It was time to change the coolant in my 496HO. Drained it and filled up through the reservoir. Problem is I drained out approx 4.5 gallons and have only put back in 3.5ish. I've run it a few times on the hose with the cap off and have not been able to get enough water temp to burp the system. Ran it easy on the lake with no change. Got close to 180 but no need to add coolant. I didn't want to run it too hard with a big slug of air in the system. Maybe I just need to run it hard for a short run and get the revs up? What do you guys suggest?

How and where on the engine did you drain the coolant system?
this will determine whether you got all the coolant or not?
Raylar sees about 4.25 gallons when we drain and refill with a complete engine take apart after drain. It is possible you just did not get all the coolant that sits in some places and does not completely drain.

Best Regards,
Ray @ Raylar

I drained the cooling system using removing both drain plugs in the block. It's all the air that seems to be trapped in the engine that is concerning me. If I drained 4.5 gallons out and didn't put that much back in, there's something wrong?

You got to 180 degrees so the thermostat under the heat exchanger had to be open unless its stuck shut but even having said that there is a bypass passage between the top of the circulating water pump and the bottom of the heat exchanger which is the highest point in the coolant system next to the coolant overflow bottle with a top vent hose that runs between the two and should always bleed any trapped air in the exchanger back to the coolant bottle. I just don't see where you could be holding that kind of air bubble in the system that would account for an entire gallon of coolant. Did you do anything else to the coolant system? Could you possibly have counted your gallons going back in wrong?

Best Regards,
Ray @ Raylar

Nope, no other maintenance performed on the cooling system.

I'm starting to think that this is a possibility based on your last post. I may drain the system again and double check my refill amount. I'd just hate to blow a head gasket or worse ...

Thanks!