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Don Foster recently posted an idea about a real-time spark tester using a neon bulb, which I thought sure beats buying a scan tool. Over the New Year's holiday, I came up with a homemade cooling system pressure tester for the Volvos. I went to a junkyard and picked up a spare coolant tank cap. I then pried out the gasket in the cap and used a Dremel to remove the vacuum relief valve and one-half of the pressure relief valve on the inside surface of the round black sealing cap. I bought a 1-1/2 inch tire stem for tubeless tires, then drilled a 1/2 inch hole in the cap and its sealing cap. Using adhesive sealant, I inserted the valve stem into the top of the sealing cap, sealed up the remaining holes in the sealing cap, and re-assembled it into the main cap with the gasket. VOILA: a cooling system pressure tester. Use it with a bicycle pump, a screw-on hose and an air pressure gauge to test whether you have internal or external leaks in the cooling system. Screw it onto the coolant recovery tank, pump it up and wait. The system spec is to test at 8psi: it should not leak down by more than .5psi within three minutes, or you have a leak. Look for leaks outside, or suspect internal leaks such as head gasket, if you have excessive leakdown. Note that the stock system is set to release at 20psi and if you pump it up beyond that you may blow out a radiator or heater core.
By the way, a real cooling system pressure tester costs a lot more than this one: it's more accurate, but this will suffice for simple home diagnostics.
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