Volvo coolant needs to be mixed with distilled water 50/50, so you need 1 gallon of each. The waterpump gasket is a pain to get off nicely, just make sure you get it off completely without scratching the block. Take your time.

The VVT on your car can be locked with a cam tool, OR, you could line up the marks on the cam and crank, continue to rotate the engine another 90 degrees and then rotate back to the TDC marks. The counter clockwise rotation will releve the pressure on the VVT.

The serp belt should be replaced more often than the timing belt. But it is easy to check as it is uncovered. A good quality belt from the autoparts store will work nicely. Compare it to the one on the car as it must be the proper width and almost as long (stretching with age).

Is the heater core leaking? Yes, it is a poor product, it only lasts 10-15 years! Longer if dirt is kept out of the core. All you should need are O rings that come with the core.
Get any cabin filter, and replace it every year or two. I usually order 2 at a time, shipping costs more than the filter.

Remember, Tasca charges shipping!

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My back feels better when I sit in a Volvo seat