High pressure on the fuel tank while racing.
 

Hi,

I just got a 88 N/A RX7 as a rally car. I am getting familiar with all the systems that work on the car, since I am more used to Subarus than Rotary Mazdas.

So far, almost all my questions have been answered by searching the forums, but there is one that I cannot find yet, and worries me a bit.


When I race the car (rallycross or autocross) the pressure on the gas tank goes up. I know this because when you open the gas cap, pressure comes out.

I thought that it was a faulty canister or check valve, so I removed the line going from the fuel tank to the canister right at the connection (rubber line) by the firewall. The result? Fuel and pressure started to SHOOT out of that line.

So the line works, but the pressure goes too high!

I know that the canister also works because I can blow through it and air goes slowly through, but I dont know what else could be the issue.

I have read that it could be a bad gas cap, but this pressure is way too high, so I do not think this is normal... I believe that the fuel might be too high on the tank. But I have checked the exhaust for leaks or missing heat shields and nothing seemed wrong.

Any ideas?

Thanks!

Anybody?

in the FSM it lists a "check and cut valve" near the gas tank. I can't tell what it does, but it might be your culprit...

http://www.rotaryheads.com/PDF/2nd_g...systems_na.pdf

I would try replacing your charcoal canister first to see if that fixes it.

We are going to check the pressure releave valve even though we already saw that it lets fumes come out.
Just in case....

Also, the charcoal canister was also replaced since the old one was clogged.

I just do not understand how the tank builds so much pressure....

replace the gas cap yet?

Yes, and the fuel pump as well.

Pressure still goes up like crazy and the fuel/gas coming out of the filler neck feels hot as hell.... I do now know how the fuel is getting that hot.

I would think this would be normal when pushing your car to the limits in racing categories such as rally and auto-x. I get pressure releasing from the gas tank when driving normal on the street. It warns you on the gas cap (or near it) to open slowly... Or something like that.

Other than the pressure being released when you open it, what else is it doing? Is it causing any harm??

Road racer
 

I've noticed the same pressure build up while racing my 1990 RX7. I race SCCA road courses and there is sometimes an amazing pressure build up that happens, especially on hot days. I haven't done anything about it and I'm not aware of any bad effects. It might be just normal expansion of the fuel due to the heat. I'm also curious if there is any detrimental effect.

Well.... I do not know how bad is the pressure increase on your cars. But on my case, on a hot rallyX day, after a few runs, I would open the gas cap and have pressure coming out of the tank for about a minute.

Yes, the vapors hiss out of the tank for a minute. So, that is a lot of vapors.... so it must be a lot of pressure.

Since we did not know if this was harmful in any way (like the possibility to blow a line) we ended up adding a "T" connector on the release pressure hose (before the valve) with a small cone filter at the end.

This stopped the pressure issue since all the vapors exit with no restriction at all.....

the release valve probably doesnt work properly. if everything else works properly, then its probably that

That is what we thought, but we blew on that piece, and air went through.

Quick question. How much air goes through the canister? We blow on it and air goes through, but slowly.