I just drove an old Humvee with the 6.2 in it from California to about 6500 feet here in Colorado. It ran great with no smoke until I got into the rockies. Now it will only go about 30 mph uphill and not much faster when level. If I give it more gas, it starts to pour black smoke. I know the altitude can have alot to do with it but this seems extreme. I have a few ideas of what it might be but I would like to hear what you experts think first! Thanks in advance.

Clogged aircleaner or restricted airflow would help exaggerate the thin air,would be a guess

I took my '90 Blazer with a 6.2 to see some friends near Denver years ago.

Once I got into the mountains, it was just like you describe.

Naturally aspirated engines rely on atmospheric pressure to fill the cylinders, so they don't stand much of a chance in high altitude.

There was a bulletin from GM back in the 6.2 days about making a "High Altitude Adjustment" to the I.P. It was to be performed in the dealership. Don't remember exactly how or what, just remember we got it because Oregon has some "high places" and the local rep thought we might run into it. Never did while I was there.

If I remember right, the high altitude adjustment basically decreased fuel in the injection pump. It also decreased power.

Not going over 35 with a lot of black smoke makes me wonder, what gear was the vehicle in? If it was already in high gear, the engine was probably bogging down. Try downshifting and get the rpms up and see what happens.

There are actually 'altitude comensated' injection pumps.

But its nothing fancy, just like D Cam says, turn the fuel down, turn the advance up.

They don't make any more power, just less smoke.

Get some more air to it to reduce the smoke:exactly:

I installed my 1984 6.2L N/A engine at sea level in 1985 and we have lived at the 5,000 ft. level for the last eleven years and I have not made an adjustment (because I didn't know about it). I have not had a smoke or power problem with our engine. And our engine with 225K miles still puts out max power with minimum smoke on the local county dynomoter. I do change the oil/filter close to 3K miles and change the fuel filter as required per Chev. maint. manual. Most of our fuel has been purchased from FlyingJ truck stops or other trucks stops when FlyingJ is not available. I have never found any water in out fuel filter, but I have seen an occasional creamy substance in the fuel filter (when I tear them apart) so I think water or moisture has been there. I don't have a computer for my truck so the water in fuel warning light is not operative. I have a manual switch for the filter fuel heater.