipper I finished my pilots license in in 1980.)

I put this fuel system back the way it has run for 14 years with minor improvements in materials. The new pump is the Italy stamped corona pump. I have a liquid filled temporary gauge in the port carb feed line to check the pressure. As soon as I’m satisfied the pump is working properly I’ll remove it and fire-sleeve that hose. 

I have read here that people find the corona pumps sometimes have fluctuating pressure they had not experienced on their previous pump. My pressure starts mid range then declines to the low but still in spec part of the psi range. I don’t have a sensor driven fuel pressure gauge on my panel. I’ve never (so far) had a fuel starvation problem in this airplane. I’m trying to see if others have this drop down to a lower pressure and if it is ok. 
ps. Yes I use the fuel injection clamps instead of the serrated. I also have Norma clamps on my silicone water hoses. Oetiker clamps where they fit. I have a pile of clamps and zip ties that came off and got replaced with better stuff. My A&P friends have almost taught me to safety wire properly. Almost.

One thing worth checking if you are having fuel pressure fluctuation problems, especially low pressure indications, is the fuel pressure sensor electrical connector boot. The Kavlico sensor, and probably others, have a ribbed sealing gasket to keep dirt and moisture out of the electrical connection pin area. The sensor measures pressure by comparing fuel pressure on the nose of the sensor against AMBIENT pressure, the difference being actual fuel pressure. The sensor senses AMBIENT pressure on the backside of the sensor in the electrical connection area. The problem occurs If that sealing gasket is too good and a higher than real ambient pressure gets trapped in that area. This often occurs when in a climb and the higher low altitude ambient gets trapped in there and cannot bleed off fast enough. As you can tell, this causes a lower differential pressure which then reads out on your panel as a false low fuel pressure. For the sensor to work properly, that "sealed" area must breath to always reflect true ambient pressure.

I had this problem for years on my Rotax engine, transient low pressure readings that always corrected themselves after a few minutes ( the gasket would eventually breath enough). Someone pointed out this potential cause to me and I made a very small cut in the gasket with a razor blade to allow better breathing and WALAH no more false low pressure readings. Try it! 

James Ott wrote:

One thing worth checking if you are having fuel pressure fluctuation problems, especially low pressure indications, is the fuel pressure sensor electrical connector boot. The Kavlico sensor, and probably others, have a ribbed sealing gasket to keep dirt and moisture out of the electrical connection pin area. The sensor measures pressure by comparing fuel pressure on the nose of the sensor against AMBIENT pressure, the difference being actual fuel pressure. The sensor senses AMBIENT pressure on the backside of the sensor in the electrical connection area. The problem occurs If that sealing gasket is too good and a higher than real ambient pressure gets trapped in that area. This often occurs when in a climb and the higher low altitude ambient gets trapped in there and cannot bleed off fast enough. As you can tell, this causes a lower differential pressure which then reads out on your panel as a false low fuel pressure. For the sensor to work properly, that "sealed" area must breath to always reflect true ambient pressure.

I had this problem for years on my Rotax engine, transient low pressure readings that always corrected themselves after a few minutes ( the gasket would eventually breath enough). Someone pointed out this potential cause to me and I made a very small cut in the gasket with a razor blade to allow better breathing and WALAH no more false low pressure readings. Try it! 

Very interesting!
I have the same problem.
Fuel pressure dropping the higher I fly.

I have a low wing and the fuel tank vents are at the wingtips facing forward and in a NACA profile.
So they get a little pressure there.
So I was thinking that it must be normal that fuel pressure drops at higher altitudes because less pressure on the thanks.

I have a likewise sensor but it is from Road Germany.  (looks just the same as a kavlico)
Are those sensors made for cars or what?
They shouldn't have that flaw!

Will try your suggestion and report back here!