81' 280ZX Runs Rough/Lean When Cold

[Yan13]

Vehicle: 1981 280ZX N/A, Automatic Transmission, 46000 miles

When the car is started it turns right over but runs rough at an idle around 500-600 rpm. If the engine is lightly rev'd during this condition it stumbles and misfires. After the engine begins to warm up the idle increases and smooths out until operating temperature where it runs fine (could maybe have a more steady idle, there is intermittent idle surging of about 50 RPM). The idle is at 750-800 rpm once the engine is at operating temperature. When the car is rev'd to around 3000 RPM (after warmed up) the idle it returns to is sometimes around the 500-600 rpm and appears to be running very lean. The timing is fine.

During the warm up stage there is noticeable engine shaking beyond normal and slight backfiring out the exhaust.

Back story: The tip of the ported vacuum switch (thermal vacuum valve) that threads into the heater housing on top of the intake manifold had a crack in the outer plastic tip somehow so it was replaced. Prior to replacement the engine ran rough when warmed up, and fine from startup through the warm up stage. Related? I dunno.

I have replaced the ignition coil, NGK plugs and wires, starter solenoid, fuel injectors, all vacuum/fuel vapor/emission control lines, O2 sensor, PCV valve, valve adjustment.

I cleaned all electrical contacts under the hood, applied dielectric grease and insured good contact with connector. 
I ran a cylinder compression test with results of 160 +/- 5 psi.
Tested the cold start valve per FSM with OK results.
Tested the air regulator per FSM with OK results.
- I removed the air regulator and inspected the shutter (little door) within the component and the shutter smoothly opens and closes; however, the shutter doesn't close all the way. Is this normal? When I removed the component the shutter looked about 3/4 of the way shut.
The AFM door moves easily and smoothly.
The resistance between the fuel injector terminals is roughly 2.5 +/- .1 Ohms
-When each fuel injector connector is removed while the car is running, there is an equal drop in RPM per each injector.
The distributor cap and rotor are fine.

EGR valve? When I use my finger to depress the plunger within the EGR valve the idle drops and induces a rough idle, similar to what I experience when the car is warming up (my reason for posting this thread). Not sure what they mean by actuated and not actuated in the FSM section about testing the EGR valve. When I hook up a vacuum gauge and apply vacuum to the vacuum port on the EGR valve, the diaphragm raises then depresses slowly until vacuum is 0 again. Should it hold the vacuum?

I am fairly confident that there are no vacuum leaks within the lines connecting all the components and the intake manifold. The exhaust manifold bolts were difficult to tighten after I changed all the gaskets and put the engine back in the car so some may not be fully tightened down. Could this cause my issue where when the car warms up and the metal components expand it closes the gap and creates a proper seal?

Any suggestions or advice would be greatly appreciated and please let me know if I am not including pertinent information.

Cheers.

[Yan13]

UPDATE:

 

I just rechecked the air regulator and there is a resistance of 70.3 which is consistent with manufacturer specifications. The gate within the component is also not supposed to close all the way. So this part seems to be working fine.

 

I really am at square one here.

 

Thank you.

Edited July 26, 2016 by Yan13

[Yan13]

Where I am now.

 

The car idles low and rough around 600-650 after start up and continues through the warm up stage until it runs fine. Then if the car is rev'd to around 3000 RPM while at operating temperature the idle drops from around 800 to around 600 but runs smoother than during warm up.

 

These things must be connected.

 

Please any help and or specialists contacts in the Chicagoland area would be extremely welcome!

[Yan13]

Getting resuts:

 

I determined that the reduction in idle rpm post 3000+ rpm revving was due to a clogged fuel filter. I hope that anyone else experiencing this issue takes that into consideration.

 

This leaves me with the issue of rough idle after startup and throughout the warm-up period. I tested the cold start valve and it passed; however, perhaps the cold start valve isn't getting the signal to perform during the cold stage and warm-up. (i.e. not enriching the air enough to provide a smooth warm-up)

 

I unplugged the double bullet connector from the thermo time switch and it did not affect the engine idle; therefore, I am fairly confident this is where my problem lies. I am going to install a new one and will update soon.