Cooling system clogged?

[hiroku]

Should also note that I pressed at the valve on the radiator cap with my fingers, and it makes a click sound on some areas, and not others. Sadly I don't know if that's normal behaviour as I don't have a known-good one to compare it against, but it does feel a little odd.

[scot850]

The caps on these Volvos do go bad over time. It may help if you can find a way to measure the pressure that is created after a run.

The caps if I recall are set to open around 15 psi (I've also read 150 Kpa). So if you are se more than that you may have a head gasket leak. If you are not getting that then your cap is bad and will cause the level to rise and leak from the cap. A repair shop should have a coolant pressure tester which would allow you to have the pressure tested.

A leak in the system will also allow the pressure/temp to rise and the coolant expand.

Neil.

[yanga001]

Free the coolant reservoir from the mounting area and tilt it such that the small pipe is fully covered in liquid. This pipe connects down to the thermostat area which will allow any air to flow back into the reservoir.Watch it with the engine on and hot and see if you see a consistent stream of small air bubbles. If you do then you may have a slightly compromised cylinder head, and issue i had on my 98 V70. The bubbles need to keep occurring and will be small if its from a compromised head.

I did successfully stopleak my head for a while, but a -31C day and bad coolant formula mixture ended my water pump, so the longevity of this fix was not properly tested. I used blue devil head gasket sealer, and its chemical composition and efficacy worked for me.

[hiroku]

Update:

1) Did a compression test last weekend. All cylinders between 150 and 160 psi (~10 to 11 bar). Pretty happy with that, especially the consistency.

2) Spark plugs looked fine color-wise with no signs of burning anything that shouldn't be in the combustion chambers. Although they did look very worn (car now runs much smoother on the new ones).

3) Inspected the cylinders with an endoscope, couldn't see very much but the piston crowns looked all good and dry.

4) Carried out pressure test of the cooling system... All it took was a couple of pumps (around 7 psi / 0.5 bar) for coolant to literally start SPRAYING out the radiator. Quite surprised at how what looked like a tiny and intermittent leak becomes so obvious when the system is pressurised.

5) I also replaced the radiator cap with an OEM one and the expansion tank has never overflown again. FWIW, the Volvo cap did NOT click when pressed, unlike the Nissens one.

So at this point happy there doesn't seem to be any signs of a head gasket failure or other internal leak, and I probably just have an aging and neglected cooling system whose components randomly decided to fail within a couple miles of each other.

Ordering a new radiator and hoses, will do another flush, and vacuum-fill with fresh coolant. Hopefully that will do!