My 1997 F150 is overheating.
This summer I changed my coolant (I hadn't done it in quite a while) The coolant appeared to be quite rusty and it took several flushes to get it to look normal.
About a week ago my truck overheated the first time. I thought that the thermostat was sticking, so I took it to a Rad shop had the system flushed again, thermostat replaced and heater core flushed.
On the way home from the shop it overheated again, They took another look at it and found the pressure had created a leak in an extremely rusted block heater, it was replaced.
It overheated again, they then did a test to see if the head gasket was leaking the test didn't confirm anything. They changed the water pump thinking that corrosion may have damaged the impeller.
Well the problem is still happening, it appears that when the motor cools down it is getting an air lock somehow in the system, when warmed up the temp gauge fluctuates in the normal range but will not produce good heat in the cab, after about 10 minutes the temp gauge spikes and the overheat light comes on. When I check the engine the upper rad hose is not pressurized, if you open the overflow bottle it releases the airlock and then the vehicle operates at normal temperature with great heat.
There isn't any coolant in the oil and it doesn't appear to show white smoke at start up or while running. With it overheating it always seems to loose a little at the overflow bottle so it is hard monitor coolant level from test drive to test drive.
Any help would be appreciated. What could be causing this air lock? Has anyone had head gasket problems I would hate to spend the money on this if I do not have to (estimated @16 hrs job) I have 260,000km (162,000 mi) This truck hasn't cost me a dime except for maintenance till now.