Too much coolant, not enough water
While replacing my tensioner, I drained the radiator, heater hoses, and whatever came out with the tensioner and cooling elbow. Couldn't have been more than 1½ gallons.
When refilling the radiator, I poured in a gallon of 100% coolant, and then grabbed a jug of water, but the system was already full. The level didn't drop much when the thermostat opened, either. Very strange, but it'* cooling just as well as it was before, so I won't worry.
I'll assume the block and heater core were still full of my old 50% strength mix, so if I lost half my coolant during the repair, I guess I'm running on about a 75% strength mixture right now. That won't cause any problems, will it?
2007 Pontiac G5
1999 Pontiac Grand Prix GT - Stock engine ate transmission.
1997 Pontiac Bonneville SE - Cammed engine ate transmission.