Chevrolet Repair: 1988 Astro Van, chevy astro van, chevy astro

Question
  Hi Mark,
I have a 1988 Chevy Astro Van with a 4.3l engine. I'm an ex backyard mechanic but this has me stumped, along with several other mechanics. It has rebuilt engine in it with approx 10k miles. The van has 136k on it. Always had a little rough idle but not concerned with that. I had head gaskets replaced two years ago, (throwing coolant when cold). PCM is not showing any codes. Other than the problem the engine runs fine and will cruise at any reasonable speed.
The engine starts to miss at around 40 to whatever mph under load, like up an incline, after 5 to 10 seconds.  If I approach an incline at say 60mph and the engine goes under load it will start to miss after a few seconds. It appears to be load sensitive rather than speed or rpm sensitive. I have to back off the pedal to stop it from missing or shuttering.  Does when cold or hot. I replaced wires, plugs, dist cap, rotor, at the onset. Then replaced dist pick-up coil, ignition module, egr valve and egr control. Replaced fuel filter and rebuilt the TBI unit.  The miss feels like one cylinder (not sure) when it happens as opposed to a sputter. I had it to the first mechanic and he checked, among other things, compression and hydrocarbon. Found no problem. Thought problem was transmission. Had it to a trans mechanic and he thought it was engine. (Disconnected trans solenoid to prevent lock-up). Still did it. Engine still misses when downshifting up a hill as long as the load is present.
  Finally took it to a dealer with three minds working on it. Replaced distributor shaft. No. Checked to see if injectors were cutting out. No. Believe he substituted a MAP sensor (not sure). Substituted ignition coil. No.  Mechanic is at a loss. Says it almost feels like a balance problem. I believe this engine does not have a balance shaft. Checked for loose harmonic balancer and flywheel problems. No. It only does it under load and after 5 to 10 seconds? Did another compression check and said it was fine. He wants to try a PCM but he has none around so I have to buy it. Already put $900 + into it but mostly troubleshooting time with no paths to take. I have a background in electronics and can troubleshoot but I need some kind of direction. Thanks.
HELP!


Answer
Hello John, Well you have done a lot here to try to repair this problem. The only thing left that I have seen is the valves in the cylnder head. I have seen them stick at highway speed after a short time under load. I have seen this on trucks with less than 3k miles. There is no way to check for this problem other then having the heads done over and reinstall them. This is the only thing left.

Hope this helps you!!!

Have a Great Day     Mark