High fuel pressure indications

I have your AFS 4500 in my IO-390 powered RV-7 that has approximately 165 hours on it. During the past 25 hours of flight I have noticed unusually high fuel pressure readings. Most noticeable after I turn on the electric boost pump for landing. Nothing unusual when I turn on the electric boost pump before starting the engine. Nothing unusual when I turn on the electric boost pump for takeoff. Nothing unusual if I do not turn on the electric boost pump prior to landing, but turn it on after landing I get unusually high fuel pressures. Even when I turn on the electrical boost pump just after engine shut down (some readings as high as 260 psi). Is this a function of the VDO fuel pressure sender?


Thank you,
Terry

Our guess would be the VDO sensor is dying. Inside the VDO is a arm that rides on a resistive coil, as the engine vibrates it tends to wear a spot into the coil and cause faulty readings. Try swapping out with a new sensor and you should see this problem go away.

Hi Terry,

Add another vote for a failed/failing VDO sensor. After 110 hours, I had similar indications as you - readings that were off the chart for a sensor that couldn't possibly measure the indicated values. Seeing that you're injected, I'd think that your VDO sensor is 0-80 PSI; pretty impressive if it measured 260 psi. I ruled out grounding issues for two reasons: 1) the oil pressure sender, that shares the same manifold as the fuel pressure sender, wasn't behaving erratically and 2) I've a dedicated ground wire from the manifold. Also, I found a discussion over on VAF about Lycoming mech fuel pumps and that, by design, they can't over pressurize.

After replacing mine, I carefully dismantled the sender just to see what was failing. There was a bit of a wear spot on the resistive coil that Shawn talks about. Looking at it with a 10x loop,the resistive wire looked in good condition. I didn't get a good look at the wiper's condition, though. There did appear to be some black contamination (perhaps from the material that that the wire was wound around) in the area that would correspond to normal operating pressure. The remaining inside of the sender was spotless and appeared to be nicely designed. I was surprised to see that the wire that bridges the connection from the resistive coil to the outside world was not better secured to the resistive coil; the wire simply sat on a contact.

Out of curiosity, when you replace the sender, could you post the numbers that are stamped on the flats? I know that one describes the sensor's characteristics and I think that one is a date code. I believe mine to be of Nov 2007 vintage identified as '11.07'.

Thanks for the quick responses. I'll call AFS today and order a new VDO and post the results. I have seen a solid state sender, sold by the competition. Anyone using that product?

Terry

We also have the new Kavleco solid state sensors, you will need to change the wiring and we have to modify your engine monitor screen for them.

The "O" ring in the pressure relif valve in the fuel Injection pump and filter assy. will swell over time, causing the pressure to spike, then when the valve is dislodged off the seat, it will return to normal, will probably happen every start cycle. Replace "O" ring with a Buena N O ring, buy a couple of extras. Problem solved if it is only going up to 38 psi or so, then dropping back to 32psi. something mor ethan that is most likely the sensor!

Regards,

Gary Brown ATP A&P IA CFI DAR

Thanks for all the replies. I replaced the VDO and the fuel psi seems back to normal, after one flight. However, I'll monitor and see if it things stay in that normal range.

Terry

After three months of flying no high fuel pressure indications and it only cost me $70.00.