Overheating due to system non pressurization

Fred Hepding

Asked by Mar 16, 2016 at 05:33 PM about the 2000 Subaru Impreza 2.5 RS Coupe

Question type: Maintenance & Repair

2000 Subaru Impreza overheated on the way home Sunday night. *I pulled it over, and sht the engine down.  The next day, I went6 back, put water in it and came home.  The car ran fine, did not overheat since it was like 4 miles to home from where it was on the hiway..  I pulled off the cap (Cautiously of course.)  I noticed NO pressure even though up to operating temperature.. Looked for a leak and found no obvious signs of leakage.. BUT.. I notice that the gasket surface of the filler neck had been gouged so that the system cannot pressurize.. (This is a sudden development, went on long trip two weeks prior with no problems, and had the car for some time.)  My question is.  With cars operating at higher temperature naturally these days, can boil off occur leading to overheating in less than 100 miles leading to an overheat condition?  The car seriously overheated before I took note, and I believe this has also lead to engine failure. (Probable head gasket.)  The car ran fine the day I brought it home.. still has water in it at the moment, and has not been driven far enough to overheat from boil off..  But on the way home from the tech that verified that the filler neck had been gouged, the car started running bad. (Maybe 6 miles total.each way, and the 4 miles from the breakdown area on hiway..  4+6+6 miles total...)  Proving the radiator was Sabotaged has been done.  I just need to show that the overheating due to boil off is what lead to damage.. The techs here are trying to claim that ad gasket is likely what lead to the overheating...  But the car ran fine prior.

7 Answers


Phew! Tricky one, as there's a chicken-egg duality going on here. Sabotaging the coolant system and resultant boilover CAN precipitate both a high-pressure internal HG failure (making the car eventually non-drivable), or a low-pressure atmospheric coolant (and/or oil) leak to the outside world. In either case replacing both HGs is in order. Techs will claim the damaged rad caused the HG failure...and they may indeed be right, as overheating the SOHC 2.5i is a major stressor of these too-thin HGs. If and when you rebuild you'll be getting the newer 3-ply HGs that will seal well. HOWEVER be especially careful to test for planeness and cracks both heads before re-use (about $200 cost at machine shop).

Fred Hepding

It seems all techs here want to immediately jump to the head gasket lead to the overheat condition. They seem to think that the only thing that happens is that the coolant goes to the overflow and nothing else. "It can't overheat because you still have coolant." I pretty much already knew the answer to this question.. This was more to help my insurance guy understand the principles. He does have to be able to explain to his boss how he is able to approve something like this if he does.... ... I think we can use this thread as further education also. System pressure is very rarely talked about from what I am finding.... I have fixed many overheat conditions in older cars simply through replacing the cap. (Old Detroit metal. :) ) I am finding out with regard to higher speeds and water pumps, they become less efficient due to cavitation.. I have to believe that such a problem becomes worse when a system fails to pressurize.... I was on the highway, and it overheated suddenly, and extremely. This is all making sense now.


Your lack of pressure effects flow rate and heat-quenching (heat sinking is the better term, but I'm thinking of your ins agt). It's well known that, for example, running a Subaru WITHOUT a t- stat will result ironically result in overheating because flow-rate is too high to provide sufficient conduction of heat to coolant. Sounds weird, but it's true. Venting the radiator inappropriately acts the same way. Hope this helps....

Best Answer
Fred Hepding

Further question now. Can I do a simple compression test for the heads. or do I need to go as far as even changing the radiator to pressure test that system as well. I can do the diagnostics myse4lf on everything that goes wrong.. Just do not want to spend the money on a radiator to to pressure test the system only to see that it needs the head gasket. (I am pretty sure it does need it....) since it just started running bad on light acceleration and deceleration.. Idles fine. This didn't start until I was coming back from a shop that confirmed that the radiator filler neck pressure seat had been poked. It has been a while since I have done a compression test.. Does the engine need to be fully warmed? Shot of oil into the cylinder before compression test? The oil level seems to be high, but has not turned milky I can't pressure test the coolant system due to the lack of a sealing surface in the radiator filler neck. Oddly enough, I really don't want to cost the insurance agent anymore money than I have to, and I really do not t5rust the service around here except for very few. ! more.. On compression test.. What should the pressures tend to fall at on a healthy normally aspired engine of its type? And severely overheated as it got on the hiway, I am actually surprised it cranked as well and ran as well as it did. how likely is it that there is other than head and HG damage? I did have to add little over a gallon of water before I started it again.


Further confirms you need HGs. You can test the radiator, but unless it's filled with oatmeal (you know....) or cracked it's probably just fine. But they ARE cheap ($120 or so?). Lots of guys chase t-stats and radiators first, but the great majority of the time the HGs are the culprit.

Fred Hepding

absolutely have to replace the radiator since it wont seal from the sabotage. I was really hoping that it didn't start to run bad so I could get away with a radiator only in this. Given what repair costs are for this, they will likely total the car out.. We don't want to loose our Unicorn..... SO we are probably going to have to repair it ourselves.... Now what is this oatmeal you speak of??? Are you talking about the lime scale and other deposits? Or are you literally talking oatmeal? Because we have been thinking something else may have been added to the coolant as well as the gouging for the cap pressure mating surface....


LOL. The cheapest Prestone and similar non-metallic coolant sealing additives are predominantly proteinaceous glues made from oatmeal and the like. Some use fish meal (the silvery ones). I dorget what Bar's Leak uses, but if used judiciously it doesn't clog ports up as easily as the cereal-based ones. The point, though, is that NONE of these stop-leak products works on Subie HGs...so are only indicated for a very rare water pump seal minor leak, for example. I suppose lime scale could build up enough to block a heater hose, but I would doubt it unless VERY high mineral content water is used. here in MA we tend to have very soft water, so even appliances like espresso machines and water filtration systems rarely scale up.

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