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Q: knock in back half of the motor on 1995 Mazda MX-6

I just recently purchased a 95 mazda mx6 and it has a knock in the motor. Evidentally it has been that wa for 5 years. We put seafoam in the oil and in the gas tank. Didnt seem to help though. Someone suggested to change the oil and use STP in it. Tryin to figure out what it could be before sicking whole lot of money into it. Any suggestions?
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I know that Seafoam in the oil or fuel will not correct or mask any mechanical problems because it's a cleaner. I don't use STP, although I did decades ago, because it's a very sticky oil thickener. It is a real mess to deal with in the engine if you have to do mechanical work, so I'd rather not have had it in a motor if I have to tear it down. It may help to quiet a mechanical noise by keeping a cushioning layer of oil between the parts that are too loose and making noise. It may even extend the usable life of a motor or gearbox that's past it's prime, even if it's knocking a bit.
The Mazda 2.5L V6 is a highly developed and precise motor and if it's knocking, it's damaged and eventually will have to be dealt with. I don't know if the engine knocks because it's got a failing crankshaft bearing, a loose (from overheating) piston or a cam knock, there's not info re: that in your message. I'm guessing it's a piston knock if it's been doing it for 5 years, and hasn't failed entirely. I don't think a bad crank bearing would last that long.
I'd suggest pricing a guaranteed used engine from an auto parts recycling center (junkyard) and getting a price to install it. I'd definitely recommend a Mazda specialist to do the work, there are too many beginner errors to make for a first-timer for me to recommend you find just anybody willing to take the job. Get a pro and pay the money, it's worth it to get it right.
If you want to find a specialist, here's a directory link for you:
check all your valve plugs one of them might have a small crack in one if it does replace it or tape it with electrical tape ive done and now there no more knocking temperarly untill i get a new valv e plug
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