Help with Legacy engine temperature & odd symptoms please

Hi guys,

We've got a 2 litre bog standard grocery-getter 1996 Subaru Legacy that has been fluctuating with its temperature readings and I'm a little concerned.

We've owned the car about 9 years and temperature always stays at the same mark on gauge. We did have an isolated event about 5 years ago where temp spiked & car was immediately stopped & mechanic cleared a blockage, since then been perfect... until recently when on a long trip I noticed the temperature needle rise from its usual position to the green line in the image below:

https://trademe.tmcdn.co.nz/photoserver/full/386967299.jpg (Apologies for the crappy image!)

This seemed to only happen under load, like going up a long uphill and only seemed to last duration of load, then return to normal. At no point did it go higher than green mark.

Took to mechanic for service & radiator flush asap, where normal engine service was carried out & radiator pressure tested and found to be fine. Mechanic recommended replacing radiator as more cost effective than reconditioning the existing 19yr old radiator.

I thought that sounds fair enough and was prepared to replace radiator, then today we did another long trip and weirdly the temperature remained in exactly the regular spot the entire trip! Now I'm not so sure about replacing the radiator, what do you guys reckon?

Other relevant info:

- car doesn't drop any fluids.

- no steam out of exhaust (fingers crossed no head gasket!)

- car does have air in heater core (common BG legacy issue) that makes rushing water sound (altho weirdly this seemed to have reduced today!)

- noticed over the last year after a long run there is a loud thud sound coming from engine bay soon after turning car off, which reduces in volume & frequency as car cools. Sound is loud enough to hear in cabin & sounds like someone smacking a hunk of metal onto rubber, almost like a deep drum sound. Strange part again is this has stopped today - so weird!

I did notice a lot of large leaves blowing around road on the trip where temperature gauge fluctuated, is it possible a few leaves stuck to radiator & reduced efficiency, then fell off later at slow speeds?

I'm stumped - any ideas?

The leaf coating is possible, but it would have to be a pretty good coating. More likely it had an air pocket somewhere that caused temp to spike, although it's stayed within the normal operating temp marks. The air pocket was also causing the heater core water rushing sound and cool down drumming noise.

When was the coolant last flushed/replaced in the 9 years of servicing? How did the coolant look this time, before getting replaced?

How does the radiator look? Fins intact, or blocked off etc?

cheers for reply Rosssub,

Coolant was definitely replaced when radiator was flushed at time of blockage (5 years ago), not sure if possibly included as part of normal engine service too? which we do at least 9 monthly / yearly. Coolant looks in good condition, bright green. Radiator is what you'd expect of a 19 year old car with the odd fin bent, but no major damage I could see in quick inspection.

So do you think this airpocket is serious, I found a post here where connecting a 10mm/12mm pipe between heater inflow & outflow pipes would remedy, think that's worth a shot? Do you think its time for radiator to be replaced?

 sooby']cheers for reply Rosssub,

Coolant was definitely replaced when radiator was flushed at time of blockage (5 years ago), not sure if possibly included as part of normal engine service too? which we do at least 9 monthly / yearly. Coolant looks in good condition, bright green. Radiator is what you'd expect of a 19 year old car with the odd fin bent, but no major damage I could see in quick inspection.?

The service manual says to replace the coolant every 50,000km or 24 months. I'm pretty sure that every 2.5 years is a sort of consensus. Not really a major issue though, the coolant air pocket has grown over the last 5 years. To now be big enough to cause the temp to rise slightly under load.

So do you think this airpocket is serious.

Keeping the coolant overflow bottle topped up will help keep the air pockets away. When the hot coolant system cools down it gulps water from the overflow bottle. If the bottle is empty or low it gulps air. Then when the engine starts up again the air is pumped through the system and gets trapped in the heater core.

I check my coolant levels, looking at the height of the coolant in the overflow bottle. At least once every two weeks out of habit. When the engine is cold, the coolant is on the cold level. When the engine is hot the coolant is on the hot level. If I ever changed to a higher pressure rated radiator cap, the levels would change slightly. But the coolant system as a whole still works the same.

But on top of regular coolant level checking. Just getting the coolant bleed, coolant changed and the coolant system checked every 50,000km/2.5years should be good.

[quote name='sooby said:

The heater core bypass is more of a band aid fix for heater/overheating problems. 9 years ownership at 19 years old is a pretty good run. Replacing the radiator, thermostat and radiator cap would be good preventative maintenance. Also thoroughly checking all coolant hoses. But it's all been pressure tested ok and doesn't show any sign's of physical damage, so I'll leave the radiator replacement up to you and your mechanic? My car is 15 years old, I would of replaced that radiator by now. But I have receipts from the previous owner showing it was done about 3 years ago.