Fuel tank has vacuum M20K LR tanks

[jwilkins]

Last week I made a trip from Prescott to Watsonville (WVI) with a stop at Orange County on the way back for a meeting. While at Orange County I opened the right main fuel tank cap for a top-off. When I opened the cap on the right main tank there was an obvious and strong vacuum in the tank.

Is this normal?  This is a 1980 K with the LR tanks. The left tank did not have any vacuum that I noticed when I opened that fuel cap.  I know the LR tanks feed the mains by gravity, but was surprised at the vacuum 'woosh' when I opened the cap. If the tanks are properly venting should any vacuum build up?

Jim

[co2bruce]

Someone corect me if I'm wrong, but you have a clogged vent.  

[jwilkins]

If this had been my F I'd agree absolutely but the LR tanks with the gravity feed are new to me. I thought there might be some pull down from the LR tank to the main. I checked the paperwork on the LR tanks but there was not a detailed explanation of venting that I could find, just a statement that the LR tanks feed the mains by gravity. Since I made only a short hop from WVI to SNA there may have been some fuel left in the LR tank. 

Jim

 

[carusoam]

Jim,

Earlier this spring I had a similar experience with my Ovation.  However, in pre-flight, my tank was slightly pressurized.

Piloto gave advice on how to unclog the vent.  I ran a piece of safety wire up the length of the vent.  I could hear the pressure relieve itself as the wire broke through a new layer of sealant that had been misplaced.

Vacuum should not exist in your tank.  This can be detrimental to the fuel pump diapragm.  Plus, it could stop the flow of fuel from that tank, or minimize it during take-off or go around.

To check you vent have some body put their ear near the open fuel cap.

Blow in through the vent, they should hear you.........

I think you can search on blocked vent?

Best regards,

-a-

[jwilkins]

I'll get my wife to stick her ear in the fuel tank Saturday. Thanks for the reply.

Jim

 

[RJBrown]

Carusoam is correct you have a plugged vent. This could create a dangerous situation. My brother in law had a boat with a non vented cap where a vented one should have been. The boat would run fine for an hour or so then start to loose power as the vacuum in the tank increased. After he changed the fuel pump 3 or so times without correcting the problem I went for a ride with him. When the symptoms appeared I removed the cap and power was restored. In our airplanes there in a seperate tank vent for each side. (Adding long range tanks does not affect this area) Insects can enter and plug the vents or a careless reseal  can cause it. An (good) open vent can dribble fuel on a hot day if the tanks are overfilled.

[jwilkins]

Thanks to everyone who replied to this question.

The safety wire cleared the vent. The only cocern I have now is if the clog will travel into the tank.

I couldn't see anything in the vent and nothing came out with the safety wire.

I used to keep a pipe cleaner in the old C vents to keep out the mud daupper wasps in NY. You could see the clog from the wasps as they plugged up the end with mud. I have no ida what this was but will check the vents.

Again, Thanks to everyone!

Jim