Fuel pressure mystery

Hi Des,  (frustration - just lost my response, will try again).

Will give your suggestion a go - thanks.

This whole fuel pressure "thing" has been going on for months. 

Despite a plumbing rework, which included a new higher pressure Boost pump and one way bypass circuit, I still get a concerning drop (2 - 2.5psi) in fuel pressure, when I turn the Boost pump off above circuit height. The pressure comes back, (4 -4.5 psi) over a couple of seconds. Had got used to managing that, when todays out of the blue drop occurred.

Hi Sean

With the return line and its orifice I would more likely suspect the system is reacting to the switch off of the boost.  Where the sensor is located and the orifice in the sensor may be an issue.  i believe someone suggested cleaning the orifice of the pressure sensor and that might be a good idea.  The sensitivity  is more likely the issue.  2 to 2.5 is ample to run the engine.  4 to 4.5 is fine.  The max that mechanical pump can do is .5 bar (about 7 psi)   

Keep us posted.

Cheers

I see quite a few planes with mechanical and electric pumps do this pressure drop then quickly recover. It doesn't seem to be a flight issue. I have also seen many VDO fuel pressure sending units hole and interior mechanism need cleaning and some exercise. I take a can of spray carb cleaner with the 6" long red straw nozzle and put that right up against the send hole. I give it 4-5  one second blast. Nothing longer. This seems to clean the orifice and exercise the internal moving parts. 

It seems to work especially if your VDO sender hangs downward and never gets any fresh fuel. If it hangs down then the fuel in the downward hose gets old and it bakes under the cowl heat. 

To solve that on my plane I mounted it on the fire wall about 2" higher than the engine. It ran like that for 1500 hrs and never an issue. Mounted up like that kept fuel from accumulating around the sender orifice and kept fresh fuel up in It at each flight. When it sat in the hangar the fuel drained away  from it.

p.s.

High wings only with fuel tank in wing. I did a research project years ago and found that the engine will run with just head pressure and gravity. Plane was a Flight Design CTSW.   The first test I just removed the in and out fuel lines on the mechanical pump and joined them together. I could get full throttle like this. Then I put the hoses back on, but removed the fuel pump off the plane and of course sealed the hole where the pump came off. So now the pump was totally dead. I ran it again and I could get full throttle without so engine missing, but you could get enough rpm in air to get 5K rpm.

You can not do this on a low wing or a high wing if the fuel isn't up in the wings.

So a momentary fuel pressure dip isn't  something to lose sleep over.

Roger/RW,

The pressure drop when I turn off the Boost is manageable BUT the out of nowhere drop, during cruise is the real issue.

Will do a clean of the sensor as you both suggest..