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PCM Knock Sensor

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RickR90s

Contributing Member
I have a 2002 PCM 5.7L HO in a MB Sports Inboard. I've owned it since 2003 and every year, I drain the block to winterize. On the starboard side, the knock sensor is removed. For years, the original knock sensor appeared to have the tip missing but it threaded in and worked fine. Last winter I replaced it with the oem part and no issues this summer.

Yesterday, I removed it to drain the block and just a trickle of water came out. In time, it seemed to drain completely but I got a flashlight and mirror to see up inside the hole and it was blocked. My first thought was that part of the old sensor was still in place and had some debris fill in the gap to make it plug.

So I got a small punch out and went around the circumference of the hole and got most of it knocked out. What was left appears to be a hole, but slightly smaller in diameter. I didn't have a tap (I measured the threads on the new knock sensor as 14mm x 1.5 pitch) but assume I can clean up the threads and hopefully push the remaining material through.

My question is, Should that hole be uniform in diameter or is it designed to prevent the knock sensor from going in too far?

Thanks so much,

RickR90s
 
I thought they were tapered pipe threads...the dimensions you 'measured' are real close to a 1/4"NPT thread...I'd be inclined to start with that tap and go slowly to make sure it fits...
 
I'll trust that is a good thing...

Do you have any idea what material was obstructing the hole - rust, scale, etc???
 
Are you talking about the npt pipe tapped drain hole in the block? If so, it is common for a film of rust to kind of form a seal on the inside area of the hole. What I usually do is, remove the plug and poke a screwdriver in and swirl it around on the inside of the hole. This should loosen and punch out the rust, then it will drain out with the water.
 
noelct said:
Are you talking about the npt pipe tapped drain hole in the block? If so, it is common for a film of rust to kind of form a seal on the inside area of the hole. What I usually do is, remove the plug and poke a screwdriver in and swirl it around on the inside of the hole. This should loosen and punch out the rust, then it will drain out with the water.
Click to expand...

That makes sense. What was strange is that this was the first year that draining that side of the block only generated a trickle of water. I did poke the hole and confirmed with a mirror that the hole was larger, so I'll tap the threads and install the new sensor and see if that resolves it. Thank You
 
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