(86 n/a)I have an audible vacuum leak coming from the part circled in red with the two vacuum hoses going to it pictured below. It’s located on the back of the UIM. The leak is coming from the plastic cap not the vacuum lines so I think I’ll need to replace the plastic piece. What is this part called?

 

you might check to see if the hoses aren't backwards, but Mazda calls that thing N304-13-980A Valve Check

 

Hope this helps. . . .

Pages 4A-57 and 4A-58

http://www.foxed.ca/rx7manual/manual...SYSTEMS_NA.pdf

 

So according to that diagram (thanks rlynchster) the thermal wax SHOULD be bypassing air out the plastic cap when the coolant temp is over 140F. So the hissing noise I’m hearing is most likely the valve operating properly. Guess I’ll keep looking for leaks elsewhere…

 

its supposed to block the vacuum leak, and release the throttle plate. try swapping the hoses.

 

Beieve it or not, I believe the top diagram 102 is incorrect. . . ..

For manual transmission, when the car is cold, the thermowax valve will allow vacuum to pull through A to B and thereby actuate the double throttle diaphragm which in turn closes the secondary flapper in the intake.

As the car warms up, it allows A to vent through to air (C) thereby allowing the double throttle diaphragm actuator to go back to neutral position and open the secondary intake plate.

If you are having the thermowax valve suck air in while it is cold, and always suck air, the effect should be a bad idle.. . It should never sound like it is sucking air through what looks like a vent filter hole on the side (C)

All this being said, on my 86 rx7 N/A the double throttle diaphragm vac hose goes on the outside nipple of the thermowax valve, and the engine block goes on the inside nipple. . . Which makes sense given the nature of the control I described above.

The check valve in line to the DTD actuator should let air escape to the thermowax valve. During acceleration, the engine will pull vacuum on the line and keep the DTD sucked in which keeps the secondary intake plate closed. While decelerating, the engine will try to pressurize the line thereby forcing the DTD actuator to open the intake plate while it is cold.. . . .The check valve prevents the engine vacuum from opening and closing the DTD actuator when stopping and going while the car is still cold.. . . .The DTD is either sucked in when it is cold, or pushed out in it's neutral state when it is warm. (through (C))

All this being said, the DTD actuator is probably a leaker due to its age, mine was, and there is no replacement. . .. Use a hand pump to check it. If it leaks, put a screw in the vac line and leave it. The DTD actuator in it's normal state keeps the secondary plate open.. . .I put a thread on here about how to replace the DTD actuator with a VW Frankenstein replacement.

To me it sounds like the thermowax valve is bad because the photo you provided shows the DTD actuator line on the outside nipple (A) going to the engine block. While the inner nipple (B) going to the DTD actuator through the check valve.. . Which I believe is correct.

One more thing, if you just plugged the vac line coming from the engine block, your idle should be better and DTD actuator would be taken out of the system altogether.

 

Most of the double throttle diaphragms have perished by now, and the system is not really useful anyways. I think the best course of action is to, as rlynchester said, plug the line coming off the block.

 

My guess is that the hoses are hooked up backwards. Try swapping them to see if it fixes it. If not, cap off those vac hoses and try again. I bet that they are either swapped or that thermowax valve is busted.

 

When I cap off the vacuum line coming from the block the engine almost dies and starts to surge down to probably 200 rpm and back which I don’t understand at all. Also the DTD is DEFINITELY busted but even when I move the actuator by hand there is no difference in idle at all is that normal?

 

Sounds normal, the regular throttle plates are closed as well, closing the double throttle too shouldn't make a difference. The surging is weird, try setting your idle speed with the adjust screw, maybe it needs more air now? It can definitely be capped off, mine is as well. Btw, normally the busted DTD doesn't cause a big leak because there is a delay valve that restricts airflow coming in, so its only a small leak.