Bleeding trapped air from Subaru cooling system

[n...@doe.carleton.ca]

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

[Mike]

<n...@doe.carleton.ca> wrote in message
news:1193791205.6...@k79g2000hse.googlegroups.com...

Most likely cause is a partially pluged radiator. With the coolant hot and
circulating feel the front of the radiator, it should be evenly hot all
over. If you find any for cold spots that's were the radiator core is
blocked.

[firebir...@yahoo.com]

Fill the radiator halfway leave the pressure cap off start vehicle let
run for 15 min with heater on high add 1/2 gallon 50 50 mix at a time
after 20 min shut off wait i min then start back up. top off system
and allow to run additional 15 min with cap off
replace cap

[rfsa...@gmail.com]

Thanks to your letter, response, and a very clever Dad we came up with
a solution to this problem.

My '95 Legacy did the exact same thing to a tee. What you need to do
is fill the engine --- not the radiator. So, drain the radiator out,
and then fill the engine up using the top hose, and let the excess
fill the radiator up via the bottom hose. Then put your top hose back
on, and you should be good to go.

Hope it helps.

[n...@doe.carleton.ca]

OK, I'll answer my own question in the unlikely event other shadetree
mechanics come across this problem in the future.

First, thanks to those who offered tips on how to fill the coolant.

The real problem turned out to be the generic heater hose I used to
replace the Subaru original, which is premolded with a 90 degree
bend. I used an EZCoil to make the bend, but even with this the hose
was a bit constricted. It occurred to me later that this engine needs
to maintain strong coolant flow through the heater core to hold a
uniform temperature when the thermostat is closed. With a slightly
restricted hose the flow was reduced, allowing hot spots to develop,
which sporadically gave local boiling, causing the system to fail.
With an original Subaru hose in place everything is back to normal.
I'm kicking myself for not installing the Subaru hose in the first
place. I was in a hurry (long weekend, needed the car, no dealers
open). I should have been more patient. Chalk up one for experience!