Black smoke when hot

[rapidoxidationman]

For the past few summers now I've been getting lots of black smoke when the engine is HOT, like during pulling a big hill (say Teton Pass in the afternoon or a long incline on I-80 on a hot summer day). When the smoke really begins to pour I also notice a real decrease in power. A scenario: Tooling down the highway at 80mph, start to climb a hill, push the go pedal, lots of black smoke and a noticeable drop in power. Sometimes the smoke goes away after a few seconds and the power comes back, other times not. Feels like something is getting stuck, but the turbo spins freely, not loose, the wastegate responds to vacuum and holds it, the wastegate vacuum solenoid works, the vacuum lines are good (one side did get replaced 3 years ago cuz it was melted). Suggestions? I'd like to not spew smoke-except when I want to of course... and I'd like to be able to use all the power this engine can and does still make at 216,000 miles.

'96 Chevy K2500 6.5L TD, 5speed manual

I'm all ears...

[gmctd]

Vacuum at the pump is 26"HG at idle and 2000rpm, cold or hot

at the wg is 15"HG at idle and 2000rpm

If it's "S" engine, check the EGR valve for easy blow-open - vacuum leak thru the sticking EGR solenoid, maybe - then the mystery gasket on the tower inside the manifold may leak at hot

Could be the fuel - try a tank of ULSD with 16oz 2-stroke motor oil and some cetane booster

Too bad ya don't gotta Boost or EGT gage for entertainment and troubleshooting events, such as rat now...........

[rapidoxidationman]

it's an F engine...

I've used B-100 and B-20 as well as pure Dino fuel, and I've also tried dino fuel with a bit of Power Service (grey bottle). I see no difference no matter what is in the tank. The problem only comes up when things are HOT, and it feels like something is getting stuck then freeing up (sometimes). Usually the truck runs like a champ, especially now that I've got the FSD mounted on a cooler on the side of the engine compartment courtesy of JK.

ROM

[gmctd]

Ok - check your vacuum system for spec, and again, a Boost gage would really be helpful, here - 19.95 from jcwhitney