(new to subarus period) I have a 1998 Subaru impreza 2.5RS, been driving fine but kept running out of coolant at this time temp gauge never went up to 'H' it just stayed at midpoint of the temp gauge. ive check to see any leaks but no leaks at all(at least from what i can see)so i did a coolant flush(no oil mixed) and changed my thermostat ran it fine for two days. also did oil change right after the coolant flush and changed my thermostat just to be safe and see if theres any coolant mixed with my oil but oil was fine also. i did change my water pump too so i dont have any idea what to do now. i hear its a high chance its my HG(head gasket) but im hoping its not because that would suck. yesterday(third day after coolant flush, new thermostat, and oil change) morning i left to work, on the highway i noticed my heat temp gauge for the first time it jumps up and down, close to 'H' but not all the way up there. it doesnt stay in one spot for long it jumps up and down, when i come to a stop it goes down, sometimes when i stop it goes up. looked and there was overflow from my reserve, so i parked the car for the night and see if it'll leave a stain from coolant but no stains at all so im still guessing there isnt any leak. if anyone has any info on this problem please help!!
also forgot to mention that lately ive notice it lag when i accel. if that helps and car idles just fine
the 2.5 DOHC is notorious for blowing headgaskets.
i cant tell you how many 2.5's ive got in with blown engines and brand new t-stats , w-pumps and radiators. usally machanics not familiar with Subarus throw everything but the kitchen sink at them in a losing battle to stop overheating.
when a 2.5 starts overheating 99% chance it is the headgasket
most likely a HG. but with that moving temp gauge that almost seems like the sensor is f* up. when you do a HG replace the ECT (engine coolant temp sensor), i think it a cheap part and easly done when doing a HG.
I've been having a similar problem. Only difference is that my temp gauge was reading correctly and my overflow tank was nearly filled to the top with coolant with this black tone to it. I have no idea how/why its doing this. Please, I need help ASAP!!!