I went to the gas station today, when I twisted off the cap, my gas tank sounded like when you open a carbonized beverage. So, is it normal for my car to do this, it hasn't done this until today.

All help is very well appreciated!!

 

Fuel tank is ALWAYS pressurized.

 

Ah, thanks nycgps. I feel dumb.

 

its normal.

 

No, no it is not.

The fuel tank has a breather that is supposed to keep it somewhere around atmospheric. There is a check (rollover valve) at the top of the tank which leads to the check and cut valve. From here, the fuel tank vent leads to the charcoal canister.

If the tank is pressurizing, then either the rollover valve has failed, the check and cut valve has failed, or there is a blockage in the line.

 

yea definitely somewhat normal, although it shouldn't be like a ton of air coming at you maybe just a little pst
if it doesn't do it than you have a problem like a leak or something

 

Here is a technical document about older style Toyota evaporative emissions systems. They are similiar to the ones on the FC. The FC does not have a coolant-controlled valve to control purge flow, and I have crossed out references to it in the document. I'm not sure if the purge flow is temperature regulated at all on the 2nd gens, although on the FD purge flow is duty controlled by the ECU through a solenoid valve. There may be some other minor differences between this Toyota system and the system on the FC, but the architecture is the same.



diagram from the S4 T2 emission section of the FSM:

 

Not to thread jack, but my 7 never used to do this, and now it does it everytime now. It started doing this after i installed my walbro, emissions delete, turbo swap. it seems like it is REALLY pressurized. everytime i get gas there is a huge release of pressure when i unscrew the cap. I capped the hard line that goes from the gas tank to the charcoal canister. could this be the issue?

 

^ I have mine vented open and I never have that kind of problem.

 

certainly sounds like it as the line going to the charcoal canister is where the air would escape

 

Isnt it suppose to be completely sealed for emission purpose ?

 

I believe so. Also when it heats up, the gas expands. If you fill your tank when it's colder out, and open it when hotter, there will be a pressure increase.

 

it gets sent in and burnt for emission purpose

 

Read the document I posted.

On a return style fuel system like the FC, the feed side of the system puts a vacuum on the tank and the return side puts pressure on the tank, all depending on engine condition. So there is an operational range of tank pressure/vacuum. That range is regulated mechanically by in part by supplying atmospheric pressure to the system, and the pressure levels are controlled by several check valves.

The check valves are what keep the system sealed for emissions purposes. They also allow vapors out of the charcoal canister or allow atmospheric pressure into the fuel tank. When these functions occur is based on pressure differentials from the gas tank or the intake manifold.

 

The tank is sealed for emissions purposes. That is the reason for the check valve.

The vent line is never supposed to dump to the atmosphere. OK, so in the strictest sense of the word, the tank is not sealed. But the system itself is closed. The tank vents into the charcoal canister through the check and cut valve (see diagram posted). Then when the engine is drawing vacuum, the purge valve opens and draw the fuel vapours from the charcoal canister, through the center iron, up through the oil filter (also taking nasty oil vapours with it) and introduces the mixture into the intake. It is then burned in the engine.

If the system is operating normally then the tank pressure will be right around atmospheric. If it is not, then the check and cut valve is probably dead or the line is clogged. As this valve is mounted under the car, it is subjected to a lot of abuse.

 

understood.

Im not that familiar with the FC system.

Thanks for clearing it up

 

That sounds like it would fix the problem, but I just am slightly uneasy over the whole fuel vapors coming out of an open hard line in my hot engine bay thing..

And I'm almost 100% sure I no longer have a purge valve, then how would the pressure/vacuum return to atmospheric? AaronCake you were saying that the check valve and charcoal canister and all that keeps the tank close to atmospheric, so if emissions stuff was removed, would I want to charcoal canister line to open air like arghx is saying? Not that I dont believe you arghx, but like I said, I dont really like the idea of venting it in the engine bay.

 

I suggest that regardless of what emissions stuff is removed, the stock purge system be left in place according to the diagram posted in this thread. I have my car set up like this (though I will be simplifying the setup a bit in a little while) and it works fine. It is a very bad idea to open vent the fuel tank, for obvious reasons. It is also an equally bad idea to just cap the vent, as the tank will build pressure while sitting and draw a vacuum with the pump running. Believe it or not, I have seen the draw from a fuel pump totally collapse a tank where the vent line was plugged. It was as if someone had crushed it.

 

On a side note, some of the 7's build up an incrdible amout of fumes that the cannister cannot handle and the excess is dumped out the valve at the bottom of the cannister. When this occurs, the gas fumes are certainly evident outside the car, and if the a/c is running, it can suck up the fumes through the condensate drain and be noticed inside the vehicle. More prevalent on the S4's.

 

Thanks aaroncake. Now if only i can find all my emissions bs..

 

Back from the dead!

I am having a problem with My road race RX7. it is a S4 NA (1987).

I currently have the oil filler tube AND the gas tank vent line going to a catchcan.

The catchcan fills up with gas after an hour or two. I now realize, I have this VERY wrong.


My question is, how do I fix this? I can reinstall my charcoal canister, but I have no purge valve.

What is the most reliable way to solve this? If it is as simple asa put all teh stock stuff back on, could you please be specific about what goes back on and where it goes and what it looks like.

Thanks!

Rob R.

 

since the tank vent on the FC just goes right to the charcoal canister, i think that is all you need. then you keep the oil filler neck hooked to the catch can

 

In that case, I would think it would fill up the charcoal canister. No?

 

since that is how its hooked up from the factory, i'm going to say no.

if there is liquid fuel coming out of the tank vent line, there is something wrong. it should only be vapors.

 

That is not how it is hooked up from the factory. There are other lines going to and from the charcoal canister from the factory. One of the lines in the system is the aforementioned purge control valve.

At the least, I guess I could run a line from the charcoal canister and tee it into a vacuum signal on the intake manifold.

This seems to be a fairly common problem on FC's that are driven hard with stock fuel systems for hours at a time (enduro races). The common fix is to install a fuel cell. I would rather not do that to fix this small issue.

I don't want to sound like a dick. I do appreciate the input. Really! You are making me think about this!

Thanks!

Rob R.