Help with my 1990 240sx PLEASE!!!!

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well i purchased my 1990 240sx(i am a drift fan) and i hear a popping sound/crack coming from 2 pistons i dont know what this is but as soon as the engine cranks on the motor pivots around crazily as if the pistons were hitting something in the motor. but if i drive it its smooth and not much pivoting/trembling maybe because its at a higher rpm. can anyone help me? can it be a problem with the pressure? please help

posted by  zooyorkn6

some more information on the conditions of the car would be helpfull.

also, when you say pivoting, do you mean as in shaking around and making a loud banging sound? or what?

posted by  Inygknok

If you are hearing something striking the piston, it can only be one of two things, an intake valve or an exhaust valve (providing rings are still seated). Everything goes away at higher rpm's, :). If you hear a distinct pop in a cylinder chamber, with the car off- rotate the crank by hand with a socket and rachet slowly and listen for the pop noise on a compression stroke. If you hear it when rotating the crank by hand you will be able to identify which cylinder it is in and more importantly its exact point of origin and contact. While a piston striking a slightly bent valve will making a distinct popping noise and the car will run, it is not good, so eliminate that, sticking valves in zero tolerance engines can be bad news, especially if the piston has already struck the valve and bent it slightly. I hope I am wrong. I have seen where a door screen screw got into a cylinder from the spark plug galley and rode the top of the piston popping as the piston moved up and down, that was the weirdest thing I think I have ever seen happen as far as 'popping' goes.

posted by  MsSnapon

sounds like a misfire and maybe a bad motor mount...a little more info might be helpful. If something was striking your pistons the car would run like absolute shit no matter what rpm you were at. If you have a misfire at idle(fouled plugs, bad wires) it is possible for it to go away during higher rpm's.

posted by  Sick88Tbird

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