Anyone a Fuel Tank Expert?

Dave · Post by **Dave** » Mon Mar 07, 2005 6:56 pm

So I got home from work today and jumped out of the roadster to get the mail. I noticed the electric fuel pump sounded louder than normal. I pulled into the garage, shut the engine off, and heard a hissing coming from the back of the car. Upon closer inspection it turned out to be the gas cap. I unlocked it, opened it, and got a huge rush of air into the tank. Obviously it's not venting correctly. I fired the engine back up and the fuel pump was now nice and quiet again.

So, not having looked at the venting system on a roadster tank, what's involved? Do I need a new cap? Need to clean it perhaps?

Help!

Post by **Datrock** » Mon Mar 07, 2005 7:24 pm

Dave , When you put an sr20 in a Roadster do you still use the return line to the tank as you would with the r16 or u20. If not, adding a return line might be worth checking into. Sure is a nice Roadster day out here in the Northwest.

.. Bill

Dave · Post by **Dave** » Mon Mar 07, 2005 9:26 pm

Bill,

Yeah, you do use a return line. As I understand it, it's pretty much mandatory with a fuel injected engine.

TR · Post by TR » Mon Mar 07, 2005 11:18 pm

Hey Dave,

On a stock 68, the vent is incorporated into the gas cap. I'm not sure of it's construction, I have never taken one apart. Maybe take a look at yours and make sure it doesn't need a cleaning (or put the stock one back on)...TR

Dave · Post by **Dave** » Tue Mar 08, 2005 9:42 am

Thanks Travis. I'll inspect it tonight. I believe I do have the stock gas cap. It opens with a Datsun key. It's got a cool little plate that slides over to cover up the key hole. It's the only cap I've had on the car in the 2 years I've owned it and it worked up until yesterday! I'll poke around and see what I can find. I guess I was just hoping 10 people would jump out of the woodwork and say "oh yeah, common problem. Here's the fix." Oh well, one can always hope for a "common" problem.

toolsnob · Post by **toolsnob** » Tue Mar 08, 2005 10:14 am

Dave,

I bet it is the cap. The rubber seal has a small cutout to let the air pass and the cap has a little laberynth cut into it behind the rubber for the vent. If the rubber gets moved around the cap the cutout will not line up and let it vent.

Alexi

Dave · Post by **Dave** » Tue Mar 08, 2005 7:53 pm

OK, the rubber gasket on the cap has a little V notched out of it. The sliding cover that hides the keyhole has a gasket behind it and is mounted to the cap with a pin and a spring. As PRESSURE builds up in the tank, it escapes past the V notch in the seal and pushes against the keyhole cover. The pressure escapes and the spring pressure on the keyhole cover snaps it back shut. Pretty slick.

This does not solve my issue though. My problem was a VACUUM building up in the tank. I can't for the life of me figure out how air gets into the tank to displace the fuel that is being sucked out and burned. I feel like a complete moron! I'm about ready to take the overflow tube that's in the filler neck and just route that to atmosphere!

So, who here knows how air gets into the tank to displace the fuel as it's burned?

TR · Post by TR » Tue Mar 08, 2005 8:09 pm

Leave the key flap open???

I would guess that the SR is not using more fuel than the stock motor, but...

I think that cap has to breathe in both directions...TR

Dave · Post by **Dave** » Tue Mar 08, 2005 11:09 pm

I agree. The cap must be able to breathe in both directions. That's the only logical way for air to get into the tank. I sure can't figure out how it works though! It looks like vacuum in the tank would just pull that key flap in tighter and seal things up even more!

Well, I went for a drive today and the problem did not repeat itself. I hate to leave it as "it fixed itself" but I don't think I have a choice!