You may not have filled it enough, so when the system is pressurized its all in the block, remember the coolant tank is more for overflow then storage. The Coolant tank overfilling is way more of a concern. Keep an out out for leaks if possible, use a OBD2 bluetooth reader with your phone to monitor engine temps.

 

To my dismay, the coolant in the reservoir was well below the min line. When I went to take the cap off, there was a hissing sound like the release of a vacuum, and then coolant from the return lines filled the tank and it was at the proper levels.

That was actually pressure - not vacuum - and the level came up when you released the pressure in the cooling system. Question then is " Where did the excess pressure come from?

Unfortunately usually that excess pressure comes from combustion gasses leaking past the head gasket.

 

When the coolant is low with the cap on (pressurized) and rises when you take the cap off (de-pressurized) that means there is an air pocket in the system. Not surprising given the recent work you did.

The coolant level should remain between MIN and Max with the cap on, hot or cold. You probably have most of the trapped air bled by now, but still wouldn't hurt to keep an eye on it the next few days.

 

Okay hol'up.
Water and glycol are ostensibly not compressible, which means no amount of exhaust gas pressure is going to push the water down low as OP said. The aluminum block would have cracked if that was the case.

If OP undid the cap and there was a hissing, it means the coolant built pressure by boiling it until about 205*F-235*F was reached and proper pressure was reached as dictated by the ECM and the cooling fan operation, which would decrease the quantity of water in the block, and drop the level, not to mention the air pockets after changing all the lines. When OP undid the cap, if the engine was hot, some of the coolant definitely boiled and filled the overflow tank. I have had times I have undone the pressure cap just after shutting off the car and the ENTIRE overflow tank filled up.

OP just keep adding coolant as the air works its way out of the system. This is normal for an unprimed coolant circuit. Be sure to add it when the car is cooled off (i.e. in the AM before the first start of the day).

Also the easy way to check for vacuum vs pressure is that your coolant will boil from about 180*F (5,000ft of elevation) to 212*F at sea level. You shouldn't have a vacuum, ever, but it can be denoted by your radiator hoses crushing in on themselves as they are def the weakest point. As for pressure, you can see if the system is letting it build by watching your coolant reservoir. If it starts showing signs of boiling as it approaches 180-205-235*F, it means pressure is escaping and allowing the water to boil.

 

Checking the cooling system with a HC detection kit is not rocket science and will reveal possible head gasket issues. The only situation when I can imagine vacuum being created inside it is a blown head gasket driving so much coolant out and displacing it with gases that when the system cools down to ambient temperature their absolute pressure drops below atmospheric according to gas constant.

Get a kit and test it, that'll reveal the presence of hydrocarbons, which can only mean one thing.

 

The only situation when I can imagine vacuum being created inside it is a blown head gasket driving so much coolant out and displacing it with gases that when the system cools down to ambient temperature their absolute pressure drops below atmospheric according to gas constant.

Yes! This exact thing happened to my V70R, I believe.

Additionally, there was no evidence of an external leak. My best guess is that coolant was being expelled during driving. Never found a drop on the garage floor or on the block.

I ran that cooling system check that you mention, but it was negative, but you can only run that test at idle. If the head is lifting a little during high boost, it could leak then, but not in the garage.

That's my story and I'm sticking to it.

 

The rubber hoses expand and contract as well with pressure and heat. So when the system is pressurized the hoses hold more volume of fluid. If the let the pressure out the hoses contract and force fluid back into the reservoir. It doesn't always mean there is an air pocket in the system.

The expansion and contraction is minor, but given the total length of hoses, it adds up quickly.

 

If the head gasket is leaking the coolant starts to look muddy very quickly.

 

When I got home today and opened the cap, it definately started to boil off! I refilled the reservoir and ran the car with the heat full on. One thing I noticed is the fluid in the return line on the right had alot of air as it returned to the reservoir. Also, when I first opened the cap and the fluid started to boil, it definatley smelled like exhaust. With the cap off and the engine running, I went and revved the car in neutral and it looked like a bunch of air was being pushed into the tank (lots of bubbles visible almost foaming) and it looks like gasses were escaping. Is this pretty much indicative of a blown head gasket? If so, what other prevantitive maintenance should I be doing if I'm going to dig in and replace that?

 

Is this pretty much indicative of a blown head gasket?

Unfortunately those are symptoms of combustion gasses getting into the cooling system. The head gasket would be the most common cause - but there are other causes. Cracked cylinders happen in R cars and I had a 01 S40 (4 cyl) that did the same.

 

I've got iPD hoses on mine and it leaks very slowly from the radiator fittings and one of the other connection points over time (despite me regularly tightening the hose clamps). Never drips on the ground or even makes it to the bottom of the engine bay, but I can see the stains and residue around the places where they leak. I put maybe a cup of coolant in it every 5000 miles or so. At one point I was worried about it... but here we are 60k miles later and it hasn't caused me any crisis. Just something I've come to accept with the silicone hoses.

If you want to test for a cracked block or blown head gasket, there are coolant test kits for that. Cheap and easy to use. Cracked blocks aren't particularly common on stock or mildly modified RNC engines like they were on some older generation Volvos. Head gasket is more likely for sure. I wouldn't worry about that until you actually test it though.

 

I used a tester kit and the fluid turned yellow very quickly which confirms exhaust gas in the coolant system. Engine overheated twice last year and I pulled over, but I heard it’s very easy to blow the gasket with over heating. Should I perform a leak down test before attempting to change the gasket to make sure the cylinders are okay?

 

Should I perform a leak down test before attempting to change the gasket to make sure the cylinders are okay?

I would not bother- you should find a machine shop that can straighten Volvo heads - it will be warped - and has the tools to adjust the valves

Sorry for your troubles

 

Yep, very easy to blow headgasket with overheating, the fact that you did it twice and it's still driving is a miracle in its own right.