Engine Cooling problem
1996 Geo Prizm 4 cyl Front Wheel Drive Automatic 140,000 miles
this car had the freeze plugs, radiator and hoses replaced at 110,000 miles due to corrosion and leaking. It has been fine since. It is driven only 10 minutes around town all week without issue. This weekend it drove a 30 minute highway ride each way and after shutting off in the garage, later nearly 1 gallon of coolant was on the ground. I can't see the exact source, but the front frame crossmember is wet on top. The block, head and radiator/hoses all look dry. I filled the coolant and got up to temerature (half way between C-H). I drove the car about town and noticed the temp gauge dropped to then running lower than usual, near C - then after sitting still at idle it got to half, Drove more and shut the motor - the coolant was still full in the radiator next morning. Drove the car around town again, still low temp gauge - then later up to mid point. After shut off no leaking. Tonight I just drove the same 30 minute each way trip on the highway and after shut off a few minutes the coolant poured out again - I missed it happen.
What could be causing the coolant to maintain in the radiator during short easy trips but gush out from somewhere after a highweay drive at sustained speed? And what part is most likely leaking? Why the sudden low temperature reading? It does not steam or overheat. The overflow jug has always been empty shortly after filling, but the run off hose is dry. Oil is clean and full. Dry around water pump and it's hose. See some signs of fluid(can't tell oil or water) on passenger side corner/front block under head, not the valve cover.
If you help resolve the root cause I will donate and post detailed results. Thank you