Vapor Lock?

[JBPILOT]

Thanks for putting the links to these videos. The elastic diaphragm controls the pressure and also the volume of fuel going thru. Fairly simple once you see what they are doing. The material the diaphragm is made of is critical. It has to be soft enough to limit pressure. If less fuel output is being required, the diaphragm just stretches as needed.

[Piper J3]

I concur with your description of the fuel system. BTW I'm the OP. I like your test for a blocked restrictor in the fuel return line. I will check this before the next flight. Appreciate the insight...

[Sink]

I got a number of emails from Vans forum members for this post and ask for some pics and insight.

Here is a nice set of pics someone sent me in their email.

http://ctflier.com/index.php?/topic/...tax-fuel-pump/

The return line isn't just for vapor. It will allow fuel or vapor through. It is there to allow cooler fuel to circulate from the tanks through the fuel lines during the engine run to help prevent vapor lock from the engine heating the fuel lines under the cowl. After the engine stops and sits the fuel hose can and does get hot enough to produce some vapor as some see with a low fuel pressure after sitting a while and starting the engine. This should go away as cooler fuel circulates through the system. The restrictor should be a #35 Mikuni idle jet and not a .035" hole. There was a misprint in the manual for many years. It is a very small hole.
If this hose was open and without any restriction the fuel would take the path of least resistance and you wouldn't have any fuel pressure. This
re-circulation line does not control your fuel pressure. What does help control the fuel pressure is the diaphragm tension or tightness. The first pumps had some high fuel pressures. This was caused by the MFG making the diaphragms too tight. This was fixed quite a while back.

The fuel pump does in fact have two springs. One external and one internal. The pump has a double check valve and an internal screen to help filter the fuel before it gets to the carbs. That said if you have an issue after the fuel pump outlet with a hose there is no more filter before entry into the carbs. The pump works on differential volume/pressure inside the pump. This is what operates the check valve as it is free floating with its movement. If there was never a volume or pressure change internally the fuel wouldn't move through the pump. As the pump operates off the eccentric in the gearbox and it pushes on the plunger and pushes on the diaphragm. As the diaphragm moves one way it allows fuel to enter the pump and into the pressure side of the pump. As the diaphragm moves the other direction the check valve slams shut and the fuel is forced out to the carbs. So as one spring pushes the diaphragm one way the internal spring pushes it the other. As the fuel moves from suction side of the pump to the outlet pressure side the check valve opens on one side and closes on the other. If there was no check valve then there would be no pressure.

Believe it or not this operates a lot like your heart with its valves from the atria and ventricles. When your valves (your check valves) in your heart leak because they don't seal you have a heart murmur and leakage. If you have a bad valve (prolapsed valve) then blood moves back and forth, but not through the heart. Then it's time to get your pump fixed and sometimes like our engine, replaced.

[Mich48041]

Good job Mike Miller. According to Sink, you are correct that there is an internal spring between the reciprocating shaft and the diaphragm. The spring must push the diaphragm to pump fuel out and the spring must also pull the diaphragm to suck fuel in. It is too bad that ctflier.com will not allow outsiders to view the fuel pump pictures.

[JBPILOT]

https://www.youtube.com/watch?v=FeP-...m-upload_owner

This is the original A C fuel pump I took apart a few years ago. Took me a while to find it. Go back to post #28. Guess I was correct to start with ( maybe huh ) !

[2johns]

John, I did take my old pump apart. I agree that at least our original fuel pump is not a positive displacement pump, has a telescoping plunger, has two springs- the lighter one to control pressure. What we don't know if our new replacement pumps are made the same way.

[JBPILOT]

I would call this a variable displacement pump. The internal spring controls pressure. The plunger moves the same amount each cycle, but the diaphragm only moves as much as needed to supply fuel.

[Piper J3]

This doesn’t fit with the vapor lock discussion but I thought I’d post here…

I had high fuel pressure alarms back in October - December and traced the problem to electrical contacts on the fuel pressure sensor. I cleaned the contacts and tightened the “pinch” on the spade lugs. Problem was solved for a dozen flights and fuel pressure readings were very consistent ~ 6psi.

Airplane has been hangered for three weeks because of weather. I flew yesterday in 30F temperature and my fuel pressure alarm problem has returned. I saw readings that spiked near 13psi with a lot of fluctuation. Toward the end of the flight the pressures settled at ~ 6psi. I’m pretty sure this is a false error. I don’t see any indication of fuel overflow at the carbs and the engine is running flawlessly. I think my next step is check for intermittent ground. I may run a new ground wire to the engine crankcase and also to the fuel pressure sensor to see if that helps. I'm running 93 octane E10 exclusively. New fuel pump 893110 S/N 13.4493 was installed a year ago.

My wife isn't too thrilled when the alarm keeps going off...

[JBPILOT]

Since 'new', I think all the electrical issues I have had were involved with connections. I went back over ever crimp, and used two hands to re-crimp them until I felt I was going to destroy them. I also added a very small dab of Vaseline on the connectors. It spreads on its own and stops corrosion. I almost never have such problems now. It has worked for me. I also changed some of the connections to Delco weather-pack connectors. Much easier to disconnect when needed.

[Tony_T]

Experience here is in agreement with John's post. All my early spikes and alarms were from poor electrical connections. If you still have the original style fuel pressure sender on your plane the spade connectors can be replaced by ring terminals. Doing this solved my fuel pressure spikes.

The sender has screws for the ring terminals. The spades are probably OK for analog gauges but the EFIS is very sensitive to spikes that you would likely never see on a gauge.
John also suggests something to mitigate corrosion. Here we are using DeOxit.
https://www.radioshack.com/products/...5#.VYAe92AdL8s
We call it the "Magic Elixir". I give all my slip-on and screw-on electrical connections under the hood a little shot of this when I have the cowls off. It is a clean spray and doesn't leave a mess or pick up dirt.