Li'l Tommy Knocker: The Joy that a New Knock Sensor Can Bring

Our Volvo 740 (1990 B230f with 368000+ miles) has not been a smooth operator, at least not in the three years that have passed since I saved it from the scrapper. Things have incrementally gotten better as I have moved through various ignored and differed repair and maintenance tasks, but it is still some distance from original performance, whereas the ChevOldsmoBuiac is usually ~90% of new.

Part of the Volvo's problem is engine related. Things got better when I replaced the long worn plugs, and stopped the oil leak into the distributor that was causing misfires. It still bothered me that the engine tended to lug in high gear, moderate speed, and moderate RPM. Pinging 2k rpm seemed unusual. I had ignored it, but this summer, it seemed to get a bit worse.

As a wild guess (that I later justified using a note on swedishbricks), I replaced the knock sensor. It is a true one-bolt job and it's conveniently located on the side of block where it's in easy reach. You can even fit a normal ratchet with a normal socket into position with clearance! It's like it was designed to be replaced! I practically threw a party when I realized the swap would be done before the coffee got cold.

I didn't really expect the replacement to improve anything; usually I'm wrong when I think a sensor is bad. However, in this case, I was immediately rewarded with an engine with smooth power delivery at low RPM.

Given that the symptoms were not super-obvious, the old sensor must have had decayed sensitivity. Inspecting the old and new did not reveal any signs of failure, and, frankly, a little cleaning would make them almost indistinguishable.