overheating at low RPM only
Discussion
Just bought a Legacy GTB which overheats at low rpm (<2500rpm). I called the guy to ask if he knew what was going on but got the expected "nah mate its well mint innit".
I have bled out the airlocks, but once it overheats it purges some coolant which is then replaced with air, so I'm going round in circles. The strange part is that I can control the temperature with higher revs. So vehicle speed isn't important therefore not an airflow issue. Its the higher RPM (>3000rpm) that brings it back down and within a matter of seconds.
Any ideas? Having searched the web for a bit it doesn't seem like the water pump is a usual suspect on these.
Cheers
I have bled out the airlocks, but once it overheats it purges some coolant which is then replaced with air, so I'm going round in circles. The strange part is that I can control the temperature with higher revs. So vehicle speed isn't important therefore not an airflow issue. Its the higher RPM (>3000rpm) that brings it back down and within a matter of seconds.
Any ideas? Having searched the web for a bit it doesn't seem like the water pump is a usual suspect on these.
Cheers
Sounds more like an air lock
Awkward, but drain the entire system. Then refill very slowly. It should take approx 7 litres ( heater matrix will hold a bit, total capacity is usually around 7.5 litres )
If you get nowhere near 7 in, then you know there is still an airlock. If you get it all in, you know it's full.
Also check the vent pipe from top of rad to header tank, it has a habit of blocking at the top of the rad which again can trap air.
But patience is generally the key when filling them.
Awkward, but drain the entire system. Then refill very slowly. It should take approx 7 litres ( heater matrix will hold a bit, total capacity is usually around 7.5 litres )
If you get nowhere near 7 in, then you know there is still an airlock. If you get it all in, you know it's full.
Also check the vent pipe from top of rad to header tank, it has a habit of blocking at the top of the rad which again can trap air.
But patience is generally the key when filling them.
I've just taken it out another test run of 20 miles with the heaters running hot and full blast. I only done this since they were already on from bleeding it again.
Anyway this time it didn't overheat but the heaters ran cold whenever the revs dropped. As soon as I picked up the revs the heater blew hot again.
Does this help in the process of pinpointing the problem?
Anyway this time it didn't overheat but the heaters ran cold whenever the revs dropped. As soon as I picked up the revs the heater blew hot again.
Does this help in the process of pinpointing the problem?
andyiley said:
I too would go with the water pump.
Had the same several years ago & when I got it off the impeller blades were all but corroded to nothing, so that it only pumped water at high revs.
I've been messing with Subarus for over 15 years, and never actually seen one with a failed water pump yet.Had the same several years ago & when I got it off the impeller blades were all but corroded to nothing, so that it only pumped water at high revs.
Of course it is possible, but very unlikely.
stevieturbo said:
andyiley said:
I too would go with the water pump.
Had the same several years ago & when I got it off the impeller blades were all but corroded to nothing, so that it only pumped water at high revs.
I've been messing with Subarus for over 15 years, and never actually seen one with a failed water pump yet.Had the same several years ago & when I got it off the impeller blades were all but corroded to nothing, so that it only pumped water at high revs.
Of course it is possible, but very unlikely.
andyiley said:
I can't disagree, as I am not massively familiar with the Scooby itself, but everything he says, generically, even after the update yells water pump to me.
Subarus are very prone to air locks if not filled correctly.However another potential issue is a poor sealing head gasket which too would cause air locks initially.
stevieturbo said:
Subarus are very prone to air locks if not filled correctly.
However another potential issue is a poor sealing head gasket which too would cause air locks initially.
What behaviour would you expect from the expansion tank? From what i've gleaned from the internet it should contract and expand by and inch or so. However mines practically fills the bottle, then each time it contracts it leave a little extra. I think this is how im losing the coolant as its not all being sucked back into the header tank.However another potential issue is a poor sealing head gasket which too would cause air locks initially.
I've done a combustion leak test which didn't show much.
Overflow tank or header tank ?
If you are getting water pushed out the header tank on top of the engine it's either overfilled or there are air locks.
As I say, empty the system and refill as described. That way you at least know the correct amount of water is in to try and eliminate any air locks.
If you are getting water pushed out the header tank on top of the engine it's either overfilled or there are air locks.
As I say, empty the system and refill as described. That way you at least know the correct amount of water is in to try and eliminate any air locks.
Water pump impeller spinning on its drive shaft sounds feasible and gives this exact symptom on VAG stuff amongst others, seen more than I care to remember of this failure 
but will admit have not seen this happen on the Subaru ...... yet 
worth checking anyway ............... 
but will admit have not seen this happen on the Subaru ...... yet worth checking anyway ............... Edited by Sardonicus on Wednesday 17th July 10:53
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