have a 97 subaru outback 2.5l, awd stick, car is over heating but without any type of regularity or consistancy as one might expect under similar driving and atmospheric conditions.... ex. can drive 12km to hardware store temp gauage comes up, to this point car behaving normally, with a stop in at store to discuss a few things and pickup a few more ( in december in northern ontario so car was now cool, not stone cold, but cool) car overheated on same trip home at about the 10km mark.... all of this with a full rad, so coolant levels okay, and with a new thermostat...now on the day previous did the same run for the same reasons with the same outside temps only the car overheated at about the 9km mark on the way to the store, i then topped up rad at store, did my business and made the whole trip home without a problem...any suggestions... overheating has never been a long term or recurring issue, and i do get consistent heat from heater, when car is not overheating there is no a sign of coolant leakage as levels stay up, also, no sign of coolant in oil and no leaks or drips in the snow....has got me mystified...thanks

You mentioned that on one trip you did top off cooling system and it was fine for that day/trip after that. WHY was it low or why could it take any?

There are some elusive coolant leaks and some may only leak while underway and gaskets (head in particular) can burn coolant unseen. Water pumps can do that as well and perhaps only when up to temp.

Pressure testing cooling system and other tests to locate leak is in order,

T
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Tom Greenleaf - MetroWest, Boston
http://acsource.net/acforum/index.php

sorry for not being clear i guess, the coolant is low only when the car actually overheats, other than that the levels are generally fine it has always held good levels, my bigger concern is the overheating that is causing the coolant loss, i thought perhaps the fact that it is not a consistent overheat might trigger some ideas of where to look for the overheating...

side bar to this, when i replaced thermostat i did change the rad fluid and even with rad cap off after opening drain cock, draining seemed rather slow - fluid just sort of piddled out rather than other vehicles i have done where it poured out. the manual makes mention of an air cock screw on lower part of engine, could this be for draining? does this info help at all.....

thanks.

Chicken and egg thing perhaps. Is low coolant causing overheat or vs versa? The recovery tank will usually contain minor overheat but not hard boiling and it would purge to ground - you'd see that too!

Understand it's cold where you are not unlike here right now and 12km (7-8ish miles) isn't real far in cold to aggrevate a marginal cooling system but could.

Just sitting and taking observations, does top hose to radiator stay "unpressurized" for a little while and stay cold while engine warms up in front of you? If it pressures up right away or not at all there could be a head gasket leak causing the issue. Trying to rule that out. Run heater - note temp change in output of that too. If you are boiling it will go cold on you! Yes - cold as air bubbles won't transfer heat for diddle vs hot liquid.

It's not conclusive how fast coolant came out while draining. Anti-freeze is thicker when cool/cold. Don't overdo that either. 50/50 or if in real extremes up to 60% is about max. Typical antifreeze is a lousy heat exchanger by itself or if too strong.

You can test for exhaust gas in cooling system or at the recovery tank. Should show any but you need to know and that suggest head gasket troubles dammit!

You already changed out thermostat - was that to fix this or just maintenance? Is it properly installed? Some can go in backwards and will screw up like this.

A good system flush wouldn't hurt noting how much junk if any does flush out.

I'm not even thinking electric fans right now as Winter in OT they may not even be necessary for most driving right now. If you have A/C try it - fan(s) may come on regardless of need and if that suddenly cools the engine it's interesting for the moment while I think up what else to check,

T
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Tom Greenleaf - MetroWest, Boston
http://acsource.net/acforum/index.php