surging idle

[black86roadster]

My 86 has been has been doing this off & on for the last year or so, however I barely put 500 miles on it this year,

When car is cold or warming up it will idle normal, once the car has warmed to say 150 degrees it will not idle smoothly, instead it will surge between 0-1000 rpm's and will eventually die unless throttle is applied.

I have replaced many sensors and relays trying to resolve this, and although the car runs great once on the road and performance has been improved with my process of elimination mentality, I have not fixed the problem that has dogged the car since I bought it..

I bought the car w 60K on it at 100% stock condition, not knowing much about vettes at the time I figured the idle surging was an easy fix, my 87 GTA Trans am w/ L98 100K miles ran incredible and never ever gave me any problems.

I have replaced: ECM
Mass Air sensor (pwr & burnoff relays)
Idle Air control
Throttle Pos. sensor
Oxygen sensor (in exhaust pipe)
Engine coolant sensor
SLP cold air induct, K&N
Plugs, wires & Cap roter,
Cleaned throttle body

The only code read by my mechanic indicated Mass Air sensor, I replaced it and relays (again) still surges. possible EGR related?

 

[WhalePirot]

Do you have normal vacuum? The ECM knows what it expects from normal sources of air but can only try (surging) to compensate for vacuum leaks or hot cams.

 

[vettenick]

Get hold of a fuel pressure gauge and scan tool. Check fuel pressure before starting. If it's OK, then leave the gauge hooked up. Start the car with the scan tool connected. As soon as the car acts up, make sure that the car is going into closed loop, then look at the fuel trim #'s( that's the block learn). See if the # is high (above 135) or low (below 120) to see if the computer is adding or subtracting fuel from the engine.
I'm willing to bet you will see something out of wack.
By the way, sometimes, if the car stalls out, you will trip a mass air sensor code even though nothing is wrong.
Let me know what you find.