I'm wondering about the impact of high altitude on engine performance.

Last year while driving from Florida through Tennessee the truck ran great all the way till I got to the jelico mountains in TN and and then somewhere around the top, even when it leveled out, the truck had no power at all. I had it to the floor and could barely hit 40 mph. Once I started getting over the mountains and to lower altitude the truck was back to normal.

It happened the same way again this year, same truck. It runs great otherwise. I noticed that none of the other cars around me at the time were having a problem so that's why I'm curious, and freaking out at the time!
The truck has just over 120k on it and has been well maintained. If it was anything serious I would have expected it to have "gone out" through the year in between.

It's a 92 s10 blazer 4x4, 4.3 tbi. was recently tuned up before both trips.

So, should High altitude have an impact on performance?

I wonder if your MAP sensor isn't flakin out at higher altitudes?

fuel injected versions should do better at compensating for the reduced air density using the MAP (TBI) or MAF (CPI) sensors, but in general if the air is less dense, then you can't burn as much fuel, resulting in less power produced.

Doing a little research, those mountain roads are only up to about 1,750 ft above sea level. I've owned a '92 4.3L, it hauled a-- and yours should too. That little difference in altitude should not make that much difference in performance. I'd have a good tune-up done, cleaning/replacing the plugs, wires, cap, rotor, and air filter. Plus, clean the MAP sensor with a spray cleaner specific for it.

Next, check the throttle body gaskets and vacuum hoses for leaks...a direct shot of carb cleaner at suspected spots while the engine running will cause the engine to stumble if there's a leak.

Next, check/clean the PCV, IAC, and EGR valves.

Finally, I'd suggest a engine fuel, oil, and intake treatment such as SeaFoam to clean out carbon deposits. Hopefully with these tips you'll be zipping up the mountains like you ought to be. Good luck!

Thanks for the info guys. I was think maybe map sensor or something like that. I will check it out. I have always kept it tuned up for the basic things with delco plugs, wires, cap etc... but not looked at alot of those other things.