Coolant tank boils over - engine at normal temp. Topic is solved

Post by **Sveedy** » 31 Aug 2025, 19:56

I thought it was just a stuck thermostat on the V70, but after replacing it, the problem still persists. After about 20-30min of driving, the coolant will start to boil over. Engine temp is normal. Fan doesn't even kick on. I even checked the thermostat in boiling water to make sure it was working properly.
From a cold start, I can watch the coolant in the tank just sitting there until the engine comes up to temp. Then I see the coolant level drop down a bit as the thermostat opens. Then it stops and sits for a couple of minutes until it just starts to rise up in the tank and boils over. Engine temp is fine through out. Fans never turn on.
I'm going to get hold of a exhaust gas tester to see if I have a problem with the head gasket. I would hope not, since I just put this back together 4K miles ago, but my gut is telling me that is most likely the cause. Seems kind of weird that it would take that long for a problem to show up. I might try one of the "gaskets in a can" if exhaust gas is detected, before diving in again.
Over flow tank and cap are also both new, but both aftermarket brands of some kind.
I'm I missing any other possible causes ?

Post by **scot850** » 31 Aug 2025, 20:02

Put a proper Volvo cap on the coolant header tank. I had this problem with an aftermarket cap that was new out the box. A junkyard one went on and is still working fine. The only other issue is the one you unfortunately lead towards, head gasket.

Good luck!

Neil.

Post by **Sveedy** » 01 Sep 2025, 11:14

Well Neil, thankfully you were right. It was indeed the crappy aftermarket coolant tank cap. Purchased 8/6/25 !!
Unfortunately it came on the coolant tank, so for me to return it, I'd have to remove and send the tank back as well. Not worth the hassle.

Frankly, all of this bad aftermarket product makes me a bit worried that some kind movement to get our "old" cars off the road is brewing (boiling). Just make the replacement parts expensive and/or unreliable, and let attrition do the rest.

Post by **scot850** » 01 Sep 2025, 11:54

Unfortunately that seems to be the case even with new parts, even OE these days. I feel Volvo no longer set exacting standards like in days of old. That is bad enough, but then they now charge 400% more for these junk parts than they were 5 years ago.

I'm glad it was a simple fix, but like you, I'm concerned moving forward on all cars of all vintages. Maybe time to go back to a 240?

Neil.