It's not clear if you've fractured the inner head gasket between a combustion chamber
and a coolant passage, resulting in fatal "percolation" ($1.4-1.7k to repair), or whether you
simply haven't bled the system correctly, or installed a too-small aftermarket t-stat.
So let's work backwards: did the new t-stat's orifice match the large ID of the original? If
smaller, you MUST chase the original size one or you'll overheat from insufficient coolant
flow at speed. But I doubt this is the problem if it overheats at idle.
Refilling and purging the coolant system can sometimes be tricky, as a trapped air bubble
near the temp sensor will cause the gauge to spike until coolant fills. Your cap-area
dynamics may still be temporary as air is purging outward. Maybe....
Start by elevating the FRONT of the car a few inches, to assure that the rear heater hoses
are BELOW the radiator cap. Then find a large kitchen funnel that fits the rad cap tightly.
Add coolant to the funnel until it's full enough to create a large visible surface to watch. As
the engine heats up and air pockets bubble out you may have to add more coolant slowly.
Keep at it until there's no more trapped air percolating out. The coolant gauge should be
in the normal zone when finished. Fill the expansion tank 2/3 full and then drive a few
times on the highway, allowing the system to cool and adding coolant to top up in the
event a bit more air purged. Rarely one will have to repeat these highway drives and
cooling cycles several times before completion.
IF, HOWEVER, the dynamic percolation of bubbles in the funnel (or expansion tank)
NEVER stops, then head over to a wrench with a hydrocarbon sniffer to see if the
bubbling gases are air (phew!) or exhaust gases (ouch!). If the former just keep watching
coolant level and adding smaller and smaller amounts as complete purging occurs. If
there are HCs in the gas bubbles throw the car away, as it's not worth this heady repair
bill. Good luck....
Another thought, as I sometimes forget this possibility: remove the rear heater hoses and
completely flush them out. Sometimes a prior attempt to stop overheating or cracked
radiator leaks via an organic coolant additive (essentially oatmeal!) can clog a rear heater
hose partially, resulting in insufficient coolant flow when running. But again, a partially
clogged hose usually doesn't result in overheating AT IDLE speed. Chronic percolation at
idle speed is usually cracked head gaskets, alas, but is most prevalent in the fragile first
gen 2.5 DOHC OB/LS 1995-1999 motor, not the more durable 2.2i SOHC motor. Yet your
HGs are 23 years old, and all Subaru engines HATE being overheated, so your cracked
radiator-provoked thermal spikes may have blown the HGs via cracking them or warping
a head. Be prepared for the worst. Sorry. (Disclaimer: I service and resell 2016+
Legs/OBs in the Boston area after 37yrs Subaru wrenching.)
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