You didn't give specific details about the engine swap, but many swappers omit the factory-style fan shroud. If this is you, the cooling isn't as efficient as is could be. We would suggest a factory-style thermostatically controlled clutch fan in place of the flex fan. It would also help to be sure fresh air is directed through the radiator core, not around it. To this goal, consider using a front air dam.
You could also have engine problems that are affecting the cooling efficiency. The engine should be properly tuned with initial timing set at 8 to 10 degrees BTDC. A radiator shop can run a carbon-dioxide dye test to check for any exhaust gases that might be escaping into the cooling system. The presence of exhaust gases can indicate a bad head gasket or worse, a cracked cylinder head. A compression test will help locate the problem area.
A Long Block
Our family car is an '04 Chevy TrailBlazer with the 4.2L inline six-cylinder engine and four-speed automatic transmission. I really like everything about the TrailBlazer. I like it so much that I wouldn't mind having another one if it wasn't for the cost.
Our second vehicle is a '93 S-10 Chevy Blazer LT four-door with the 4.3L V-6, four-speed automatic and four-wheel-drive. The S-10 Blazer has more than 185,000 miles on it, and the engine is getting a little tired. The rest of the truck is in excellent condition.
This got me to thinking: What if I swapped the old engine and transmission for a 4.2L inline-six from a wrecked TrailBlazer? If I included the transmission and possibly a few other niceties from the TrailBlazer, I could have some of the TrailBlazer's best features for a whole lot less money. The 275 hp and 275 lb-ft of torque in the 4.2L six should make a great improvement over the tired V-6. How much trouble would it be to make this swap? Could I use my existing transmission?
We agree with your praise for the GM 4.2L inline-six. It's a wonderful engine with excellent torque. The idea of a straight-six seems a little retro, but the execution is great. BMW builds some fantastic cars with inline six-cylinder engines, so these engines aren't holdovers from the Fifties.
The 4.2L six is actually lighter by about 40 pounds (407 pounds) than your existing 4.2L V-6. The big negative is the overall size of the inline-six. At almost 33 inches long, it's 3 inches longer than a typical Chevy 350 small-block V-8. A V-8 in an S-10 is a snug fit, so some serious firewall work would be required to accommodate the 4.2L six. The 4200 six is also quite tall. At 32.62 inches tall, you would probably need to raise the hood of your S-10 Blazer.
Part of the height comes from the deep-sump oil pan, which would likely interfere with the front transfer case and related components in your Blazer.A two-wheel-drive Blazer would be less troublesome.
Using your existing transmission would be more trouble than it's worth.The 4200 I-6 is designed for a 360-degree bellhousing, so a custom adapter would be needed to mate your current 700-R4. If you moved the transmission, you'd also have to modify the 4x4 driveshafts. A final snag would be adapting the 4.2L six's electronics that are required for its variable valve timing.