I have found my Double Diaphragm Valve, which is vacuum activated by the thermowax, is bad. The effect is that I get a vacuum leak when the car warms up. (i.e. thermowax valve opens up at temp, tries to pull vacuum on the Double Diaphragm Valve which is a complete leaker and now non-functional.)

Does anybody know where I can get a replacement that works? . . . .I am having a hard time sourcing this part. .. .Can I make a S5 unit work?. . .Is there another alternative part?

It seems as if this DDV is only used to keep the secondary's from opening up when cold. . .. . .. . Is the solution to simply plug the line and hide it so that the smog guy when he does his visual doesn't catch it?

Thanks.

 

yeah you could just plug it, but put something in the hose so it looks correct.

part is NLA from Mazda, N326-13-W7Y

 

I read through the training manual for my 88 GXL NA and it explained what this entire system is for.

It is to prevent hesitation from a too lean air/fuel mixture when the secondary throttle plates are opened while the engine is still warming up (below 140 degrees f).
This is for better drivability when the engine is cold.

For the system to work properly all the components need to be in working order.

Let's start with the thremo valve which allows vacuum from the secondary intake manifold to retract the diaphragm which closes the #2 secondary throttle plates.
It is made to allow vacuum through it while the coolant is below 140 degrees, above 140 the vacuum is blocked off and the spring pressure in the diaphragm hold the #2 plate fully open.
I tested mine by attaching two vacuum hoses and submerging it partially into a heated pan of water. Using a thermometer, I blew through the hoses as the temperature rose.
At 140 degrees the hose closest to copper part should close off, this means the valve is working properly.
Also make sure the coolant hoses attached to the thermowax valve are open and not kinked or blocked!

Next, is the hoses with the Delay valve that run from the outside connection on the thremo valve to the diaphragm housing connection. There is a test in the FSM where you attach the delay valve to a meter long length of vacuum hose, apply so much vacuum then release the end of the valve and measure how long it takes for the initial vacuum to go down over time.
A new delay valve is over $50 so try to see if what you have is working!

Now for the fun part, all the stock diaphragms I have come across (I have five now) do not hold vacuum. I took one apart and discovered a vertical cut through the rubber membrane. I think the cause is by the sharp spring end that comes into contact with the membrane over time.
This membrane has to be able to hold vacuum! With any leak in it the #2 secondary throttle plate will never fully close! Causing hesitation as explained above,


damaged membrane

This was the only faulty area on either side of the membrane.
After a third of a century, I do not think that any used diaphragm is in working condition.

I am currently adapting an EGR valve diaphragm to replace the stock one. Search my posts.

 

Yea, getting a working unit is basically impossible at this point. I tried, and just decided to make my own.

When it comes to the EGR valve, I would just blank off the EGR valve port. The EGR only gets activated in 5th gear. It's not activated during the smog test cause you are never in 5th gear on the dyno. It's only a visual check which is why you land the EGR valve back on top of the plate and re-land the vac line there. You can land the EGR valve right back on top of the blank off plate using slightly longer screws. Then you place a set screw in the vacuum line to seal it off from the system cause that will be a diaphragm leaker just like the double diaphragm valve. As you know, the EGR valve is also no longer available.

The real issue for FC owners in terms of rubber diaphragms is the brake booster. . . .. That one is also getting old and replacements are getting harder to find.