This vehicle has been designed for principal use at high altitude.

[CrustyJunker]

I've never seen this...Pretty interesting! I wonder if it's just a minor mechanical adjustment they did, or if there's something different in the tune?

 

[BBSteve]

Hello, my 1988 K3500 with the 7.4,TH400 and 4.10 gears has a high altitude emission sticker on it. It was originaly sold from a dealership in Wyoming, I purchased the truck last year and brought it to the Midwest area which is much lower in elevation. The truck ran OK but acted lean on cold starts and engine temp always hovered around 210 at highway speeds and just generally seemed weak for a 7.4. I had the PROM ID'ed before ordering a PROM from Harris performance. The PROM info was that it was for a 1988 5.7, pretty strange. The PROM matched the ECM so I am for certain that it had not been changed. Anyway after replacing the PROM with one that I ordered from Harris, the trucks lean while cold, hot running issues are gone and it acts like a 7.4 should act.

 

[August76]

My truck is a 97 5.7 z71 we moved to PrescottValley Arizona at 5000 feet mile high. I go into Phoenix often. I see no changes in the elevation on my truck but people here say gas milage changes. I'm more affected than the truck. Thin air sucks. Breathing