2.5 L Turbo I4 Plastic Plug, valve cover gasket, gasket set

Do you happen to know what generally causes this problem and what I need to do to stop it from occurring again?

Thanks

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-----Question-----
I have a 89 Plymouth Acclaim with the 2.5 L Turbo I4.

I'm by far not a car person. If you're looking at the engine you'd have the oil fill cap on the left. Follow that section of the engine to the right and once you're on the side of the engine a couple of inches down there is a round plastic plug about 2-3 inches or so around. I was wondering what this plastic plug is?

I ask because it has come loose and oil is all over the place. It'd be helpful to know the correct name for what I'm talking about when I talk to my mechanic.

Thanks in advance for your help.

-----Answer-----
this is a very simple and common problem with that engine. it is called the circle plug or the camshaft plug, believe it or not. it will come with a valve cover gasket set and is fairly inexpensive to buy and replace.

Answer
really the only cause is age. due to the constant contact with engine oil, which can become caustic, and the heat cylces they just dry out and crack or skrink. nothing you can really due to prevent it from happening. the important thing is to catch it when it first starts.
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