spamrestricted

After driving hard, I can hear a bear growling in the trunk. When I open the gas tank, it pukes out fumes heavily for about 5 minutes. This car has 90k miles on it and has been garage queen. So I’m thinking from sitting around that something has clogged up. I’ve driving around with the gas cap barely snug, which helps release the pressure. When driving slowly or in the cold it doesn’t affect it, but driving hard and heated days, it does it bad.

Anyone have any ideas or experience it yourself, looking for things to look at. I’m at a loss. TIA

Turbonut

iTrader: (1)

The '88 TII we purchased new had that problem, it was so bad that after driving in hot weather
then shut off, the high pressure would be released through the valve under the car. That, and
as the trailing plugs stopped firing because of a design problem, Mazda bought the car back,
then got the 89TII.

Now on to the '89TII, never had a fuel tank problem in the first 27 years, but after the car was
in the body shop for 3 months the problem reared its ugly head. With all the tests, couldn't
locate the problem, so I resorted to drilling a hole in the fuel cap, that alleviated the problem,
couldn't tell you when it happened, but out of nowhere it returned to normal and has been fine
for years.

DR_Knight

iTrader: (23)

I’d start looking at the evap system diagrams to get familiar, and probably inspect the charcoal canister, purge control valve, the check valve for the purge control valve, and vent valve (if the car has one).

can start here but this is t2: https://www.rx7club.com/2nd-generati...ussion-825093/

full service manuals here:
http://foxed.ca/indexmobile.phppage=rx7manual

the fuel tank is supposed to generate pressure, then ingest those fumes through the engine so it can burn them to reduce emissions.

professionalpyroman

The evaporation system is supposed to maintain the fuel tank pressure. There are three lines that run from the fuel tank to the engine bay; the high-pressure fuel line, the return fuel line, and the evap line. The evap line is the one you want to inspect. The FSM has diagrams that show you what they look like.

There are 3 parts of the evap line you want to inspect:
-Check and cut valve - This is under the car, next to the fuel tank. The evap port on the fuel tank runs to this check valve. It has a diaphragm in it that closes off the fuel tank evap in case of a rollover. It's a safety feature to keep gas vapors isolated to the tank unless the car is upright. Disconnect the lines to this and see if you get low pressure airflow through it (blow through it). It doesnt handle high pressure. You can remove this valve and clean it out (gently). Mine was stuck shut. Be aware that it is made of brittle plastic, so if you do have flow through it, I'd leave it alone.

-Evap hard line - There is a metal line that runs from the check and cut valve, along the drivers side frame rail, into the engine bay. I was having the fuel tank pressurization problem in my car because this was completely rusted solid. I went to O'Reillys, got a hand bender and some tube of the same size and bent a replacement. It was a pretty low effort install, with some patience. Took an afternoon, and made it fit the same.

-Charcoal canister - This sits in between the evap hard line and the intake manifold. Vacuum lines connect the evap hard line to the charcoal canister, and the charcoal canister to the intake manifold. When the car is sitting there, this canister allows the gas vapors to be either sucked straight through to the intake manifold with the engine running. When the engine isn't running, gas tank pressure is vented through the bottom of the charcoal canister, with the charcoal trapping the gas and allowing the air to leave. When the engine is started, the intake manifold sucks up the accumulated fuel in the charcoal canister and the cycle repeats. The charcoal in the bottom of the canister can degrade with time and plug the bottom vent hole.

I would say the first thing to check is flow from the gas tank to the charcoal canister in one go. If you don't have any flow, its easy to track down where the plug in the system is. Remember, this is a low pressure line, so don't go putting more pressure in than about 10psi or so if you are checking with a compressor. Let us know if you have other questions

spamrestricted

Wanted to add that the car won’t start unless I release the pressure. This is a 1989 Mazda RX7.

j9fd3s

iTrader: (3)

you might start by removing the small hose from the Charcoal Canister, if it fixes it, replace canister and recheck. if not, then its the rest of the system (pipe, roll over valve etc)

spamrestricted

Thanks very much Everyone! This has been so helpful. I will update when we can finally do some diagnostics/testing.

Turbonut

iTrader: (1)

Did the '89 car shut of when running, or just hard to start? If I shut the '89 off after a drive when this
phenomenon occurred, I needed to keep the pedal to the floor to get it started. The car actually
would also cut out when driving as air wouldn't be replaced inside the tank, so eventually no fuel
to the engine, release cap, drive on and it would happen again, so in went the hole in the cap.

Just to add, the '88 TII, the fumes were so bad that the charcoal canister couldn't hold the pressure and
would release it out of the bottom valve, and to make things worse, the negative pressure in the HVAC
system, would draw the fumes into the car through the condensate drain. Been many year ago, but it's
no wonder they bought the car back.