Chevrolet Repair: 1995 Chevy Astro, chevy astro van, throttle position sensor

Question
Hello. I have a 1995 Chevy Astro Van with a 4.3L Vortex. I have had it less than a month. It has a surging problem. When in park it will sometimes idle way up and then way down. It doesn't do it all the time. Sometimes it stops completely. It also causes it to run funny when it is acting up. It will lunge forward even when you have your foot on the brake. It runs rough and sputters in park and drive. We have done a total tune-up, replaced the throttle position sensor, the oxyegen sensor, and tested the idle position sensor. We have gotten 2 codes from it. The throttle sensor and the oxyegen sensor, but even after we replaced them the problem still will not stop. It is so weird because sometimes when you first start it in the morning it will start and run like nothing is wrong at all, but it will warm up and start doing it, or it won't do it at all again until you shut it off and start it again. Sometimes it will surge up and down until it almost dies and then it will catch itself and even out sometimes. But you can tell when you drive it, it is interfering with the way it runs. It is a good running van if we could just figure out what this is. Any suggestions would be very much appreciated.
Thanks, D";  

Answer
Hi D,
I would check the fuel pressure. It should have throttle body injection, and call for about 13 PSI.

You could look for vacuum leaks, and clean the EGR valve. Check the intake manifold gaskets while you are looking for stuff, but I doubt that is the problem. You can spray some carb/PCV valve cleaner around it while running, to see if it causes an effect.

Run some injector cleaner in the fuel tank.

The cold running fine thing is because when cold, it does not use sensor readings like oxygen sensor, throttle position, etc, but rather uses default values, and just adds plenty of fuel.
After warming up, it goes into closed loop, using the sensors to decide what fuel to add, and where to set the timing.

Kinda makes you wonder why you can't just disconnect some of the sensors to keep it in open loop.

Van