Help with 1999 fuel tank and charcoal system
 

Hey guys. I could use some help with my 99 duel tank connections and charcoal system.

My tank is no no food so I’m swapping it out with a tank from a 2002. The 99 tank has 2 electric connectors relating to the charcoal system. One is a pressure sensor and the other I have no clue. The 2002 tank has the pressure sensor but does not have the 2nd electrical connection. Pics at bottom.

Basicallt, I want to know if I can simply leave both connectors unplugged if I’m deleting the charcoal canister.

2nd question is relating to the hardlines. 3 hardlines run from the tank to the front. If I am correct only one line actually carries fuel...the one coming out of the fuel filter. The other two hardlines are connected to the charcoal canister I believe, correct? If this is the case can those two lines be completely removed from the car, so in essence only 1 hardline would run from rear to front? I know I would have to cap the intake manifold vacuum fitting that attaches to the vapor canister upfront.

I hope this makes sense. Any help is appreciated. I am not worried about the related CELs/fuel smell as long as the car will drive the same. The car is “off-road” only use.

Thanks
https://cimg3.ibsrv.net/gimg/www.mia...4faf3fbc6.jpeg
99 tank
https://cimg7.ibsrv.net/gimg/www.mia...f38a59323.jpeg
2002 tank

There should only be two hard fuel lines on a NB, big one is fuel and small one is vapor. The third is brakes.

The NB2 gas tank has no external vent on top like the NA and NB1, instead it's a bunch of plastic bullshit on the inside of the tank, only one small vent tube that appears to just be a pump shutoff when the tank is "full." If you remove that fat hose and cap it with a 7/8" plug your tank won't take fuel without feeding back up the filler neck, or at least it does on mine. I've verified the neck is clear, the tube inside the tank is clear, and the vent on the neck is clear, modded the cap to vent, still will not vent air while fueling but will not build pressure. Super annoying, but it works. My bet is all this bullshit is to force all fueling vapors through the fat ass NB2 charcoal can to then vent. Fuck that.

Also, if you instead remove that pressure sensor, small thing tee'd off the large hose and instead cap the other side of the large T, and run that hose over to the filler vent and tee it in there it still won't take fuel. NA (maybe NB1?) had two lines here, seems one was higher and could actually vent while fueling. I want one. I would keep the NB2 white T, plug the end that runs down to the charcoal can, and then run the old pressure sensor port over to the second higher tube on the filler neck.

https://fc47ae86-a-62cb3a1a-s-sites....attredirects=1

Hope this helps!

You are right. Only 2 hard lines. So basically I can remove the one line completely right?

im following what you are talking about. I have both 99 neck and 02 neck. Based on what you are saying I might leave the 99 neck in the car. This would allow me to run the line from the T to the large upper port on the filler neck, and then the small fitting on the tank I can run to one of the other filler neck fittings...and the 3rd filler neck fitting will get plugged. I think this is what you are saying.

You've got it, that's pretty much my plan. Let me know how it works!

Awesome. I’ll let you know. Only thing I see as problematic is the lack of a vent. The gas cap will hold in pressure. I’m wondering if it might be beneficial to install a T at one of the fill tube lines and have a little VTA line.

You can pull the cap apart and remove the spring that holds pressure. If I remember right there's still a little plastic disc flap you can leave in that might somewhat work as a rollover valve. I seem to remember my na6 having a large steel bearing instead, would be a much better rollver valve due to the heft.

Is the only point of holding pressure in the factory system for the evap emission system to function properly? I’m guessing yes as on my small engine builds (minibikes) the caps don’t hold pressure at all.

Yes, the tank holds pressure for government stuff, not supposed to vent unburned fuel vapor. The pressure sensor lets the ECU know when it needs to cycle the purge valve, or if you left the gas cap off.

Here are some pics, differences between NB1 and NB2. Looks like I need a NB1 fuel neck.


NB2 emissions stuff...
https://cimg8.ibsrv.net/gimg/www.mia...14bdabf9d4.png

1999 miata fuel neck, shutoff and vent.
https://cimg0.ibsrv.net/gimg/www.mia...3b7906cd25.png

NB1 Emissions stuff
https://cimg1.ibsrv.net/gimg/www.mia...f8fdc2594f.png

Good info. Thank you for the help. I’ll see how it works out in the next couple of weeks hopefully. I’ll be using the nb2 tank with NB1 neck.

I got the rear of the evap system deleted and the new line routed. I pulled the “T” on the NB2 tank and it’s simply a union. No internals. So I deleted the “T”. I ran a larger line from the factory outlet to the large upper port on the NB1 fuel neck. Installed the tank and subframe. It definitely cleaned things up a good amount.

https://cimg2.ibsrv.net/gimg/www.mia...b2ce569d8.jpeg
https://cimg3.ibsrv.net/gimg/www.mia...b88db1659.jpeg

Had the impression that the purge valve was either "open to manifold" or "VTA" in the subframe; apparently it also has a "sealed" position?

The purge valve is what opens to release vapors to the charcoal canister I believe. There is a VTA line on the stock tank setup. The line runs up by the underside of the passenger shock tower. It has some circular contraption that the line ends in. It’s some sort of vent. No check valve or anything...it flows in either direction. Or sure if it functions as a breather as well as a vent.

When you say purge valve I think #8 or #1 from the NB2 and NB1 diagram I posted earlier. It's only got two ports so it can't vent to the subframe. It's cycled based on ECT and tank pressure and apparently runs PWM to target a fixed tank pressure and to keep from messing with the engine operation too much.

Strictly speaking NB here, not NA. I guess technically the NB can under the hood isn't a charcoal can, it's a catch tank. But, it only has two ports, not three like a NA.

https://www.ebay.com/itm/99-05-NB-MA...-/322585236599

https://www.ebay.com/itm/90-93-NB-MA...75.c100623.m-1

https://www.ebay.com/itm/94-95-96-97...IAAOSw-ctbe5bl

Just recalled a discussion we had with a mechanic friend a couple of months ago. EUDM NB2s are totally different on that aspect compared to USDM. There's no rear mounted charcoal can, and the one on the engine bay is a 3-way unit (line from tank, line to intake manifold, line routed inside the front subframe), controlled by THIS solenoid valve next to it.