What is the durability of a plastic valve cover?

Question

After a change of the valve cover gasket didn't fix a rather persistent leak (than managed to cover my entire engine in oil) I took my car to a mechanic. They called up saying that they have pinpointed the problem to one corner of the cover that they say seems to have warped slightly. Now this is a thick plastic cover, I am just wondering, is this warping common or is it indicating a larger problem? Is there anything I can do to prevent this in the future?

NoCarrier · Accepted Answer

This would depend on the type of car you have.  The majority of valve covers i've come across are aluminum.

You can try some RTV sealant or Hondabond sealant (pretty popular solution for this) in place of a valve cover gasket.  The sealant should mold itself around all the gaps and irregularities in your valve cover, making for a better seal.  
If that fails, see if an aftermarket aluminum valve cover is available for your car.  They make them in colored/polished/chrome finishes for all kinds of cars - chances are they may have one for yours.

JeremyP · Answer

A few year ago, I remember Stacey David (when he was hosting the "Trucks" show) commenting that he thought the plastic valve covers on the old Jeep Wrangler must have been designed from the factory to leak because he had never seen one that didn't. That doesn't sound promising. Maybe you can find some aluminum or steel replacement valve covers from ebay, car-part.com, or from rock auto.  What kind of car is it? That will help.

EDIT:  Here is a picture of the new valve cover gasket that I think will be needed on your car. I don't see any aftermarket valve covers (of any kind) for this car, so I guess plastic it is.

Here is a LINK that describes the process of replacing it. It all sounds pretty accurate, except for the part where they tell you to speak with a Saturn mechanic... doh!

William FitzPatrick · Answer

Be carful of how you go about fixing that. It is all too easy to overtighten the bolts if they do not have shoulders. There may be a crack. Plastic can be much stronger than a comparable steel/aluminum part, or it can be much weaker. All depends on how good of a job they did designing the part and selecting a good polymer.
For example, if you put the Moly bearing oil into a motor where the valve cover is a plastic sensitive to Moly (many plastics are) then the plastic will become brittle and crack. Though I would hope car manufacturers would know better than to make such a critical oversight.
In your case, I would put a swatch of RTV sealant on the plastic valve cover and see if it sticks. If it sticks you are good, if not then I would buy a whole new valve cover.

juhist · Answer

The probability of warping depends on many features:

What kind of plastic is used
How well the cooling system of the car has been designed
Has the car ever overheated

I used to have a 2011 Toyota Yaris mk2 for five and half years. It had a plastic valve cover. But, being a Toyota, I assume the plastic type has been carefully chosen and the cooling system has been designed to work extremely well. Thus, it never overheated and the plastic didn't warp. No valve cover gasket leaks.
I also used to have a 1989 Opel Vectra. It went to the junkyard at bit over 21 years of age. At that age, the valve cover gasked has started to leak but was never replaced due to it being a minor annoyance and not a major issue. I assume the fault mode was rubber deteriorating. The valve cover was steel.
So it's not a given that a plastic valve cover will warp. However, it is a given that rubber deteriorates.

What is the durability of a plastic valve cover?

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