88 K1500 4x4 350SB Overheating

[Doug O]

I am having trouble fiixing a overheating problem. I have changed out the thermostat twice, replaced temp sensors on the intake and the head, flushed radiator, and changed out the serpintine belt. I think that it may be the waterpump, but after it got hot on me one day I raised the hood and the top radiator hose was completely sucked down. If the waterpump was going bad would this happen? When I remove the thermostat the truck runs cold like it should.
This is a new motor out of the create. I had a 305 in it and switched it for the 350. I'm using the intake off of the 305 with the throttlebody injector. Any information would be greatly appreciated. Thank you Doug O.

[truckdude239]

can i ask a really sutpid question? why did you post this in the computer forum lol might want to post here http://67-72chevytrucks.com/vboard/f...splay.php?f=12

[Palf70Step]

Welcome to the board Doug O. I moved your post to the area your more likely to find an answer.

[ChevyTech]

Here are a few ideas:
When you put the intake gaskets in, did you put the coolant passage opening holes to the front where they should be, or to the rear of the engine?

Did you change the injectors in the TBI unit? 305 and 350s use the same TBI but have different flow rate injectors in the TBI. The 305 injectors will make it run lean which will make it run hot.

As for the radiator hose getting sucked down, make sure the hose to the coolant recovery tank is open and not pinched. If the recovery tank hose is open then your radiator cap is bad. The radiator cap should allow coolant to be sucked in from the tank through the little valve in the middle of the cap.

I see it’s your first post. Welcome to the board.

[Mr_Rich]

Speaking as a person who has done this swap; the 305 injectors will run fine with the 350. The 350 will run better with 350 injectors, but it certainly doesn't cause overheating. I have a Hypertech chip in my computer which may have compensated for the 305 injectors I ran for a couple of years. That may have been a factor. I bought a low-mileage '90 roller-cammed 350 many years ago for another project of mine, and what I've ended up doing is using a lot of the peripheral parts from that 350 to keep my truck running.

The only thing I can think of that's weird is the water pump situation. Could you have wound up with a non-serpentine belt water pump which would turn in a clockwise direction? I think the serpentine belt water-pump runs counter-clockwise if I'm not mistaken.

Make sure you don't have any coolant leaks, and I'm sure you shouldn't have an overheating problem on a new crate engine.

Thinking about the previous post; make sure you have the correct radiator cap so that coolant can travel both ways. When hot, coolant flows into the tank, and when it cools the coolant should get sucked back in to the radiator.