90 burb, New motor, water pump, overheating only when t stat is installed

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Spider

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Montana
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Craig
Truck Year
1990
Truck Model
gmc v2500 sub
Engine Size
5.7 tbi
Alright, this is beating me up. Fresh rebuild, I have burped it, ran 3 different t stats, different temp t stats, verified the correct water pump, new fan clutch, and it still isn’t right. Without a t stat it sits at 120-130, Idling or driving. Ive been driving it as many as 50 miles at a time at highway speed and it doesn’t overheat sans t stat. I’ve shot it with a temp gun, the gauge works. And one of the t stats was actually cycling, but at 240 and then it would drop down like a good gm should. 240 is way too hot. I’ve driven tbi 350s throughout the years and usually they sit pretty consistent at 180 on the gauge. Radiator worked before I pulled the motor and it didn’t have overheating problems. Never seen a 350 act like this. It doesn’t take more than a couple minutes for it to get hot with the t stat installed. I’d run it for the summer without but I’ve got a couple more issues I’m working through and coolant temp needs to be at operating temperature to continue. Plus I like quick heat in montana spring time.
 
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Strick

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Make certain that the belt routing & fan blade orientation is correct. Make certain that there's correct tension on the belt as well. I ran into a similar issue with an old NC Forestry K2500 years back.

HS
 

Ricko1966

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Drill a bypass hole in a T Stat 2x1/16th hole Let's see if the bypass is closed off.
 

Spider

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Montana
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Craig
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1990
Truck Model
gmc v2500 sub
Engine Size
5.7 tbi
Make certain that the belt routing & fan blade orientation is correct. Make certain that there's correct tension on the belt as well. I ran into a similar issue with an old NC Forestry K2500 years back.

HS

I’ve done this and checked the fan blade. Thanks for the reply.
 

Spider

Junior Member
Joined
Nov 24, 2024
Posts
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Location
Montana
First Name
Craig
Truck Year
1990
Truck Model
gmc v2500 sub
Engine Size
5.7 tbi
Drill a bypass hole in a T Stat 2x1/16th hole Let's see if the bypass is closed off.
I’m going to do that this morning while she’s cold. Will report back
 

Spider

Junior Member
Joined
Nov 24, 2024
Posts
10
Reaction score
20
Location
Montana
First Name
Craig
Truck Year
1990
Truck Model
gmc v2500 sub
Engine Size
5.7 tbi
Well, it got up to the first line past 210,so 240ish kicked back down to the first line, 140ish and stayed there. Better than before, but do I have a blocked bypass? And where is that located at?
 

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