Gas tank venting
 

So I am currently stripping out an NB to build a track only car. And when I say stripped, I mean everything that is not necessary for the track must go, including all of the stock emissions components.

I am going to run two lines to the engine bay, one for the feed, and one for the return line. I am planning on removing all of the pressure sensors, canisters, filters, and solenoids from the system and the gas tank. I'll add back a fuel filter of course.

All this I get. What I am wondering about is venting the tank, and how to do it properly/safely. What I plan to do above will seal the tank, except for the gas cap and the vent fitting on the top of the tank. I am not sure if this is sufficient to prevent the tank from building excessive vacuum or worse, excessive pressure.

I googled around and searched and what not and came across a few threads discussing some aspects of this.
https://www.miataturbo.net/general-m...anister-67673/
https://www.miataturbo.net/engine-pe...-others-65940/

Some have suggested teeing the vent line into a loop between the two small steel lines that run to the filler neck. See pict below. I get this, but with that setup the only way the tank can breathe is through the gas cap. All other paths are blocked/sealed.

https://www.miataturbo.net/attachmen...g-fueltank-jpg

So, what is the NB race-prep experienced crew doing for tank venting? Discuss.

I have it on good authority that the loop-tee method in the above picture works and that the system vents through the gas cap. Anyone else?

The cap is a one-way vent, so it would prevent vacuum but not pressure.

From my limited knowledge on how the factory manages the EVAP system, there is a MAP sensor that monitors tank pressure. The OE ECU monitors this pressure and under certain conditions, it will command a pressure purge via a solenoid. The outlet of the solenoid is routed through a charcoal canister to collect vapors and then dumped to atmosphere.

It's uncommon for a standalone management system to monitor or control tank pressure or control a tank purge. However it should be relatively straight forward to do, if one were inclined. Even with relative ease, I'm not really sure it's necessary. My feeling is to gut it and never look back, however the approach I have with safety equipment is "Use all that you want and can afford." So if you feel the car is safer with a purge, go for it.

:beer:

Since the MSPNP soes not handle purge, I just vent it to atmosphere through the charcoal canister.

Evaporation canister is now gone. As far as removing non-race-car-essential stuff goes, that evap canister, the one under the car, is heavy.

I should mention that the engine is a LS1 with a (flashed) GM ECU so I do not have a programmable aux output.

I wonder if the factory pressure sensor and pressure release solenoid can be wired together. That, or just find an aftermarket pressure relief valve. Then just run a line from the factory vent fitting (on top of the tank) to some safe dump spot under the car.

Vent to genitals.

The Hustler Vent To Genitals (VTG) system. Genius. That would certainly be a motivator to drive faster.

The charcoal canister is now gone. As far as removing non-race-car-essential stuff goes, that charcoal canister, the one under the car, is heavy. So that is not going back in if I can help it.

I should mention that the engine is a LS1 with a (flashed) GM ECU so I do not have a programmable aux output.

So looking at the Mazda shop manual, the tank vent or 'rollover valve' looks like it flows in either direction (it definitely flows under tank vacuum) unless the valve (car) is upside down. So that is good, leave that. The Tank Pressure Control Valve (TPCV) flows passively, in either direction, when pressure thresholds are exceeded (either tank vacuum or pressure). The pressure threshold is .98 kPa or about 0.14 psi. The solenoid part of the TPCV just opens it up to free flow.

So, the answer here could be to simply plumb the rollover valve directly into the TPCV, then vent that (through a filter) via a hose to some safe point under the car. Actually, that is pretty much the way the factory has it hooked up (through the filler neck lines) except that the TPCV goes to the charcoal canister instead of venting directly to atmosphere. So, just reroute the TPCV outlet to some safe spot on the car, put a filter on it to keep out dust, and call it done. No need to power the TPCV solenoid.

I am sure that is clear as mud to everyone without the benefit of the shop manual.

Common racecar method is to gut all evap and put a vent line in with a rollover valve that just vents to atmosphere (filter if you want). Some just use filler neck vent. Some modify gas cap. Lots of methods work.

I have yet to remove those parts but planned to just run one of the existing vent lines up to trunk level with a valve inline and be done with it.

Yep I hear you. Normally I would just rig something up but since this is a FUEL system issue, I am analyzing it to death, no pun intended.

I need to look at mine when I did my V8 swap. I think I just cut everything out, looped the one line and cut off the second pressure regulator in the tank on the "return" line as the NB is essentially return-less.

My car was running very high fuel pressure The 5.0 runs at 42-45 psi and we were seeing 60-70 psi or something. Running pig-rich until we found the extra regulator. Cut it off and all was well. If the LSx runs higher than 70 psi, disregard.

I only say this because the NB is different from the NA's. I didn't even know that they had a pressure regulator in the rail and a second in the tank.

Oh, and I've had no trouble with pressure or vacuum in the tank with the stock filler cap.

I ran braided fuel line from the tank to the front and back with cleanable 20 micron filter with a 5 micron filter after that in-line. All Summit parts.

Yeah. I ran in an endurance car with a bad vent system and my eyes were burning by the end of my stint. We just moved the filler neck vent line exit further back. In the same race, a fuel system failed causing a different car to shoot a stream of fuel on every right hand turn. Passing that car sucked and it took a while to black flag them because it was at night.

Either way the run a line and vent it method is done at pro levels with a rollover / fluid valve. All the stock crap makes you think there is more to it or needs to be more elaborate but fortunately a simple valve with a vertical line that then runs into the air stream or routes back below the vehicle is enough.

Definitely run what you feel comfortable with as you pointed out, it is a fuel system issue.

I ran a rubber line from the vent line up where the canister was back down the frame rail to VTA in front of the rear diff. The extra distance kept me from smelling it up in the cabin. I also drilled a tiny hole in the filler cap because pressure was building in the tank. I'm not happy with the drilled cap because of possibility of fuel drizzling out of tiny hole in cap after a rollover.

If you have a VTA line from the tank, I don't see how any significant amount of pressure can build in the tank, making the hole in the cap unnecessary.

We just looked at the puge system on a NA6 Miata I picked up over the weekend. Never really studied it before. It appears to me that the tank purge is directed to the engine, through the charcoal canister. I think the line that is open to atmo on the canister is not an outlet, but an inlet for when the purge is occurring. It prevents the engine from drawing too much vacuum on the tank.

Conclusions drawn from casual observation. Didn't actually test.

10:1 the hardline is clogged if you still have pressure in the tank. I had that same problem and didn't find it until I deleted the hardline up to the engine bay completely. It was fill with a black tar substance. I guessing remnants of the charcoal canister.

Yeah FWIW that is the whole deal with charcoal canister systems. The fuel vapors vent from the tank into the charcoal canisters, which absorb and store the vapors. Due to the charcoal they can store much more than just an empty tank. When engine conditions are right, the system opens up the purge line and dumps the vapors stored in the canister into the intake manifold, where they are then burned by the engine. It is more environmentally friendly than just dumping the vapors directly to the atmosphere.

I think you are right about the valve that pulls air into the system. It would have to have something like that so there could be flow through the canister without pulling a vacuum on the tank like you said.

Anyway, all that stuff is gone now. Because racecar.

Dan, just curious, was (is) this on the black S/C car, or a different one? Agreed about drilling out the cap. That reminds me of a Formula SAE story I will not get into. It involved some creative engineering on my team's part.

I don't know how it can smell so strongly of vapors sometimes up at the terminus of the line, and yet have significant pressure at the cap, but it did. When loosening the cap it would spray fuel vapor under great pressure and you could hear the metal of the tank unswelling. This was common in the heat of the summer after running for hours straight. I know the fuel going from the tank through the hot fuel rail and back to the tank over and over again was heating it up over the hours and building pressure.

I've seen a buddy's rubber fuel filler line burst from pressure while he was on track. He barely got the car off the track and was puking and whatnot by the time he got out of the car. Needless to say it could have been a lot worse if anything had sparked. Venting the pressure is serious business...

robert

I'm not suggesting its the answer to anyone's situation but is there a spring loaded gas cap for our cars. My 95 cap has a spring and some stuff goin on inside the cap. It still seems to build a bit of pressure during the hot summer months. I'm looking for just a cap with a spring loaded, preset release to atmo.

Btw, does anyone have a capless system? Not so much on our Miatas but newer cars, specifically a 2011 Navigator, they have a gas flap with a seal on it and a surface on the fender that just sits there. I don't think it ever pulls a vac or anything. Got a cel from it the other day. I just don't see how it keeps rain or pressure washing out of there. The vapors are most likely taken care of down in the tank or system somewhere but its mind bottling to me. Someone explain to this tard.

Sorry to resurrect such an old thread but it had the most pertinent information of any I could find with my searching.

So ZX is this what you ended up doing?: