Just a couple of days after it passed its annual emissions test, my old Mazda B2200 pickup suddenly started running really poorly and making a very inauspicious knocking noise. I thought it might be about to throw a rod! The first thing I did to try to figure out just how bad it was was pull all the spark plugs and ran a compression test. The readings: #1 170 psi, #2 160 psi, #3 and #4 both 30 psi! Judging by the loud knocking noise I figured I had some kind of severe mechanical problem, like a broken lifter or rocker arm, but on two adjacent cylinders? I didn’t know what to make of it (I’m just a wannabe mechanic) until the next morning. The diagnosis must have come to me in my sleep: The head gasket is blown out between the #3 and #4 cylinders.
Not the usual blown head gasket where the oil and the coolant mix and the engine overheats. The coolant came out clear, and I didn’t even drain the oil before I took it all apart. Here’s a YouTube video with the same problem: https://youtu.be/_GTL5GbMNJc
Taking it all apart was, of course, a non-trivial undertaking since this is one of the very last carburetor-equipped vehicles sold in North America. The carburetor is computer controlled and has dozens of vacuum and electrical connections, There were three rigid pipes from the exhaust manifold back into the air cleaner, and coolant is routed all the way around the engine and through the intake manifold. I’ll be adding more posts to document the disassembly, inspection, and If nothing more serious is found, the reassembly.