Fuel Pressure fluctuations? problems?

I have had many fuel pressure senders act up. The ones I see like this a lot are ones mounted on the firewall hanging down with the hose going up. The issue with this is that the fuel sitting in that hose never gets refreshed and it sits there flight after flight cooking and cooking after you land. If you use 91 oct. auto fuel then sometimes you can even smell the old varnish smell of old fuel. This old fuel clogs the tiny hole on the sender. So I remove it and take a can of carb cleaner with the long small tube mounted in the spray nozzle. Then I put that tube up against the sender tiny hole and give it a one second blast. I do that 4-5 times which cleans it out and exercises the internal workings of the sender. 9 times out of ten this works. That said yes senders can just go bad and need replacement, but it never hurts to try.

p.s.

I mounted my fuel sender up high on the firewall above the engine level with the hose going down away from the sender. This way when the engine is off the fuel drains back away from the sender and each flight gets fresh fuel. In 1500 hrs. I never had an issue.

Good information here. Relatively new to Rotax 912's. They appear to be awesome engines and I am now sold on them. I have over 50 hours of trouble free operation. I did not know about opening up the fuel psi sensors electrical plug seal to allow it to breathe. About 8 months ago I bought a Zenith CH750 with a 912 ULS. I use 91 no lead / no ethanol fuel. The engine has a Facet cube pump on the firewall as well as the new style mechanical pump. It builds 2.5-3 psi on its own before start-up. The fuel psi looked like trouble from the beginning. The previous owner / builder is a friend, and he stated that he has had trouble with those sensors since he built it 10 years ago. The engine has never given him or me any trouble. It is doing the same thing for me as it has always done for him. He has replaced the sensor once or twice, and I replaced it when I bought it with a new Kavlico model. The sensor is located on top of the engine right in front of two NACA vents, in the fuel block that splits the fuel and also has the return line with the orifice in it. The psi seems somewhat stable at run-up, but fluctuates all the time from 1.5 to 6 during flight. It seems to be more stable with the electric facet pump on, but still fluctuates constantly.  Along with that, if I stop somewhere to get a friend or eat breakfast, sometimes it shows 15-16 psi when the Dynon is powered up again up before any pump is pumping. at all. Then it goes away and back to its wandering ways sometime later. It runs fine but drives me nuts. I replaced the sensor with a new Kavlico model and it continues to do the same thing. I've checked everything electrical and fuel related and all is good. (Although I did have an issue with what I believe to be vapor lock/ 0 psi at idle due to "winter blend" Wisconsin fuel and a warm 70 degree day recently. Engine quit while taxing after landing. We can discuss that elsewhere)  With that said, the electric pump is new three days ago, the fuel system all gone through, and i am going to replace the mechanical pump this weekend. Again, it runs great.  I honestly think the sender gets too much vibration on top of the engine, and also gets heat soaked when parked, which may be causing trouble and errant readings. I would like to relocate it to the firewall with a hose, some fire sleeve and an adel clamp. What are guys using for that hose? I prefer aviation type hoses. The fitting is 1.8" NPT. And do you think the hose should have a restrictor jet installed in case of a failure? I am always checking fuel psi., and would like to get a more solid reading as I am a little gun shy with regard to engine outs either on the runway or in the air.

I fly experimental - this means I am not limited to an aircraft  manufacturers service items.

I use, almost exclusively, Gates coolant/oil/fuel hoses. I am careful to research their specifications & purchase those that equal/better Rotax supplied items.

"..........do you think the hose should have a restrictor jet installed in case of a failure?."

Restrictor jets are usually installed in the fuel return line. The size of the jet is determined by the ability of the pump(s) to maintain a minimum of 2 psi  (usually 3-5 psi, max 7 psi) in the fuel delivery to carburettor system.  Rotax systems commonly use a #35 Mikuni jet (other sizes are available).

I stand to be corrected, I do not think the restrictor jet has anything to do with a failure in the fuel supply system. 

The return line is to aid in minimising the effect of fuel vaporisation . It does this by allowing the vapour to be bled out of the fuel system. The location of the return line is usually as close to the carburettor distribution manifold as can conveniently be managed (aften part of the manifold itself).