I wondered if anyone knows of special procedures for bleeding trapped air when refilling the cooling system on the 2.5L Subaru Boxer engine (in particular for a '98 Subaru Legacy Outback).
To make a long story short, in August the car blew a small hole in the heater hose. I replaced the hose and topped up the coolant. After that time the car would occasionally (for no apparent reason) suddenly overheat in normal driving. The temperature gauge would rise from its normal midrange position to full scale in about a minute, and coolant would boil off into the expansion tank. After letting the engine cool and topping the radiator, the car would then run for many miles without a problem- until the next overheat event. In frustration, I drained the cooling system, treated with descaler, flushed thoroughly, removed the thermostat, got a new pressure cap, and refilled with de- ionized water and Waterwetter. The idea was to check the flat-out cooling capacity of the system, and in particular to check if the radiator was clogged. After this there were no cooling problems, even on long, steep hill climbs under load.. In fact, as expected, on cool days the temperature gauge would barely budge from the bottom position in highway driving. I concluded that there was nothing seriously wrong with the radiator. There is also no evidence of serious problems such as a blown head gasket. I also tested the thermostat in boiling water- it opened to a gap of about 1 cm, which should be about right (I don't have the exact spec for a Subaru thermostat).
With winter coming on, last weekend I reinstalled the thermostat and refilled the system with 50/50 water/gycol antifreeze. I first removed the top radiator hose and filled the engine directly with coolant. I then reconnected the hose, removed the vent screw at the top of the radiator, and slowly filled the radiator to the brim. Following the instructions in the Subaru owner's manual, I warmed the car up with the cap off, then put the cap on and ran the engine at
2,500 RPM for 5 minutes, let it cool completely and topped the coolant (it took very little). I then drove off, and the temperature gauge held nice and steady at midrange for a few miles... but then suddenly, under light load, it started to climb to the overheat position. I cooled the car down and limped home. The next day I installed a new thermostat, and carefully refilled the system as described above. I encountered exactly the same problem. It's hard to believe both thermostats could fail in the same way. My guess is that there is trapped air in the system which is somehow sporadically causing the coolant to stop circulating (although I can't quite understand how, since the pump is at the very bottom of the system, so gravity should always bring coolant to the pump). When the car overheated today I found that the upper radiator hose (carrying coolant from the engine) was really hot, while the bottom hose (returning coolant to the engine) was barely warm, which seems to confirm that the coolant had stopped circulating. Any advice on how to bleed out the trapped air (or other thoughts on what might be causing the problem) would be greatly appreciated. I'm out of ideas!Thanks in advance,
Garry