In another thread we have scratched our collective heads and come up empty. So let me try it this way. Can a blown head gasket or cracked head only leak some of the time?
I have driven 1200 miles with no problem. I can accelerate heavy with no problem. I can't make it happen like you think it would. But at cruising speeds my 1995 V6 gets air in the cooling system, at the top of the motor. No water circulates through the heater core and the temp gauge reads the air temp, which is hotter than the water actually is. The radiator will overflow into the overflow bottle. After 5 - 10 minutes the air bubble goes away and everything returns to normal and I can not make it do it again, like hard acceleration would if the head gasket were blown (right?)
The first time this happened was the first time the outside temp hit 16 degrees. I did not drive it the previous two days. I had driven in 20 degree weather before. I was cruising at 35-40 miles an hour on my way to work. I had been driving about 15 minutes when it happened. After about 10-15 minutes everything returned to normal and I was able to drive back home.
After replacing the radiator and thermostat I drove the car 770 miles to Florida. 400 miles into the return trip, after stopping about 25 minutes, it happened again as I pulled out of a rest area. It was 50 degrees and I had been driving continuously for about 300 miles before I stopped for gas. I made another 20 minutes stop maybe an hour later and after the 20 minute stop about an hour later the problem happened again.
In July the water pump developed a leak and I replaced that and the thermostat. In August the radiator cooling fan died and I replaced that. Nothing happened until that 16 degree morning in December. I replaced the radiator and the upper and lower hoses and the serpentine belt. I still got the air bubble problem. The local Ford dealer told me some mechanics in town prefer Ford thermostats over aftermarket parts so I tried that but I still get the problem.
I am getting air in the upper part of the cooling system, causing nothing to flow through the heater core - the heater blows cold - and the radiator still overflows. The problem goes away in about 10 minutes and I never know when it will happen again. Before this happened the heater always blew good and hot. The heater core can't be blocked, because in the summertime, the A/C cuts off flow to the heater core and this doesn't cause the radiator to over-flow.
I know there's air in the system because I open that air bleed thing on the intake and air escapes, not water. When everything is operating normally, water escapes from the valve when I open it. The Ford dealer says he doubts that bleed valve could be bad because he has never sold any replacements. But the air isn't there when I first start the motor - the heater blows warm. After a few minutes - if it is going to happen - the heater starts to blow cold and the radiator will over-flow.
If I can get anybody to buy this car what motors should I avoid (like the '94-'96 V6s)?