1990 Suburban dies intermittently

[SoCalSlaughter]

The vehicle is a 1990 Chevy Suburban V2500 5.7 - TH400. The truck just passed Calif smog, and did pretty well. But its been having a problem recently, doesn't do this all the time. Say if I drive to work, work for 4 hours and go to lunch, on my way home, if I hit a stop light the truck will surge, I will hear a "whistle" type sound and the engine will shut off. It will start right back up and drive. It also does this sometimes when I come to a stop after being on the freeway. Also, if I come to a stop and am sitting for longer then 5 minutes, the truck will surge, I will hear a whistle type sound and the rpms will raise, and it will stay that way until I shut the truck off and turn it back on. Ive already swapped out the IAC, the ECM, distributor, cap&rotor, wires, plugs, tps, coolant temp switch and ICM. Also the truck last year was getting a code 42 (Electronic spark timing circuit open or shorted) But I havent seen the code in more then 5 months. I work next to a speed shop and know the owner pretty well and I have access to many parts. But I just want my truck to run normal. haha Thanks guys

[cableguy0]

With no codes at all its going to be tough to find. What condition is the throttle body in? With as old as it is it could very well be a worn out throttle body. Replacements are available. Also just because a part is new doesn't mean its any good. I would lean toward the IAC myself because the symptoms your describing lean toward a bad IAC. Do you have access to a known good part?

[SoCalSlaughter]

The throttle body is in pretty good condition. The truck has 161k miles. I rebuilt the TB little over a year ago with new injectors and all. I went to my friends shop and took 4 IAC's out to my truck. I pulled the ECM fuse on each IAC swap and the same result for all. Also I have a vaccum gauge attached to the intake manifold and Im at a steady 19psi at idle while hot. When the whistle comes, the vac doesn't drop at all. Also the mpg hasnt been as.. bad? as it used to be. haha

[72MARIO]

Check the power wire and knock sensor from the starter up to the firewall I recently had the same problem with my 1990 suburban. The wires had started to melt inside the steel tube that's supposed protect them, because it was bent and touching the manifold

[SoCalSlaughter]

Ive checked all the knock sensor and starter wires, I had the transmission off not to long ago to install a new flex plate and I rewrapped all those wires. I work with a Ford master tech of 35 years, and he says it sounds like its my brake booster. Because the whistle and the dying only happens when I come to a stop, he said it can be a large vacuum leak from the booster. But I dont have a hard pedal, its actually quiet soft. Goes almost to the floor and the truck stops pretty well with having 35" tires on it. He said to try putting the truck in neutral next time im coming to a stop and dont hold the brake and see if it has the whistle or dies. hmmmm