Help! SBC overheating only with heater core connected.

[Toms63SSQB]

I'm almost afraid to ask this question, is the thermostat installed correctly? Reading what is happening the thermostat seems to open very late, but in a pan of water opens correctly. That would cause the high water temp and then once open, all ok. Please dont shoot the messenger.

 

[ssleeper]

hey no worries. Yes, all the thermostats I've tried were installed correctly, thermo 'sensor' end down into the intake.


I just drained all the coolant, hooked up the garden hose and flushed the system both ways. Nothing came out as I expected. I checked the passages coming out of the front of the cylinder heads and both are clear. I decided to go back to the full flow Mr Gasket 180 tstat, and I drilled 3 small holes around the perimeter just for good measure. Topped it off and used the Lisle funnel when filling and idling/revving. I got as much air as I thought I would be coming out, so I put the rad cap on and went for a drive. This time the gauges reported 175-190 for the whole trip, no fluctuation, but the car was already up to temp when I left. I did note that temps go up by about 10 degrees when I open the water valve. I still don't understand why that would make a bit of difference. I reset the low speed fan to come on at 185 and the high speed to come on at 195. There were times, even when idling, that neither fan was running.

I'm letting it cool off and I'll go try again. I did verify the water pump is pumping...it's moving plenty of water across the top of the radiator, so I'm ruling that out. Seems like I've whittled the list down to the possibility of a seeping head gasket, but only time will tell. The combustion gas test isn;t showing gasses in the coolant (tested again today)...but I would be surprised if a freshly decked block with MLS gaskets and new heads would be leaking. Anything is possible I guess.

Thanks everyone for reading and contributing.

 

[MRHP]

Have you done your combustion gasses test on a cold engine? I have had them leak when cold, and not when hot.

 

[ssleeper]

Have you done your combustion gasses test on a cold engine? I have had them leak when cold, and not when hot.

I have not done a test when cold, but I did a 4th test this afternoon because the theory of an internal leak was nagging at me.

Drumroll please....



The first three times I did the combustion test, the fluid didn't change, or did't change enough for me to notice a difference. This time I took the car for a drive with the high flow 180 thermostat, and although it idled at 180-185 on the mech gauge, the temps jumped to 220-240 out on the road and remained there longer than they ever have with the other thermostats. It's odd that with no thermostat, it ran 100% normal, no fluctuations at all. My theory is that it was not getting hot enough to make things expand where they need to be to leak. Once the higher temp thermostat was back in, it was enough of a temp change to get things to move around. I let it idle for probably 45 minutes and the temps ranged from 165-210, fans coming on and off as they should, and each time the temp would swing I made sure to squeeze all the hoses. Sometimes I got no air bubbles, other times I got several bubbles.
So it's good news and bad news. Looks like more busted knuckles in my future


Thanks again everyone for all the help and suggestions.

 

[El Rat]

Sell the car!

 

[catdoctor]

Is the radiator in the car now the same one used before engine was disassembled ?