Bubbling coolant on the reservoir
 

So I just bought a 940 turbo wagon with 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 possibilities?

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

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 operating temperature?
Also doesn't smoke from the exhaust.

Any particular reason you opened it?

Did you crack it off and allow the pressure to bleed off before removing the cap or did you remove it quickly? If you didn't let the pressure bleed off then yes it will overflow as all the pressure comes out very quickly and it will bubble up.

Normal operating temp is bang on middle of the gauge.

Honestly I opened it because I bought a high mileage car with an engine that has been rebuolt before and I'm super afraid I might find engine issues.
I do recognize I kinda opened it a little too fast.

What about the steam/smoke? It seemed to be coming from the side of the tank that connects to the radiator. Does this symptoms warrant a HG test?

Here is a video: https://www.youtube.com/watch?v=QSk4vw3sk3E
I started filming right after I opened it. Coolant looks dirty too no?

Looking at the reservoir that cap is leaking every time you drive it. This means it will draw air in when it cools down which will cause airlocks and the resulting bubbles.

I'd suggest a new cap, do a coolant flush and change (i've typed the procedure out many times, when i find it i'll copy and paste it here) and then keep an eye on it. Chances are if it was leaking during the previous ownership, they simply topped up with water, hence the dirt. This can cause other problems such as corrosion inside the block and/or head which can cause blockages, hence the suggestion of a coolant flush (reverse flush) and change.

What you see probably isn't several leaks over a period of time, it's just me having opened it several times in the last few days. I was under the impression that the level shouldn't rise when you open the cap even if the car was hot. I'm starting to learn hiow dumb that is...

What about the steam/smokle coming out? Is that normal?

I was considering doing a flush anyways. But I?m also considering doing a head gasket test. Do you think I'm being too cautious/paranoid? The car doesn't have other symptoms. No smoke. Also the echaust doesn't have any smoke build up. No overheating at all. And today I drove it up the mountain for quite some time...

I saw the recent opening evidence, i also saw the much older, crystalline deposits on the seam where the top and bottom halves of the expansion tank meet.

The level will rise because the system is pressurised which compresses the air in the system - this expands when the lid is removed dropping the pressure in the system.

Given the coolant is going to be in the region of 88-92C, steam is perfectly normal, yes. Also if you took it up in the mountains where the air is at a lower ambient pressure, there is more of a chance of it boiling due to the pressure difference. If it behaved during that then the chances are the HG is ok and so is everything else except maybe the expansion tank cap.

Given the evidence, i would try a new expansion cap first and once fitted, leave it alone unless the engine is cold. Top up to the "MIN" mark when cold, it should go up to about the "MAX" mark when hot and whatever you do, DO NOT OPEN WHEN HOT!!!!! :nah: :nah: :nah:

If my suspicions are correct and the existing cap has gone weak, when you fit the new one and it pressurises correctly to 1.1bar/kPa or 1.5bar/kPa depending what the existing one is rated at, this is either 16psi or 22psi - that's a lot of force behind scalding hot coolant! :eek:

Opening the cap with that force behind the coolant would see you getting a hot shower - scaldingly hot!

Thx for the tips. I'm still considering doing the flush anyways, becuase it looks dirty. But I'll hold off the HG test.
I'm thinking if I do a marking tomorrow on the tank of the current coolant level when cold, and after driving check it again, if it's higher that's also a way to know if the cap is letting air out right? When does this become also a sign that the HG might be failing? Or should I only worry about the HG once I have other signs too?

You see where I'm going? My noob brain just thinks "if pressure in the coolant reservoir is a symptom of HG leak, every it happens must mean the HG is faulty". But that's probably no the case is it? I'm just not understanding how much pressure is too much pressure. Or when bubbling is a HG sign and not just decompression from opening the cap when it is hot. Because if I search these symptoms on google the results are "for sure HG" but probably it has to be accompanied by other symptoms.

If it's not overheating it's fine, Just stop opening the cap when it's hot :teeth_smile:

It's difficult to say this without sounding sarcastic (and i'm not being) but the clue really is in the name - expansion tank. It's designed so the coolant and any air in the system expands and contracts within certain limits, controlled by the cap. There is no need to mark the coolant level, simply top up to the minimum line on the reservoir, refit the cap tightly and leave it alone. If the HG was gone, you would have a serious amount of air/liquid being forced out at a great rate and would really know it.

You're in danger of becoming an "automotive hypochondriac" and to be honest, searching online for percieved symptoms will almost always lead to the wrong conclusions. Also Google isn't necessarily the best search engine to use for this as i've found (and proved) on many occasions that the information Google holds isn't always accurate.

As i've said several times and Mark has also pointed out above, stop opening the cap when the system is hot! It's not designed for that and will lead to problems.

In the first instance, check the level with the engine cold, top up if needed to the "MIN" mark and refit the cap tightly. Next order yourself a new cap, making sure it's the same pressure rating which should be on the current cap. When you have the new cap, remove the old one when cold, top up the coolant if needed to the "MIN" mark and fit the new cap tightly.

After that check the level visually when cold and ONLY remove the cap to top up if needed. Repeated removal of the cap in between whiles will cause more problems so unless you need to remove it when cold to top up, leave it be!

In the fullness of time, yes a full flush including reverse flushing the system is a good idea but get everything on an even keel first.