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98 Subi coolant overflow tank keeps overflowing

I got a 98 Subaru Forester w/130k miles with a weird problem. The coolant overflow tank fills up over time, and if left alone will eventually overflow and then the engine overheats. I’ve been pretty good at remembering to pull it out then dump the coolant back into the radiator, oh, maybe once a week and everything stays fine. But I’ve been lazy for the last 2 weeks and yesterday it started to heat up while on the road. I pulled over (I can usually see it coming before it overheats because the temp gauge starts to bounce around), let it cool for a short time( strange but there wasn’t that much pressure even though I’d been driving for a while) did my pour-back thing, added a little extra coolant to replace what had blown out the overflow and went on my way. No other symptoms, car runs fine, & coolant looks and smells clean.What gives, guys?? Please don’t tell me head gasket—pretty please?? Oh hell, I can handle the bad news- give it to me straight. Thanks for your help guys- great info on the EGR problem and the wheel bearing. Glad you’re here!!

It sure sounds like a blown head gasket. Here’s what to do. When the engine is cold remove the pressure cap. Start the engine and watch the coolant. If bubbles start to appear in the coolant it’s a sure sign of a breached head gasket.


Maybe it is just that the cooling fan isn’t coming on that is causing the overheating. When the engine overheats, is the radiator cooling fan spinning like a banshee?

The other question is why isn’t the coolant in the overflow tank being sucked back into the radiator when the engine cools? That could be caused by a radiator cap that is leaking the slight vacuum that pulls the coolant back into the radiator as the engine cools.

It might be the head gasket. But before assumng the worse, check the cooling fan and the radiator cap.

Blown head gaskets are common on 2.5l Subarus of that age, like your Forester. Follow Tester’s advice. Also, shops can check the fumes coming out of the radiator for exhaust.

I just clicked agree on Tester’s post. This is textbook.

Thanks guys. Tester- I’ll check for bubbles in the coolant. George- yeah, the fan spins when it’s overheating and I thought the same thing about the radiator cap, so I replaced it- no joy.

Crap! As you can tell by my initial post, I suspected the head gasket. It’s a good car, but not worth paying for a head gasket. I guess I’ll just keep the trips short and keep doin my coolant pour back routine until she dies.

Have you considered the possibility of an iffy thermostat or a clogged radiator? I wouldn’t get too wound up over a head gasket issue at this stage of the diagnosis.

How handy are you? The major expense with a head gasket job is the labor. If you do it yourself, it could cost as little as a few hundred dollars. And take a couple of weekends. I say that, because my typical method at home is to disassemble the first weekend, have a machine shop check and prep the heads during the week as I work, cleaning other parts after work, and reassemble the next weekend. Last time I did this, it cost me about $250 in parts and $300 for the machine shop to shave the head flat and reseat the valves for my cousin’s Volvo.

Start simple and cheap. Check the radiator cap and hose to the reservoir first. It may be allowing the coolant to flow into the tank at too low a pressure and not allowing it to return to the cooling system when the engine cools off. Best of luck.

Ok- I kinda thought about that. The thing that gives me hope its NOT the head gasket is the coolant is clean and green. Wouldn’t it be oily and/or smell of exhaust gas?

And that leads into Knuckles post- yes Knuckles I’m very handy. In my younger days, I had a 70 GTO with a 455HO that spun a rod bearing and I rebuilt the engine (totalled it a few months later-I was 19-no surprise, huh?). I’d prefer to do it myself, but the thing is I currently don’t have a garage and my 2nd vehicle is brokedick in Colorado. I’m moving in July, so maybe I’ll find a place that’ll work for ripping an engine apart.

Ok- Even though R&R a thermostat and/or pull a radiator isn’t a big job, the car needs a timing belt (so might as well do water pump), spark plugs (and they ain’t easy to get to without some teardown), and then there’s Murphy laughing when I discover it IS a blown head gasket. So What I’m gettin at is that I’m to damn old to be a shade-tree mechanic! Call me a pussy, but I need a garage.

What you described is precisely an issue that I recently had though not in a Subaru. And the coolant stayed clean. It was a one way leak of exhaust into the water jacket. I could pop the coolant overflow anytime after driving and find bubbles coming up in there. I’m not precisely sure that this was the dynamic, but basically the coolant would expand and hit the overflow as normal. On my car, as the coolant cools & the level drops in the radiator its kept full by siphon back out of the overflow. The exhaust bubbles would rise to the radiator cap & kill the siphon. So I was never really losing any coolant, but the radiator was getting low b/c it wasn’t pulling back from the overflow.

You should be able to verify this easily - after you drive it next time, just leave it running when you stop, pop the hood an take a gander into the overflow for bubbles. If you don’t see any, then start looking for an alternative explanation.

Cig- That makes good sense. I couldn’t come up with a reason why the coolant would not be drawn back into the radiator from the overflow on cool down, but exhaust gases rising up and breaking the siphon sure explains it. I’ll be checking for bubbles shortly. Thanks!!

Some have said that on a Subby you can try to re-torque the head bolts and this may buy you some time. The main issue that I belive the motor must come out on that car to get the heads off. There is not enough room between the block and the strut mounts to pull the heads.

As a band-aid make it OK now, fix for real later type of deal. Try Blue Devil headgasket sealent availible at most autoparts stores. There is a ton of videos of people useing it with great success, and I know of two people who also had success with it. For $60 its not cheap but worth a shot…

It is indeed worth the effort to re-torque the heads before you pull the engine to replace the head gaskets…

I believe many if not most of these Subaru head gasket problems could be eliminated if the heads were automatically re-torqued at 40,000 miles and after ANY over-heat episode…

Just one more little detail to check, the hose that goes from the overflow cap down to into the reservoir, or molded tube as the case maybe. If the hose is missing or has a crack/hole in it, or the molded tube is cracked, you will have this problem. Its rare, but it happens.

I had the same problem!! Check the reservoir cap! Over time they will get worn and the cap needs to fit properly to insure a good seal if not loss of vacuum will cause it to overflow!! I just had this same issue so i’d check wouldn’t hurt!! :slight_smile: Good luck!

It is interesting to see caddymans input because when I was in college with enough money for food and friends with cars in the same place I have torqued head bolts in this same way.LOL. I must be old. It can be a first fix. Time will tell. Every engine has its own specific issues and none of us are there. Good luck.

Thanks for all your input, everyone. It’s a blown head gasket for sure- air bubbles are coming up in the coolant overflow. Maybe I’ll try both ragtop & caddyman’s advice - re-torque the heads and Blue Devil sealant, maybe that’ll hold it until I can replace the head gasket myself. I sure ain’t dumping 2 grand into this car to have a pro do it. I’ll pull the engine and replace the clutch too. From what I’ve read, they should rename my car the 1998 Subaru Blown Gasket!!

Instead of paying $60.00 for a stop-leak product for head gaskets, contact your local pharmacy and order a bottle of sodium silicate for about $20.00. This is the primary ingredient in most of these high priced stop-leak products. You can read how sodium silicate stops head gasket leaks here.


Tester- WOW! That is very freakin cool! I’ll follow-up on that. Thanks!