Hi guys, my forged sti has a strange overheating issue.

When I'm cruising at around 50mph, the temperature needles starts to rise and the goes about 2 thirds up the temp guage. when I press on/drive spiritedly the temperature goes back down to normal. The temp also rises when driving around down at slow speeds (through town etc)

I have burped the system several times, checked coolant for leaks and the fans are ok (i have a manual switch for the fans i use in warm weather etc)

Can anyone point me in the right direction for what could be the problem? Am guessing thermostat or water pump?

Thanks

Lewis

 

Had the water pump been changed?

 

water pump has been replaced a couple of years ago, could it be a sticky thermostat not opening and closing as it should?

 

Good chance it could be that, fine the thermostat and check the pipes both side too see if both hot, if one isn't hot when temperature in increasing then sounds like your right.

 

Why the manual switch you should'nt need it all your doing is cooling the water down too much then the stat shuts and hence no circulation to cool the engine so it overheats.

 

Headgasket more than likely toast. combustion gas entering the coolant

 

There always one that trys to put the s**ts up people isn't there

No its not the headgasket. No contamination what so ever in the oil or coolant, fluids all fresh with correct levels. Zero white smoke from exhaust etc

Motor is fully built using RCM head gasket. Am sure that can handle a slight raise in engine temps on 2 occasions.

 

I only use the manual switch on extremely hot days after a spirited drive etc. it still doesn't explain the rise in temps when cruising at 50mph

Cheers

 

Far from it, Having built more subaru engines than i care to remember I've seen a few things over the years. Just because its fully built doesn't make it indestructible

Ask yourself this, Why are you continually burping the system, a water tight system wouldn't induce air itself.
Thermostat when failed usually stick open not closed from the ones I've seen.

if i was gonna put the wind up you id get the bores checked for a split liner if you wanna go down that route

 

Stick torque app on it and track the temp after/during few good hard runs

Was able to see mine starting to go as temp increased up near 105 mark but dash temp wasnt moving at that temp

Top hose started to bulge also but that might not happen for you if running silicone hoses

O aye and mine was a 2.5 built engine with rcm gaskets...it lasted 5yrs before starting to show signs...only caught during mapping session or I'd have been none the wiser

get it checked dont assume it cant be the headgasket.....just because 'built'

Never any need either for a manual cooling switch

 

If you want to rule the thermostat being stuck closed out without spending any money you could try running without it.

A coolant tester is probably worth doing as a first step once you've done everything you can try that doesn't involve spending money, unless when you say "no contamination" you mean it's been tested and not visually.

 

Thanks for your comments guys.

Drained all the coolant and fitted a fresh OEM thermostat. Carefully filled the coolant and bled the system carefully using a funnel attached to the header tank.
Cars temps are now rock solid and don't budge, taken the car for several long drives with a mixture of driving and all seems ok

Think it must have been a classic air lock within the system.

Cheers

 

Never had any airlocks using the fans coming on twice refill and bleed system, literally fill with water and leave expansion tank cap off, let car warm up with heaters on until the fans have kicked in twice, keep checking levels while this happens, once the fans have come on and gone off twice replace cap and drive car, literally done hundreds with no issues.