Is this vapor lock?

Sid Lambert · #1 03-19-2012, 06:41 AM

Are these the symptoms of vapor lock about to happen?

Yesterday my wife and I flew over to Bama to visit her folks. My carbed 320 sits on the ground in the sun for about 4 hours. We get in and the air temp is about 84. We taxi back, do a nice run-up and then blast off.

Climbing at 115 KTS and about 4500 MSL the low fuel pressure warning alarm comes on. The pressure is 1.8 PSI. I turn on the boost pump and it goes to 5.1 PSI. I turn the boost pump off and it goes back down to 1.8. I switch tanks and the pressure stays about 1.8 PSI. We level off at 7500 MSL and I notice the fuel flow was about 12 GPH, way too high for 7500 MSL and 2400 RPM. I stay there for a few minutes and the fuel pressure starts to rise back to the normal 5.6 PSI and flow goes down to 7.8 GPH.

The engine never stumbled and the rest of the flight was uneventful but rough as a cobb.

I just installed the engine monitor at the end of last summer so the airplane could have been doing this from day 1 but I never notice because I had no FF and just a FP gauge mounted down low and almost out of site before.

Should I wrap my fuel lines in heat reflecting foil or just chalk it up to warm fuel on a warm day, no harm no foul?

Geico266 · #2 03-19-2012, 07:25 AM

The way you describe it would say yes, but now that you have gages it might be normal. Leave the boost pump on until you level off and allow things to cool down. Shut it off after 10 mins of straight and level at cruise power and see how things balance out after a few mins.

I would wrap the fuel lines in any event. Stack the deck in your favor.

Your mechanical pump might be getting weak also. How old is it?

Sid Lambert · #3 03-19-2012, 07:31 AM

Quote:

Originally Posted by Geico266

Your mechanical pump might be getting weak also. How old is it?

14 years with about 915 hours. (12 years of flying)

Geico266 · #4 03-19-2012, 07:32 AM

Quote:

Originally Posted by Sid Lambert

14 years with about 915 hours. (12 years of flying)

Time for an upgrade, IMHO. You got your money's worth out of that one.

Your wife will be happy that you did.

My -10 did this and it helped to replace the fuel pump. Has not happened since.

Walt · #5 03-19-2012, 08:23 AM

Vapor lock is generally caused by the fuel boiling in the fuel line from the firewall to the eng driven pump and will usually show up early in the ground run or takeoff phase (the worst time). This is why it's especially important to run the fuel pump on TO as the electric pressurizes the fuel in the line up to the eng pump and will effectively prevent VL.

Once the hot slug of fuel has worked it's way through the system the chance of VL goes way down. Assuming you are burning about 16gph at TO, it would take you about 1 minute at this setting to burn a quart of fuel which would easily purge the system of hot fuel.

My guess would be the mechanical pump is getting a little weak, however a carb will run fine on 1.8 psi.

All fuel lines FWF should be firesleeved and additional reflective tape cannot hurt.