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

[Spider]

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.

 

[Strick]

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]

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

 

[Spider]

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]

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]

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?

 

[Ricko1966]

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?

Drill your T Stat holes out to 1/8th inch. What year is the replacement engine block? What year waterpump are you running. Check back I'll put up posts when I find them of the bypass. Is tge heater core hooked up and working? Got you some pics. That 1 hole allows water to circulate until the thermostat opens.Not all blocks are drilled,not all waterpumps are drilled,some people block that passage on purpose,sometimes it just gets plugged with gunk. And sometimes in some situations the heater core itself is the bypass. If you buy a no drilled waterpump they advise you to drill 2 holes in the thermostat. I believe 3/16ths 180 degrees apart. I'd rather sneak up on it to see what I really need. IDK if your bypass is plugged,but the fact that adding bypass holes in the T Stat is making the problem go away makes me think it's a definite possibility.

 

[Spider]

It’s the original motor, just rebuilt. Heater works like it should. I had the ac bracket and water pump off today and the third hole is open on the passenger side. I’d assume it’s open on the block since it was just rebuilt by a reputable builder.

I’ve got a 160 in there now, I’m going to try the 190, but……I’m pretty sick of antifreeze for a moment.

Drill your T Stat holes out to 1/8th inch. What year is the replacement engine block? What year waterpump are you running. Check back I'll put up posts when I find them of the bypass. Is tge heater core hooked up and working? Got you some pics. That 1 hole allows water to circulate until the thermostat opens.Not all blocks are drilled,not all waterpumps are drilled,some people block that passage on purpose,sometimes it just gets plugged with gunk. And sometimes in some situations the heater core itself is the bypass. If you buy a no drilled waterpump they advise you to drill 2 holes in the thermostat. I believe 3/16ths 180 degrees apart. I'd rather sneak up on it to see what I really need. IDK if your bypass is plugged,but the fact that adding bypass holes in the T Stat is making the problem go away makes me think it's a definite possibility.

 

[Ricko1966]

Did you reuse your original heads? I did locate the predrilled thermostats they use 3x3/16ths holes. I still think I'd use 2 and sneak up on sizes until I had just enough bypass,too much is going to slow warm up in winter. The fact that no Stat = no overheat and a drilled Stat means no overheat still makes me believe it's a bypass problem. Head gasket blocking bypass port,waterpump not drilled or not drilled correctly,head not drilled,etc. If a drilled thermostat fixes it,I wouldn't tear my hair out investigating.

 

[Spider]

Did you reuse your original heads? I did locate the prefilled thermostats they use 3x3/16ths holes. I still think I'd use 2 and sneak up on sizes until I had just enough bypass,too much is going to slow warm up in winter. The fact that no Stat = no overheat and a drilled Stat means no overheat still makes me believe it's a bypass problem. Head gasket blocking bypass port,waterpump not drilled or not drilled correctly,head not drilled,etc. If a drilled thermostat fixes it,I wouldn't tear my hair out investigating.

Yeah, same heads, im not gonna pull my hair out, just glad it’s running at a better temperature. A drilled t stat is a cheap easy fix.