Van

Yesterday I noticed that when I turned off my car, I heard a liquid gurgle coming from back near the filler neck of the gas tank... I got out and went to check the gas cap - only to find that when I took it off, air pressure came out of the tank.

Anyone else have that happen? I'm assuming the vent line is some how clogged up.

I'll let you know what I find.

harveyf

It happens to my track car all the time i.e. the tank develops a positive pressure. This used to be counterintuitive to me, as I thought the tank pressure would reduce as gas was consumed. But I've since decided that the gas bypassed at the fuel pressure regulator under hard hot driving conditions is heated up, returned to the tank, and the total volume of the gas is increased due to it being heated.

My track car has the line that would normally go to the charcoal canister vented locally at the rear of the car. There are multiple chances for failure in the stock setup. I don't profess to understand how the system at the front of the car works but for instance if the heat activated vacuum valve at the rear engine heater hose connection stops working, I'm not sure the system would ever activate. Various check valves at the rear of the car. And on and on. It seems like a lot of hardware and complexity compared to a gas cap with a hole in it!

87944turbo

Certainly think you are on the right track here. Are all of your stock items still in place? Curious to learn what you find. I deleted all of mine several years ago and just have a filter on the end of the line under the hood now.

odonnell

Having a filter on the open line under the hood sounds like a recipe for disaster....don't most people feed the line to the dirty side of the air box?

Van

Yeah, on my track car I have the vent line open to atmosphere are the back of the car.

This car, however, is my street car, which has all of the factory emissions stuff in place.

Here it is, 4th of July parade ready!

87944turbo

In reality (at least on my car) it's probably not as bad as it sounds. To be honest, I wouldn't be surprised if the line is not continuous from the tank to the filter I have installed. There isn't even a hint of fuel smell even if I put my nose right up to the filter. So long as it's vented and not leaking, I'm not going to spend too much time worrying about it.

Cars look great Van!

MAGK944

I disconnected the line going from the tank to the emissions under the hood and fed it to the dirty side of the air box like odonnell. It's a tidy solution for a street car.

On my track car I fitted a tip valve on the tank and removed all the vent lines. The tip valve allows the tank to vent to atmosphere when it is not on its roof

StoogeMoe

Isn't it normal to have pressure come out of the tank? All my vehicles have done that. Except no gurgling that I recall.

jhowell371

My 87 Scirroco used to build up so much pressure it would make people jump back when opened. My 84 944 builds up pressure too (rare un-cracked tank , if the S2 does it's only slight.

400hp944

Van - check the vent line that comes out of the tank. It might be pinched between the tank and the frame. I have this issue with my race car from time to time. If the tank starts to expand it pinches the line which make it worse. There is zero clearance between the tank and the frame but somehow my vent line found it's way between there. Might be worth looking into.
SC

thomasmryan

I believe the rear 'vent' is an inlet as fuel is consumed.


you might give the charcoal canister a shake to see if the pellets have been set free, thus clogging up the plumbing.


shake any jug of fuel and it will build vapor pressure. shaking up your fuel tank should push the excess pressure towards the charcoal and, of course, after you are warmed up, the engine vacuum 'purges' the vapor from the charcoal.

generally speaking.

Tiger03447

Good call on the vent line from the tank to the dirty side of the airbox. Might have to run a new one to do that...pulled mine out when I pulled all the brake and fuel lines..Getting rid of the emissions stuff, canister etc.. still like the idea of a hole in the gas cap though..

Arominus

The only bad part about that is that you lose gas, as in thats money evaporating on a hot day. I left my stock system in place, despite its complexity for this reason alone. I get 15.5mpg already, i don't need it to be worse lol.

Van

I've taken apart the little 3-way fitting from the vent line to the charcoal canister to the recirc valve / intake manifold... and, after removing the fuel cap, used compressed air to blow into the vent line and the charcoal canister. The vent line seemed clogged and took some pressure to clear... the charcoal canister had some water in it that blew out.

So far, it seems to have solved my gurgle noise.