therotaryrocket

ok, i did a search on gas tank vent and read some threads. It sounds like the charcoal canister to me, so how do i check it? whenever i leave the gas cap on and then take it off..fumes rush out of the tank, whenever i leave the gas cap off and take a right turn hard gas pours out of the filler neck (i've probably lost an eigth of a tank doing that). tanks giys

inittab

Same problem here.

Rx7carl

Disconnect the small hose from the cannister and see if it corrects the problem. If the problem persists, then its either a clogged line or the check/vent valve is bad.

Glazedham42

iTrader: (4)

Carl (or anyone else that thinks they posess equal knowledge ),

While we are on the subject of the check valves under the car I have a few questions as well. I am fixing all of my rusted fuel lines and other equipment under the car while I have my tank off. I have noticed these two check valves that and started doing some research but I've still got a few questions. I found a rather old post where some of you guys were discussing this but I didn't really find the answers I was looking for.

1) First I have one overall question about the whole charcoal and tank venting system. How does is work, what does it do, and why is it necessary?

2) The check valve that is in the tank vent system (Check & Cut Valve??). Is this thing necessary or not? I saw that Jeff20B recommended just putting a "T" fitting in place of this valve. What would this do? Is it okay to use a plastic "T" fitting or should I find a metal one?

3) The third line on the check and cut valve empties into a little round, gold can that is mounted on the frame of the car on the driver's side. What is this can thing for? It is the only thing left that I don't know what it is.

4) Lastly, I have identified the fuel cut valve that kills the supply in the event of a rollover. I would like to remove this for simplicity's sake, but I'm wondering if this is a bad idea? Should I put it back in for safety when I finish my repairs, or is it just one more piece to go wrong in the fuel system?

Thanks!

Jamie

NewRXr

Hey guys get a load of this, I have a '85 GSL-SE and it had the same problem. It was so bad whenever I drove the car the pressure would build up in the tank to the point that it would whistle out somewhere and I could hear it inside the car until it cooled down. I've also been working on this engine to bring it up to full power since I've bought it a few months ago (see my thread titled "Calling GSL-SE tuners, need diagnostic help"). It's been a series of baby steps but I've gone from somewhat slow to pretty fast and would you believe that "insignificant" check and cut valve was worth a big chunk of horsepower.

Let me explain, I was flipping through my Haynes manual and came across the fault diagnostics section in the front. In section 7, "Lack of power, poor acceleration", step three says "Examine the crankcase and evaporative emission control systems and in particular check for defects in the check and cut valve and for clogged hoses in the evaporative line." Now I'm a professional mechanic who actually specializes in emissions. On any other car I can think of a problem in this area would only be a minor pollution producer and this would have no significant effect on how the engine ran. So like you guys I found this hard to understand and studied the Haynes manual, the factory service manual, and my car to try and determine how this could cause a low power condition but I couldn't figure it out. There is a serious lack of engine control theory information about first gen RX-7's available. I had been advised on the board that it was a non-issue on their cars. Finally I just determined I had nothing to loose and at least I could possibly get rid of the annoying whistle by repairing the stuff.

So I removed the check and cut valve from my SE and my '81 parts car. Both were clogged up according to the testing procedures in print. The '81 car ran super good, it would smoke fast cars and it was stock, this just adds to my confusion. So I disassembled both and started cleaning them out with carb/choke cleaner. The build up was stubborn and it took a while to get it all out. During this time one of the valve bodies was accidentally knocked off the workbench and cracked apart when it hit the floor, these parts have become brittle over the years so handle them carefully. Upon reassembly of the cleaned out valve I found it still wouldn't open at the pressures listed in the book, which was low enough to be checked by mouth pressure. So I grabbed a portable 12V air compressor with a beach ball type nozzle adapter and forced air and carb cleaner through the ports used for testing until this freed them up to the point it would pass the tests. I slid under the car and used the carb spray and air pump technique on the vapor lines also. Then I reinstalled the valve. I went under the hood and removed the purge valve and used carb cleaner to flush it and it's attaching hoses. I then checked for vacuum and free flow on the proper attaching hoses. All was working properly so I reassembled the system. I started the car and let it run for a while and everything seemed fine, the whistling never started.

The next day on the way to work I found the engine was making significantly more power across the entire range. It was enough to really feel the difference and I'm no rookie, I've been building seriously fast streetcars for over 19 years. When I say a significant difference there is one. I would estimate at least 10 HP across the board. Now tank pressure is always normal and it's been about a month. The bump in power is still there also. While this is obviously a good thing and I recommend anyone to duplicate my efforts I still would like to know what the heck happened with the fuel injection because of a clogged up check and cut valve and or purge valve.

Good luck,
Vernon

faye x7

Crazy, I too have the same problem. When I go fill up for gas I take the cap off a huge rush of fumes/gases come flooding out. Sometimes it's pretty damn loud too.

NewRXr

Jamie on your other questions, that gold can is either your fuel pump or the fuel filter. Both are in that area.

The charcoal canister and purge valve are part of the evaporative emissions control system. This system prevents hydrocarbons from venting into the atmosphere as gas in the fuel tank evaporates. There is the check and cut valve on top of the fuel tank that is connected by tubing to a canister full of activated charcoal, which is attached to the firewall on the passenger side of the engine compartment. As the fuel in the tank evaporates the fumes travel through the tubing and into the canister where the charcoal collects the hydrocarbons. Attached to this canister is a vacuum line connected to the engine intake at the purge valve. This valve is shut when the engine is off and not producing vacuum. When the engine is run warmed up vacuum is routed to the valve to open it, allowing the engine vacuum to pull fresh air through the charcoal, pulling the absorbed fuel vapors out in the process then burning them in the engine.

If the purge valve stops opening fuel will collect in the canister until the charcoal is saturated with it. Then it will drip out anytime your tank gets hot and builds up pressure, sitting in the sun or running on a long trip. If it stops closing fuel mileage will go down and drivability will be worse.

Vernon

Glazedham42

iTrader: (4)

Thanks for the attempt, but I am POSITIVE that it's not the fuel filter or fuel pump. I know both of those by heart. Also, they are located in the fuel supply line. This gold can I'm talking about is coming off of the check and cut valve. It has a hole on it and it mounted to the frame of the car. It is held on with two screws that have a 10mm head on them. I just took it off actually and it is nothing more than a round shaped piece of metal. There is NOTHING inside of it. It just has a hole in it that the hose off of the check and cut valve fits into. What is the purpose of this thing if there is nothing inside of it? When I replace my check and cut valve with a "T" fitting can I just leave that part of the fitting open since the gold can thing is empty???

BTW: This is on an 85 GS. SE's may be different or may not even have this thing, I'm not sure....

Carl where are you?? I know you'll know the answer to this...

Rx7carl

1- The theory is that this is not a closed system (IE- no atmospheric connecton). The tank must be able to vent so it maintains equal internal pressure as the ambient environment. Because of the laws, you cant have a vented gas cap anymore cause that pollutes. So we have this elaborate system that minimizes pollution. The charcoal is there because you cant pollute the air with gas fumes, so the charcoal "scrubs" the air that vents from the tank.

2&3- its all part of the vent system so as not to pollute the atmosphere.

4- your choice. Odds are youll never need it, but if you do and its not there.....................
Its only failure mode that I could think of is that it doesnt open back up after the car rolls and is righted back on its wheels. But youll have bigger problems at that point.


Vernon- It can cause a huge performance problem. If the tank pressure drops (cause the valve is clogged and the tank cant suck in air to equalize pressure), then its harder and harder for the pump to pull the fuel out of the tank, making for lower fuel pressure and flow to the injector/carb.

Wankelguy

The tank purge line also helps to keep the tank in an air-starved (rich) condition to lessen the chance of the tank exploding when the sender unit inside the tank arcs (which is normal). This is partly why you never want to run the car without a properly sealing gas cap. The setup on the older cars with the canister in the aircleaner lid is quite a bit simpler.
Just my two-cents.

NewRXr

Jamie, I don't have that on either of my cars. But I think I know what it is now that you've described it. It's either a small vacuum reservoir or a pressure spike dampener for the check and cut valve. Small canisters used in pressure systems (positive or negative) do either. I would say if you're removing the check and cut it can be tossed without issue.

Carl, I can see your point about the pump pulling against negative pressure but most of the time the tank was under high pressure from the fume build up like everyones been describing. I know the pressure at the rail was good because I drove around for a few days with a gauge taped to the windsheild and it was always within specs. I'm not saying you're wrong but that there's something more to it.

Vernon

Rx7carl

Oh well, so much for that theory.

therotaryrocket

well i guess either my line is clogged or my check/cut valve is bad. Probably the line seeing as how now my fuel line is also clogged. If you guys ran your engines for 15 minutes and then shut it down, jump out of the car and disconnect the main fuel line wouldn't fuel pour out? Mine pours NO fluid, well I got it to one time, i just replaced the filter 20 minutes before this, it also did this yesterday and it had a 3 day old filter then. So how do I clean out the fuel line? what about the vent line? whenever the car was running ok i did get a personal opinion for any body running auto-lite spark plugs, compared to my new NGK's autolites suck. my autolites were less the 500 miles old and model 2526, i slipped in my NGK BREQ-7... and it ran awesome, a lot smoother idle and revs up faster, until that fuel line clogged up.

therotaryrocket

How do I clean out my fuel lines? I was told to replace them. I hope I don't have to remove the gas tank because that thing is freakin hard to put back in.