Help Needed - Running Hot After Heads Replaced

[mark460]

1998 5.7 Rebuilt about 75K miles ago, running great until a few weeks ago, radiator had leak and overheated engine slightly which cracked head. I put in a new radiator, ran cool like normal, but had engine miss due to leaking coolant in head. Had 062 heads, machine shop found me 906 heads, mag checked, valve job, surfaced, etc. Put everything together, temp gauge keeps rising past 210 degrees. Tried bleeding air out system several times, ran with heater on, cracked lines, etc. Pulled thermostat, put in boiling water and it opened. Top hose going into radiator feels hot, but air being blown thru radiator doesn't feel that warm. On another car, I once had an impeller come off a water pump, but the odds of this being the problem, now? Just not too sure about that. Any ideas would be appreciated. Need to get this back on the road.

[ChevyTech]

If the intake gaskets are installed on the wrong sides the coolant passages to the intake will be blocked. They warm up quickly when this happens.
You might be able to see if that happened if you remove the thermostat and look down the hole toward the passenger head.

[mark460]

hmmm. interesting on the intake gaskets. I think I installed correctly. I decide to take out the thermostat for testing. Start it up and immediately temperature gauge reads in the normal range. Under the hood, I can see the water flowing good in the radiator and hot air is coming out the back side of the fan. So apparently the thermostat even though tested in a pot of boiling water didn't work like its supposed to. Off the parts store to get a new thermostat. When I pull the water neck again, I'll see if I can look down the thermostat hole as you mentioned.

[mark460]

After a new thermostat, its beck to spiking up in temperature. Factory gauge up to 210 and climbing and then I have to shut it down. Removed thermostat again, took it for a test drive for a few miles, temp stayed unusually low, which is expected. I have no idea what is going on at this point.

[ChevyTech]

Does the new thermostat have a little air bleed hole in it?

If there is no air bleed hole in the thermostat I would drill a tiny hole in the thermostat to help bleed the air from the system. This also helps if there is air getting in the cooling system from a crack or other leak.

Did you look at the head gaskets to make sure they looked good and all the holes were there before setting the heads in place?

[OutlawDrifter]

I believe TBI and Vortec rigs had different water pumps. The Vortec rigs had an extra hole for constant flow, if memory serves. Maybe you have the wrong pump?