I have been losing coolant recently, and as I examined all the likely culprits it is pretty clear that the coolant is being lost through the top of the coolant reservoir, bubbling up past the cap; I actually saw the coolant being forced past the cap with the car sitting at idle. Do these caps just suddenly go bad? I pulled the cap and looked it over and (to my untrained eye) it looks just fine. The reservoir itself was replaced a couple of years ago (and appears to be completely intact), but the old cap was reused.
I'd appreciate any words of advice from those of you who may have dealt with this before. If the cap is a common issue I'll replace it; I just don't want to get the cap if the reservoir itself is the actual problem.
850 wagon - bare bones manual with 124K and going strong (I thought) until grinding up to the top parking lot of Mt. Washburn in Yellowstone last August with a load of teenagers, when it overheated and lost a lot of coolant. I let it cool off and put about 3 qt water in there and drove home to Seattle without any issues then, or since (until two weeks ago). After August, my mechanic replaced with proper mix for the winter, and told me I'd likely need a new radiator this year. Last week in 55 deg weather, stuck in a traffic jam heading toward the Mariners game (except for me!) - coolant loss again: lots of steam and smoke, but NOT really overheating (barely a tick over midline on the gauge - a little warmer than usual, maybe). So I crank up the heat, pull over, bring temp down to midline, turn off and read my book for an hour, put a gallon of water in this time. Again, it's running fine for two weeks since. I'm thinkin maybe gradual coolant loss through a cap that no longer holds pressure (that whole gray cap vs green cap thing that's mentioned on several threads here) resulted in the overheating episode in August? My mechanic says if the gray cap failed it would be obviously broken or cracked - really? No oil in the reservoir, oil on stick looks fine. Now he's doing days of diagnostics and talking head gasket failure. So, is it possible that the single overheating incident last August warped or cracked or thrashed the block or cylinders, and now combustion gases are overpressurizing the coolant system - causing coolant loss without overheating? Doesn't the coolant "boiling" indicate not enough pressure in the system? I guess I can't tell if it's overpressurized or "boiling" just by looking (I'm a girl). If it's head gasket failure, does it run great, like mine does right now? Anyway, he did the block test today but he never called to tell me, so maybe it's a puzzle still and I'll hear from him tomorrow. Thanks for your thoughts or experience on anything similar.
My car started having boil over issues shortly after replacing the head gasket, despite the temp gauge showing a normal reading right smack in the middle of the gauge, and I thought for sure I had a cracked head or block (and of course was freaking the frick out at that point, lol). Went through all the normals - tested the brand new thermostat, flushed the block and radiator I don't know how many times, new cap, new water pump, etc etc etc. I even have a long thread either here or on Volvospeed concerning it.
To make a long story short, turned out that I had a hairline crack at the base of the threaded part of the resevoir, and I found out that these cars NEED to have pressure in the cooling system in order to circulate properly (went through most of a tube of JBWeld to cover the crack, and the cooling issues instantly went away). The crack in the resevoir bled off the system pressure, thus slowing or stopping the flow of coolant, and how I got the coolant boiling over in the resevoir without an overheating condition being shown on the gauge.
To goodrun: coolant boiling is exactly what it implies - coolant hot enough to boil, and it will be obvious as you'll see the bubbling in the reservoir from the outside, and most likely will hear it as well. In my case, I bought the car with a failed head gasket, and I remember seeing the coolant cap sitting just a bit off level, but I never really thought about it before. When I finally found the crack, the cap was sitting quite a bit more off level, which is what let me to look at the top of the reservoir more closely. I now think that the reservoir developed the crack before I got the car, caused the system to bleed off pressure, slowing the coolant flow, and eventually the car overheated.
From my research here and on other forums, it looks like if your head gasket did in fact blow, chances are real good that it just warped the head which can be fixed with a planing at a machine shop, and the block will be fine. When I tore mine down, it was an obvious head gasket failure, involving cylinders 4 and 5, and the machine shop took the head down to level. Block was fine. Put it all back together, and all is well after finding the cracked bottle and replacing it
Thanks for your reply. More food for thought. I'm going to check out that reservoir myself. So, do you believe the gasket on yours blew because the engine overheated due to coolant loss, or loss of pressure? I got my car back tonight. The block test revealed a "slight" exhaust leak into the coolant. My mechanic put 2 bottles of Subaru stop leak into the coolant and he says it sealed up! OKay....a boon for the single mom, for sure!
You might be interested that he also discovered a sticking relay on the cooling fan that's supposed to kick in at low speeds. When I was in stop and go traffic 2 weeks ago and overheated that was *probably* what happened? Maybe the leak wasn't even showing up yet? I'm still wondering why my engine temp displayed normal during the "overheat" recently. Was it because the coolant overheated because the fans weren't working which caused it to boil and create so much pressure that it blew thru the cap? Sorry, this is all new to me. This car hasn't needed more than a tuneup and oil change in 15 years.
My mechanic is a skeptic on the gray vs green coolant cap failure theory. Is there any more/new information out there about the caps and/or reservoir failures? how often? If I do replace my radiator this year, should I replace the overflow tank and cap as well?