So I just bought a high mileage car (500000km) with an engine rebuilt 24k km ago. Today it was the first time I really drove it. About 100km or so. Well, when I came back home I parked and turned off the car and I popped the hood and opened the coolant reservoir and the level arose and it kept bubbling.
I also think I see a bit of smoke or steam coming out of the reservoir.

Is this a blown headgasket for sure or are there other posibilities?

Car doesn't overheat. At least I believe it does not. The temperature gauge needle seats in the middle. Doesn't have temp written on it. Is middle too much for opperating temperature?
Also doesn't smoke from the exhaust.

You Wrote: "Today it was the first time I really drove it. About 100km or so. Well, when I came back home I parked and turned off the car and I popped the hood and opened the coolant reservoir and the level arose and it kept bubbling."""


Never do that. The coolant is UNDER PRESSURE and Very Very HOT---You release the pressure and the Coolant gets past the boiling point.

You are lucky it didn't spray your face---with 212DegreeF or 100 Deg Centagrade liguid. Equals 3rd Degree burns.

Leave the coolant cap off, let the engine get to operational temperature and see if bubble come up into the coolant reservoir. Compression testing of
the cylinders will tell you more ....Pressurized coolant system is so the boiling temp is higher when pressurized. Releasing the cap when system is pressurized it's normal theses bubbles.

Here's a video I made. Started filming right after opening the cap. https://www.youtube.com/watch?v=QSk4vw3sk3E

Hi,

It’s never been condoned to remove the cap when the engine is hot enough to boil water!
If you use a microwave to boil water or other liquids do not break the surface violently or you can see an eruption as well! The substance is still vibrating and creating friction.

You are very lucky that the designers have changed things!

The design incorporates a separate zone indirectly connected from the larger coolant mass and not on the very top of the systems mass.
A rule was developed back on solid radiators that never even had recovery bottles on the side as a reservoir.
The caps back then got a second safety catch to stop a person from totally twisting the cap off loose in one turn. At least your threaded one cracked open first!
This all helps keep the a boil water geyser from flying out at hitting the hood and spraying all over the unsuspecting consumers.

The coolant in any system technically gets hotter just after a shut down than it would be normally. This is because all circulation stops and therefore cooling stops but the engine still has some heat generated that never got time to leave the block.
A higher Pressurized mass or vapor holds more heat within its own containment environment.
In this case, just like a food pressure cooker or the earth, with its volcanos! It’s the same!
You don’t open those either!
Its energy that would like to get out and move elsewhere, with more freedom! Don’t we all?

What is going on is a thunder storm, down in the block and that’s the noise you’re hearing. Rumbling Molecules are changing states from a pressurized liquid off into a vapor gas and then collapsing (condensing) back and banging into other similar conditions elsewhere in the coolants mass.

What I’m getting to see, from the safety of your camera lens, looks very normal to me!
If you are NOT losing coolant due to a leak somewhere, you are probably ok!

I have written here or in another thread recently, that there is a hand held tool for checking for combustion gases coming up and out of coolant.
The monoxide reacts with a liquid and changes color as it exits the coolant system from the cap opening.
The vintage ones I use creates a slight vacuum above the coolant, with a hand bulb, to draw gases up through the detecting agent. A very visual operation.
I see Today some kits are more sensitive to even detect residual gases in antifreeze.
No experience here with them.

A simple test for confidence that all is tight below.
Just watch the coolant precisely for awhile and engine oil for coolant to be safe.

The car is used but a new one can do exact the same thing! More than we ever hear about!
At least this one is past those pricey and complicated new vehicle warranties. (:-)

Sponge bath time means get closer and feel better for it!
The car quietly says thanks! (:)


Phil

Hello Phill. Thx for the great reply. For now I'll stop opening that cap when the car is hot.

Here in Spain i can only find those tests on Amazon. They cost 22€. The cheapest one is a rubber plug with a hole where a you mount a clear plastic escape circuit with a P trap. The chemical is in the P trap.
I think I'll buy one to put my mind to rest.

I'm with the others on not opening the cap when the engine is hot. Not only does it tell you nothing about the health of the head gasket, it is potentially very dangerous.

The now lower pressure means the boiling point of water is lower and the residual heat of the engine block starts to boil water and the bubbles find their way up to the highest point. And if you're living at a somewhat higher altitude, then the boiling point of water is lower than the 100 degrees C it is at sea level.

Once the engine has cooled down fully, take the cap off the reservoir and start the engine. If straight away bubbles appear and keep coming, then you have a big head gasket leak.
Smaller leaks can be detected by the device already mentioned.

In your video, I couldn't tell if there was oil floating on the coolant, but if things start to look like this in the reservoir, it's not OK either:

What kind of car is it?
--
Art Benstein near Baltimore

Just found 2 lumps on my car battery.
Had them tested.
One came back positive.
Hope it's not terminal

Sry I'll edit. It's a 940 turbo wagon. 1997

OK, this is a little out of my depth. The 7/9 cars run the cooling at a higher pressure than my 240s, and the added heat contributed by the turbo cooling circuit combines to make it plausible in my mind that when you release the pressure in the reservoir tank, your coolant could boil, especially now that there's no circulation through the radiator and perhaps the electric fan has shut down.

Bottom line is I expect this is normal. Wait for the 940T owners to chime in. Marked Model and Year.
--
Art Benstein near Baltimore

“Take your Army to the Haunted Forest and bring me that girl and her dog! Do what you want with the others!” -WWW

Hi Art. I think others have it right, likely quite normal after a good run. Starting from a cold/warm engine with the rad cap off and seeing if it suddenly rises and bubbles within 5-10 mins as soon as the thermostat starts to open is more definitive of a head gasket leak into the water jacket -I was quite impressed the first time I witnessed that.

In my 940T, between the hotter engine environment with a turbo, a plastic sided radiator, failure prone heater valves and failure prone heater cores that are an absolutely bear to replace, at the first opportunity I changed from the stock 700/900 green (21 psi) cap to the lower pressure 240 black (11 psi) cap. I believe at one point there was a white (15? psi) cap used with some early 740s that has been NLA for years, also a grey (21? psi) cap. For my 940 NA I decided to live dangerously and keep the green cap, but wouldn’t you know it, that was the first of these ‘95 cousins to have a cooling system failure. If I ever trip over a free black cap I’ll likely switch it too.
--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now