What is this thing
 

I have an 88 22REC pickup and I've been chasing an idle problem and miss through checking of all the various components on a newly rebuilt motor. The only two things I haven't looked at are the air valve and this thing in the picture. I can't find any reference to it in my 85 FSM. Does anyone know what it is and how to test it for correct operation? Would it have any affect on a idle or would it cause a misfire?

When cold, the engine starts right up and idles smooth for about 10 seconds then starts missing. If I hold it up around 1200 for a minute, it will take over and hold the high idle up till it warms up. Once it warms up, it drops the idle to normal and it goes through this cycle of normal idle then a slightly higher idle then back to normal, pretty slowly, maybe a 15 second cycle. Engine runs fine other than the cyclic idle and the missing at idle only.

Thank you.https://cimg7.ibsrv.net/gimg/www.yot...6e81338c21.jpg

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Ec-19 air suction system

How do you test it?

With vacuum on the diaphragm the reed valve passes air, no vacuum no flow thru the port to the right.

Let me see if I understand your comment.

Are you saying, apply vacuum to the diaphragm to cause air to pass through the reed valve. If no air passes through the port on the right, there is a problem?

Is that right?

Correct.
This just checks the diaphragm is free moving and there are no obstructions. There are other parts to the system, a check valve (one way valve), vacuum switch, misc piping.

The full system checks start on Ec-19 (around page 150 of the FSM)

https://cimg5.ibsrv.net/gimg/www.yot...9513656ea0.jpg

Ok, took it apart and cleaned it and checked the operation of the diaphragm (perfect) and blew through the read valves and they seemed fine. Put it back together and the same thing happens. Runs rough at normal idle speed then slowly ramps up to a smoother idle at about 1200 and drops suddenly to normal idle. I monitored a vacuum gauge while it did this and it starts at 14.5 inches and builds to 18 inches and suddenly drops back to 14.5 inches. Where should I look next?

So far I've got:

a new motor
adjusted throttle stop screw to 1/4 turn after closed plate
adjusted TPS as per FSM
replaced faulty harmonic balancer
timed to 6 degrees BTDC with T&E1 shorted
working EGR diaphragm
working modulator
new BVSV
new 190 thermostat
new coolant temp sensor
new coolant
cleaned coolant temp sensor contacts
flushed fuel system with sea foam, berryman chemtool and injector cleaner
new fuel filter
new intake gaskets
new air filter
new spark plugs, distributor cap, rotor and wires
new ignitor and coil
new air flow sensor
new fan clutch
new oxygen sensor
checked fuel pressure regulator diaphragm (holds a vacuum)
cleaned AS reed valve and checked diaphragm (holds a vacuum)
checked connections at ECU for corrosion (shiny)
checked vacuum switch for AS (working)
checked AS check valve