Don't know if anyone else has experienced this. I have a 1992 Blazer S-10 with the 4.3 Vortec engine which I completely rebuild. I had to over size the pistons and related engine components. I broke the engine in correctly and it seems to work fine until I need to tow my boat (3,000 Lbs). It loses power while towing. On the highway, just going up a moderate hill the speed steadily decreases until I have to press the gas all the way down just to maintain 45 MPH. Of course, the engine temp climbs very high.
Does anyone have an idea what the problem could be? The trans was also rebuilt and isn't slipping (automatic). Do I have to update the computer chip or something? Timing is fine. The dealer and mechanics here are at a loss as to the problem. Any help is appreciated.

 

Have you checked the o2 sensor and the catalytic converter?

 

are you driving a manual or an automatic?
if you are in a manual, i would say shift at higher RPM an hang around 2800 instead of 2200, but if you have an automatic transmission it is possible it is shifting normally without knowing you are towing. Does your s10 have a tacometer? i know some don't.
if it is an automatic, Tow/Haul mode was an option. are you equipped with tow/haul?