Right I guess this is where my noobish attempts digging at this thing end.
So check valve instead of solenoid even worse idea?

Do you have an idea how to get some use out of the canister? ​​​

At the minimum, since you can't write custom logic/firmware, you would need a pressure sensor on the tank, and only activate the solenoid when the pressure inside the tank is above barometric and the intake manifold pressure is between 80kPa and barometric pressure. This will allow you vent pressure from the tank to the manifold at a "controllable" rate. You will also need a failsafe valve that will allow fresh air into the tank should the vacuum inside the tank fallow below approximately 90kPa and also set a CEL if this persists. This is somewhat doable on an MS3 if you have enougj inputs, outputs and loops left.

Link pnp ECUs all come with functioning purge capabilities. I don't personally use the function, but here's a copy of the help section on the topic:

Where is the in-tank pressure sensor on a 1991 miata?

There isn’t one

https://cimg3.ibsrv.net/gimg/www.mia...3a1273d4f0.jpg
​​​​​​'94 1.8 fuel and vacuum system





https://cimg6.ibsrv.net/gimg/www.mia...6e6f077f51.jpg
'92 1.6

exactly

Thought I might chime in with my recent experiences:

I just got my car running with the new built engine and turbo setup; car is still technically a MSM, but is built like a run of the mill turbo NB. BP4W with forged rods and pistons, flattop intake, LS coils, FMII 2560R kit, with AC/PS

In the process of removing and re-installing the engine I decided to delete the EVAP canister and solenoid in the engine bay to make room for a catch can. Everything in the rear of the car remains the same, except with a DW200 fuel pump.
I originally did as a couple of users suggested and ran the fuel tank vent line straight to the intake manifold with one of the crappy little plastic check valves inline to prevent pressurizing the intake. I found that after cruising around in high vacuum for a long time, the car would start to crackle and pop on decel as if my decel fuel cut weren't working properly. Additionally, the car would begin to mysteriously and intermittently idle on 3 cylinders when hot and would have a dead miss on cylinder 1 under very light throttle. When this occurred my AFR's would go pig rich and the car would idle like garbage as my O2 control tried to pull the fuel back. As soon as I dipped into boost, cylinder 1 would wake up and everything would be fine.

My investigation proceeded as follows:
Disconnected the coil for cylinder #1 while idling on 3 cylinders and found no change in idle; pulled the plug and it was black, fouled and reeked of fuel. Coil appeared to be firing with a healthy spark jumping to the mounting bracket
Fresh plugs, new day... no change. After driving for about an hour the mysterious misfire and bad idle had returned. Pulled plug #1, fouled again.
Checked compression suspecting bad rings or valve problem; all cylinders were an even 150 psi.
Swapped coil 1 and 2 suspecting intermittent coil issues with heat soak... No change.
Changed fuel injectors from FF 900's to FF 640's, suspecting a leaky injector... No change.
Removed PCV and vented both sides of the valve cover to breather tank, suspecting oil flow from PCV to be fouling out the plug... No change.
Disconnected fuel tank vent line and capped the intake port... PROBLEM SOLVED

This past weekend I drove 200 miles and ran an autocross event and experienced no issues while the vent line was disconnected and venting to atmosphere. So far the only plausible explanation I have is that the engine was sucking extra fuel from the tank under high vacuum and at idle, fouling the plug and causing the obnoxious burble tune noises. As soon as I got into boost, the check valve would do its job and the issue would disappear.

Right now my plan is to either run the vent line back down into the frame to keep the fumes out of the engine bay or to implement a solenoid that is only open for a few seconds at a time during steady state cruise.

Check valve goes to evap canister, not the tank.

I removed the small canister and solenoid valve in the engine bay. I connected the smaller diameter hardline in the engine bay to a port on the intake manifold with a check valve inline.

To clarify, I deleted what is labeled as the "catch tank" and the "purge solenoid valve" in the engine bay. The large charcoal canister at the back of the car remains. For some reason deleting the solenoid valve and catch tank seems to causes issues on an MSM, or at least on my particular car.
https://cimg7.ibsrv.net/gimg/www.mia...3592eaeb42.png

Would that be because you're now pulling 20hg of vacuum at idle on the fuel tank with a direct connection between the hard pipe and intake manifold? You've bypassed everything that controls vacuum flow rate if I'm not mistaken?

Yes, I believe that's why I'm having problems. What I don't understand is how people can do that on an NA and not have similar issues.

On an NA it looks like the charcoal canister has a vent line going directly to atmosphere, which would keep the pressure slightly below ambient but probably wouldn't suck hard enough to get a substantial amount of fuel. It is unclear to me how the check valves before the charcoal canister operate.

On an MSM the vent line is present, but I wonder if the leak detection pump is normally closed and is allowing the line to get down to substantial vacuum.

Depin the fuel tank pressure sender and wire it to an analog input; you’ll be able to monitor tank pressure.

if you’re stock, it’ll be in the OBD2 data.

Does an NB2 have a fuel tank pressure sensor? There's one listed on the 99-2001 evap system diagram but there doesn't appear to be one listed in my '05 evap system manual or on NB2 the ECU pinout. The EVAP leak detection pump doesn't appear to have a pressure output.

​​​​​​Also the FSM shows that the leak detection pump should be normally open if there's no power. Further investigation is required.

Either way, it appears that NA6, NA8, NB1 and NB2's all have EVAP systems that are different enough to require different strategies or deletion methods.

Why not get a GM FTP sensor and tap the top of the fuel sender assembly like GM does? Should be a linear 5V signal that you could use.

@jawatkins95
The NB2 does have a pressure sensor for the tank. It is mounted above the gas tank. Currently I am working a return system on my 2001. MS3PnP Pro and Trackspeed turbo'd for years now. Would have trouble putting gas in the car if on a road trip. Ended up doing a switch on the dash so could hit it as coming up on gas station. Worked well for that. Engaged and small throttle conditions it will definitely add fuel. if at idle could kill the motor. I don't have a solution just yet to mine. Limits of Megasquirt are a problem. One thought is a low pressure switch( idiot light type oil switch?) and Use ECU to control power to it during desired engine parameters.