make sure you use a good primer and paint that is labeled to withstand the engine heat. give it a good wash, light sanding with hi grit sand paper and wash again, make sure it is completely dry before the first primer coat, try to follow the firections on the cans as close as possible and try to get a nice even coat when doing the actual painting, remember alot of light coats is better than a few heavy ones.
lastly alot of the high heat paints recommend you cure the piece when done painting, wich involves puting in the oven, since the cover is plastic you can try a few quick times at like 150-200 in the oven or just give it more than the recommended drying time if not heat curing
