Purge valve solenoid control with MS3-Pro?
 

Hi! I want/need to have a functional evap system in my 1.6 NA running a 2003 VVT engine with MS3-Pro. How would i operate the purge solenoid?

My idea was to use a generic PWM output or open loop boost control (PWM) with a RPM/TPS% table. Whenever throttle is closed (TPS=0) and the engine below 1200 RPMs, I'd run a duty cycle of around 40% and 0% anywhere else.
This would relieve fuel vapor pressure into the manifold when idling and would otherwise not interfere with the intake system.


Or should I simply apply a switched 12V source (blue wire from ignitor, e.g.) and ground the second wire at the chassis? This way the purge valve solenoid would stay closed with the engine off, and open with the engine running. Or would intake vacuum draw too much additional air from the tank via the evap system?

It's already connected to a switched 12v, you can control the ground through the MS.

You'd probably want to rethink your PWM table, IIRC you really only want to purge during cruise.

thanks a lot! i just found a table at MSextra

https://www.miataturbo.net/attachmen...ine=1454249989

bear in mind that his car uses inverted duty cycle values (100=fully closed, 0=fully open). I'd give this a try, albeit with inverted values (100 - *his value*). Or do you have a better idea (what do i know..?)?

I was thinking about the same thing. Removing the evap canister could have me fail inspection (depending on how awake the inspector is), and if it's there I might as well have it operational.

What is known about the behaviour of the stock ECU on this? I remember reading somewhere about it only purging on cruise with warm engine every now and then, but not sure where I got that from (or really what it said).

Also, I would guess that having the valve "open too much" at the wrong time on a stock ecu would mean the engine is getting unmetered air, but if running speed density, would it matter to the engine if it's getting "evap canister air" or not? (assuming it's not shitloads of gas fumes)

I had thoughts about using an Arduino (reading data off the MS3-pro over CAN, which I'll be doing anyway for other reasons), in which case it could also consider coolant temp (is engine warm yet) and/or how long it's been running. But if it doesn't bother the engine otherwise, just having Megasquirt open it at "cruise loads" on mid rpm's maybe works just as well and would be much simpler.

i think all you thoughts, jstck, are spot on.

If I end up with a stock ecu car in my garage anytime soon I will try and scope the purge output in different states. If I remember.

Ah, of course. Measure reality.

I still have the stock ECU in, and as soon as winter is gone here and the MX-5 is out of hibernation (in 10-12 weeks...) I should rig a scope and maybe a diagnostic LED to it (easy to see when driving) and see what gives.
I want to inspect a bunch of signals anyway before I swap ECU to have some kind of baseline for later when I'm trying to figure out why it won't run on the MS3-pro...

put it on jackstands and drive?

Oddly enough, I'm putting MS3Pro in a completely different car and I was looking for examples for purge control... forgot that I had an account here :)

Pre-OBD-II (meaning "dumb evap") GM products just run a 50% duty cycle at anything over idle and 5mph. No purge at idle or when stopped, this is a simple way to say "only purge when engine is loaded".

Hope this helps!

Good stuff, thanks!