[AndyRV7]

I have some issue or issues with predominantly my left tank. Initially, I had a leaking filler cap. When the fuel would heat up and the tank was full, it would vent fuel out of the cap. I presume I have some type of venting problem. Then, in flight, I would see streaks of fuel on the top of the wing from the cap. I presumed I needed new cap o-rings but I never considered that the tank might be forcing fuel out. Then, my fuel gauge began to function intermittently. Most of the time fine but some of the time, zero, regardless of fuel quantity.

Let's stop there for now. I'd like to try to fix this problem or these problems, if they are not related. I believe I have float type senders and Van's gauges. The plane is an RV-7.

I looked through the older threads and my plans (I am not the builder), but I only have an idea of how the tank vents. I see that there is a vent tube that runs from near the fill neck to the wing root but I am not sure where it ends (possibly at the cowling/fuse connection on the bottom of the fuse???). I'm sure I can check connections at the back of the gauges. Beyond that, I am going to need an education.

Thanks! Andy

PS. For complete disclosure, and not to confuse this issue, my right tank gauge read zero this past weekend for the first time. I cannot say however if they read otherwise at master-on or engine start though.

 

[wirejock]

Vents

It depends on the builder but the vents should go through the root and end up below the fuse. You could open the tank and apply very low pressure to see if they are clear. Could just be a mud dauber built a home in the vent opening.
The senders can be measured with an ohm meter but I don't know the resistance value. Seems like empty might be a short to ground.

 

[noelf]

I also had fuel venting out of one of the fuel caps in flight, but only when that tank was full. The fix was to dissemble the fuel cap, put a light coating of fuel lube on both the large and small cap O-rings and reassemble. No more issues.

I believe the fuel was being sucked out due to the low pressure on the top side of the wing while in flight.

 

[BobTurner]

IIRC the Vans senders run from 30 ohms to 270 ohms or something like that.

Andy, please find those vents. I check them every preflight.

 

[Taildrgr]

The vent line terminates on the floor just behind the firewall. There are two, one for each tank. The plans show an AN bulkhead fitting that is cut at an angle that goes through the floor. Look on the belly just behing the cowl and you should see them. Inside the plane they are behind the rudder pedals on the floor.

The connection to the tank is just forward of the spar about 8-10 inches and goes throught the side of the fuselage with a bulkhead fitting.

If you remove the wing root fairing, you should see 2 aluminum tubes, a 3/8 inch one near the spar that is the fuel line and a 1/4 inch line that is about 6 inches forward and that is the vent line.

Make sure those lines are clear. You can disconnect the vent line at the tank and blow air through the fuselage bulkhead fitting out the end by the firewall. Do not blow air into the tank unless you have the filler cap off.

Sounds like the ground on the fuel gauge is not working well. The gauge is grounded by its mount to the tank. Sometimes the tank sealer can interfere with the ground connection. Check the ground by using an ohmmeter with one lead on the plane and one on the sender mount. If the ground is good, then you may have a failing sender.

 

[AndyRV7]

The vent line terminates on the floor just behind the firewall. There are two, one for each tank. The plans show an AN bulkhead fitting that is cut at an angle that goes through the floor. Look on the belly just behing the cowl and you should see them. Inside the plane they are behind the rudder pedals on the floor.

The connection to the tank is just forward of the spar about 8-10 inches and goes throught the side of the fuselage with a bulkhead fitting.

If you remove the wing root fairing, you should see 2 aluminum tubes, a 3/8 inch one near the spar that is the fuel line and a 1/4 inch line that is about 6 inches forward and that is the vent line.

Make sure those lines are clear. You can disconnect the vent line at the tank and blow air through the fuselage bulkhead fitting out the end by the firewall. Do not blow air into the tank unless you have the filler cap off.

Sounds like the ground on the fuel gauge is not working well. The gauge is grounded by its mount to the tank. Sometimes the tank sealer can interfere with the ground connection. Check the ground by using an ohmmeter with one lead on the plane and one on the sender mount. If the ground is good, then you may have a failing sender.

This is a great help Bob. Thanks. I am going to run down all the things discussed here. I'll be back! Andy

 

[AndyRV7]

Some additional information

I got near the plane yesterday and tried to check everything I could. I wasn't able to find the ground for the gauge (maybe it was staring me in the face?)

Here are a couple pics. I started at the gauge and checked all the wiring. I couldn't find anything loose. Then I took the wing fairing off and had a look around. Again, everything looked clean and dry and the only wire present seemed to have a solid attachment at the tank. I guess this is the sender's wire so the gauge can get an Ohm reading?? I wasn't able to check the resistance because I couldn't get this thread to come up on my phone at the airport. Maybe someone can describe the process using the picture I posted so it is a little more clear to me next time I am at the airport?

As far as the pressure was concerned, I separated the vent hose at the fuselage and blew air through it to the belly exit of the line. It was clear. Then I put the line back together and blew air into the tank from the belly end to the tank end of the vent tube. It was clear all the way through. I presume that the tube was completely clear (not partially blocked) based on the amount of air I put through it using my Wal-Mart tire inflator but I can't say for sure.

As far as the gauge itself, I studied it a little this weekend. It seems to come on virtually all the time (maybe actually all the time) when I flip the master switch to on. The problem seems to be when I begin to crank the engine. Sometimes it works but a lot of times, the needle drops to zero as the engine is cranking but before it starts. I'm not sure if that is some electrical problem or something to do with the shaking of the plane (or both). But at least I got a good look at everything so if anyone has any suggestions, I am now better able to understand just what we are speaking about.

Any ideas for a next step? As far as I can tell, I still have a left tank that won't vent in spite of what appears to be a clear vent line, and an intermittent left fuel gauge.

Thanks for your patience!! Amdy

I replaced the back light after I took this pic. It was loose so I pulled it out to make sure it was still connected well.

 

[wirejock]

Sender

Assuming you have a volt/ohm meter, touch the leads together first to verify the meter reads correctly. It should display a really small number close to 0.0 or no resistance. Place one lead on the sender connection and one on a clean surface of the plane or ground. You can disconnect the lead at the gauge and test there if its easier. Record the value then crank the engine. Watch and see if the reading goes to 0 (short to ground). It sounds like the sender lead to the gauge is touching ground somewhere when the plane shakes.
Vent is an interesting problem. Maybe you fixed it when you blew the line.

 

[Taildrgr]

I am at the office so your pictures are blocked by my company. I can have a look later from home. So excuse my reply if it is addressed in your pictures.

The ground for the sender is the plate that is screwed to the tank. If you have a gasket under that plate, then the screws are the only ground path, but sometimes the proseal also shortcuts the ground path. Check the ground by touching one probe of a multimeter to the plate and the other to the tank and make sure you have no resistance.

The single wire that connects to the center post on the mounting plate is your signal wire and would connect to your gauge. I would check the mounting lug of that wire first for a loose connection. Sometimes there is corrosion under the crimp or it is just loose. Recrimping the existing lug or cutting it off (if you have some slack) and putting on a new connector could help.

Remove the signal wire at the tank gauge and use a multimeter probe on the sender center lug and another on the tank or sender plate should show a resistance value proportional to the fuel level in the tank. It should read betweeen about 30 and 240 ohms. If it doesn't then the ground is bad or the sender has failed. Do not measure the resistance with the wire still attached because it will show the resistance of the gauge, not the sender.

You also may have a failing fuel gauge.