This is an Update to the original thread about the cold engine stumble that early Z31 (1984-1985) owners sometimes call the "lean surge". After figuring out that everyone was overlooking the EGR circuit as a possible cause, I was able to free up the sluggish solenoid that was allowing exhaust gases into the fuel/air mixture when it should have been completely shut/closed at cold engine startup. The EGR-Valve Solenoid was/is still available from some on-line Nissan dealers so I ordered it and just installed it on Saturday.
Sometimes you do things to the car and it causes other problems or issues to appear. Been there, done that! Sometimes it makes the original problem even worse.
A member on another site I frequent commented that I should "take it the f*** off and through it away because it's not needed." "Or block it off with a beer can." 
I just thought it was funny and amusing that someone can get so intense. Anyway, back to the story, after closing it up I got into the car and started it up for a cold engine run down the road. The first thing I noticed was that the fast idle was up 100 rpm to 1,400 rpm. 
When I pulled off the driveway in first gear I immediately notice a faster throttle response and a stronger pull like never before. It chirped into second gear and continued pulling like a freight train. My face was in pain by the time I hit third from the big grin on my face (even my butt felt happy and tingly). This 1985 300ZX GLL only has 32,900 miles on it and is barely broken in. When I bought it a 1 1/2 years ago I thought it ran perfect. I had no idea that some of the ponies were grazing in the meadow waiting to be brought into the corral to join the rest of the stallions. I really can't explain why the car has a faster throttle response or why my idle (both cold and hot) is up 100 rpm or why there appears to be more horsepower/torque. I just know that changing this solenoid on MY car fixed the "stumble" and then gave me a better running and performing fun machine. Now I'll have to try to control my right foot so I don't upset law enforcement.
"NEVER GIVE UP!" and "NEVER SETTLE!"
