Help with engine temperature & odd symptoms please

sooby, May 31, 8:36am
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!)
sooby, May 31, 8:36am
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?
pettal, May 31, 8:41am
Replacing the thermostat is always a good starting point .
mugenb20b, May 31, 8:43am
Take your car to the garage for a cooling system check.

If you're lucky, you'll have a stuck thermostat, blocked radiator or electric fan problems. Otherwise, your car will have all of the above problems and blown head gaskets.

In the meantime all you can do is not use your car for long trips, keep an eye on the coolant level and keep your heater on. Good luck.
3tomany, May 31, 8:44am
I suspect you need a mechanic to diagnose the problem. Thing is cooling is hard, it could be partialy blocked radiator, thermostat, or worst case head gasket or cracked head so it really is a process of elimination. Only fact here is you have a problem so don't ignore it.
shakespeare6, May 31, 8:52am
Get the heater core flushed, the coolant that feeds the thermostat housing on the bottem of the engine is fed via the heater core, they block with age , causing cooling issues, that ultimately lead to other issues.
franc123, May 31, 8:53am
Go ahead and replace the radiator, its well past its use by date. While doing this make sure there isn't any external obstruction either, its all too easy for debris to get caught between it and the air con condenser in front of it. The gauge behaving in that way usually indicates the amount of flow through it is barely keeping up with the engines demands, ambient temperature ie if its a cold day outside or not can also determine to what extent it happens. It would have also been interesting to know what the actual temperature of the coolant leaving the engine was and what the fans were doing at the time.
pettal, May 31, 8:57am
I would of thought the green line is where normal running temp is indicated .
shakespeare6, May 31, 8:58am
It's important to bleed the air out of that heater core.
Good indication of hg failure in those is engine fans run all the time.
Note that hg failure is not always caused by over heating the composite hg break as down with age between the firing ring and water jacket. You don't get any oil/water mix hard to detect with out a tk test.
gmphil, May 31, 9:04am
replace radiator at the least .
if its a cold day u may not notice it yet and flush may have help/cured but wait to start summer or a good day in u may notice to late . as ambient air temp does change things
sooby, May 31, 9:38am
Not so, temperature gauge in both our Subaru reads normal with the needle bang on the 'bulb' part of the thermometer logo on temperature gauge, green line in photo is about a 1/4 above that!

cheers guys will take back to mechanic for further investigation & radiator replacement
franc123, May 31, 9:51am
Yes, for any Jap car to be reading above half way is very suspicious. If you live with a vehicle long enough you should get a feel of what's right and what's not.
msigg, May 31, 10:07am
Yes to those that said replace the radiator, thermos and check cooling fans for correct operation, if this thing does ever go into the red it will be a bigger bill than replacing these items, If you want to keep the cat then it deserves it, $500 spent now is nothing. Do it.
tamarillo, May 31, 10:18am
I'm confused. You say you took it garage who did a full service,mradiator flush, and pressure test and they said it was ok. And now car is fine?

So leave it. Maybe it did just need a lean out and bleed. If it's running fine again why spend money on new radiator on an old car with low value?
Sure it will ensure radiator is up to scratch but if you worked like that on an 19 yr old car you'd end up overhauling the whole thing top to bottom. And it's not worth it.
If it's not overheating leave it I say.
xs1100, May 31, 10:18am
and get them to bleed properly that's the waterfall effect and would be trapped air so possibily not reading true
gmphil, May 31, 10:22am
because even tho its bein cleaned it will not be moving correct amount water and next stinking hot day or long trip op will not be Impressed with ur last comment sitting on side of road
sooby, May 31, 10:23am
Sorry I was vague - I meant normal engine service was carried out, radiator pressure tested & decision made by mechanic to replace rather than flush radiator. Strange part was temperature seemed to behave itself today even without a flush being done!
tamarillo, May 31, 10:26am
Cool and thanks for more info. Mechanic must have given reason why it needs replacing though? Didn't it pass pressure test? Was it rusted out, not flowing enough? Must be a reason they didn't go ahead and flush it to at least try.
gmphil, May 31, 10:30am
flushing a radiator is a mith really !
needs go and have tank removed and professional rodded out to be of any good !
franc123, May 31, 11:59am
Correct. The issue is that blockage, or partial blockage is only one issue that a 20yo plastic and alloy radiator is going to be suffering from, degradation of the tanks and cracking around where the tubes enter the ends of the core are the biggies. There's sod all point in spending $100 on cleaning and $50 on a new top tank when $180 gets you a brand new one. In fact once its out the radiator shop man might take one look at it and say its scrap.
aredwood, Jun 1, 6:31am
Common problem is thermostat not opening enough. Reason - not enough water flow through heater and bypass hoses. Commonly caused by slow headgasket leak. Air builds up in heater pipes. A big air bubble goes into waterpump. Waterpump cant pump air so waterflow slows to almost nothing. Fans switch on so bottom radiator hose goes cold. Cold water causes thermostat to close. If the waterpump doesn't manage to push out air you get instant overheating. And since it is caused by the coolant pipes air locking. It will either be fine or instantly overheat. And it will be seemingly random as to when it will do it.

Since you get the sound of air bubbles in the heater core that never goes away. This points to a small head gasket leak. To fix it properly will mean new head gaskets needed. And it has been said in other threads on here that you need MLS head gaskets instead of organic type ones. For a permanent fix.

If you want to do a bodge fix, just remove the thermostat. But that causes it's own problems.
saki, Jun 1, 8:52am
Heater controls thermostat, drain and flush system disconect the heater hoses and give it a good reverse flush, only use one brand of anti freeze mixing brands will cause coagulation and end up in the heater.
gs1220, Jun 2, 5:56am
We have had a similar set of symptoms. Gurgling in heater, overheating on a trip, fine around town. Diagnosis was that the heater core was blocked and stopping flow of coolant in entire system. Fix (possibly temporary) was to fit a metal tube by-pass between heater hoses. Total cost of parts was about $100. Radiator shop had a bi-pass in stock as did our subaru guy. Fitting a new heater core was quoted at $1000 - mostly labour. If the actual cause is a head gasket leak then there hasn't been any obvious recent symptoms - though we wouldn't be surprised. The car has suffered really high temperatures (clouds of steam) back in the day. As well as being stolen and recovered. (Recovered abandoned because of flat battery.)
shakespeare6, Aug 10, 1:33am
Yep most common fault for random subaru over heating ,as I mentioned above flush the heater core, the thermostat won't open if the coolant can,t reach it. The bypass mod is quite effective if they left the heater core in service and put the bypass in you still get some heat,the other plus is very easy to bleed with a bypass tee. In some cases the heater core will leak it get that bad. However as someone mentioned above a bad hg will push air up into the heater core and you get the water fall sound ,most causes it's just taped air from last time the coolant was changed