5.3 coolant won't flow, help!

[OKGMC4]

The upper hose gets hot all the way into the top of the radiator, but the bottom stays ice cold. I thought maybe a new tstat would help but it turns out the old one may not have been stuck as the new one does the same thing. I have an aftermarket compressor but have double checked my belt routing and all looks good, pump turns counter clockwise. It's full of coolant and the hoses pressure up. I let it get to 210 dgrees it just doesn't open up and flow. Any ideas from experienced folks? Thanks.

[ls1nova71]

First start up? If so, how did you fill it with coolant? You need to take the upper hose off and fill it through there once the radiator is full to insure there is coolant in the block, otherwise the thermostat won't open because there is no coolant in there.

[BR3W CITY]

x2, I filled my rad, block and hoses then bled and topped off. If you fill rad or overflow only, you probably have another 2-3 qts to add.

[OKGMC4]

I checked and it seemed to be full all the way to the top. It just seemed like the pump wasn't working. I took off the pump and removed the cover to see if the impeller was tight and I swear the fins are turned the opposite way from what they should be. I compared my belt diagram to the factory one and it shows to be the same. All 5.3 engines should be the same right?

[harnalien]

When I had this problem, I had to fill it up through the upper hose to get water in the block.
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[Clyde65]

I agree with harnalien, you need to fill engine first, if you fill from rad, the water can't get into the block cuz it can't displace the air in it. The thermostat isn't in the water so it will never open
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[AJII]

I filled mine from the top hose and still never got water to flow.

Drilled a couple of holes in the Tstat, and everything worked as should. Heat is still nice and hot as well.

[OKGMC4]

What's odd is I put it all together and ran it for 30 minutes and...
The lower hose never got hot, maybe just lukewarm. The top got hot, I used a digital meter at the cap hole and the water ran 195 to 200. At 200 my fan comes on, it runs till 194 and then off, but the lower is still only warm at most. Gauge never got above 210. Maybe I'm just over analyzing this and it's OK?

[ls1nova71]

Originally Posted by OKGMC4

What's odd is I put it all together and ran it for 30 minutes and...
The lower hose never got hot, maybe just lukewarm. The top got hot, I used a digital meter at the cap hole and the water ran 195 to 200. At 200 my fan comes on, it runs till 194 and then off, but the lower is still only warm at most. Gauge never got above 210. Maybe I'm just over analyzing this and it's OK?

If it's not getting over 210, then you're fine. And yeah, you're probably over analyzing it.