89 Grand Marquise LS Gasket leak

Hi,
I have an 1989 Grand Marq LS (5.0 Engine and AOD). I recently changed my valve covers gaskets myself and everything was fine, but now the left valve
cover gasket (facing the car) is leaking again. I changed gaskets to original parts from the dealer and I am sure I tightened everything properly. Do I have to change the gaskets again?
Thanks
Pooya
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Bob
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Pooya Jalali opined in

Are you sure your PCV system is in good shape?
With engine idling, a sheet of paper held over the filler should suck down..
This doesnt mean you dont have blowby at speed but if that test fails, you have a serious problem.
The Oil separator screen BELOW the PCV may be clogged with gunk.
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I am not sure what you mean by filler. (The engine oil filler spout?). Also could it be my PCV valve (and is easy to change?).
Thanks

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