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Disappearing coolant no leaks when pressure test- is it head gaskets

1998 subaru coolant has disappeared 2 times with highway driving last June and November but it exhibited no leaks visibly or under pressure test. Put flourescent dye in.

Found a hairline crack in neck of radiator under hose and replaced in december. End of february with local driving after siting for 5 days it overheated, sucked up reserve tank and needed a quart of coolant in the new radiator.No leaks under pressure test again and tested ok on turnpike. No smells oil ok. Doing tailpipe heat dye test to see if it’s burning coolant and needs head gaskets. Big bucks and want to be sure that surgery is really needed. Help !

I don’t know. You should get the car into either a local dealership or radiator shop and have it diagnosed and repaired. It might be that the car overheated once too many times and a head gasket has been damaged.

How many miles on your car?

Disappearing coolant is never good, and your car is on the list of Subarus with head gasket problems.

How many miles?

Head Gasket

Thanks. I’m working with reliable repair shop I’ve used for years trying to diagnose and repair. Dealership offered to a diagnostic check for an additional $104 but seems like repair shop is checking the same things without charge. As head gaskets are so expensive to repair, I’m wanting to know by a definitive test before doing it, as there are no visible leaks, no bubbles, no odor, no white smoke, it’s ok on pressure test and car runs okay for a few months and then exhibits problem.Any thoughts ?

98,000 miles. previous owner replaces water pump, thermostat timing belt tensioner pulley oilpump and seals at 55k. 2 times disappearing coolant occurred on highway third time short local slow drive ?

12 year old car. When was the last time the coolant hoses were replaced? I’d inspect every single one of them, including the small ones that feed the throttle body and the medium ones that feed the heater core. Also, look under the floor mats for moisture from a possible leaking heater core. Coolant can escape the system may ways when it is hot.

How was the pressure test performed? Many times a pressure tester is connected and the car is given 15 seconds to see if it leaks or not. I always give them 10 minutes at a minimum.

With the proper adapter it’s a good idea to pressure test the cap also. If that adapter is not available then a new cap might be in order.

Have you verified cooling fan operation in both modes? (based on engine heating and operation when the A/C compressor is engaged)

Lastly, have you inspected the engine oil to make sure it’s not diluted with coolant?

Hoses inspected. No pinhole leaks found. Upper radiator hose replaced when hairline crack in neck of radiator under hose found and radiator replaced. Will check under floor mats. Thanks.

Will double check with repair shop on these questions to verify answers. Thanks.