Had a problem with bubbling in the expansion tank setting off the red low coolant light (my wife calls it the fan in the water bottle light haha) which led me to question whether the headgasket was going. Started noticing noise on startup that turned out to be the waterpump. Also noticed that my amateur timing belt change where I skipped the waterpump left the timing slightly off. I was getting intermittent timing over-advance codes every few days/weeks. The waterpump was a cheap aftermarket should have replaced it at the timing belt change but it seemed fine with no leaks. now getting no problems and the car has much better pickup and no codes so far. my question- is a bad waterpump/mis-timed engine capable of making that many bubbles? the pump impeller was still in one piece and wasn't loose on the shaft it was just noisy bearings that were getting louder all the time. I took my time on the timing job this time and locked the cams from the rear side with the aftermarket cam locking tool. Weird thing is I used the tool last time as well but I think the crankshaft moved a tad last time.

 

Hey, don't see any replies to your query and I'm not any kind of an expert, but on the 1998 V70R I acquired, could not get the cooling system to "burp". I had the bubbling issue in the expansion tank. No oil in coolant nor water in oil, but try as I might, if I put the cap on tight, the car would overheat in an instant.

I found Block Tester BT-700 Combustion Leak Test Kit in Hard Case on Amazon - quick test for exhaust gases in coolant. The result showed that there is combustion gas getting into the cooling system apparently from the head gasket.

Perhaps you have already found the answer to your problem.