Contour Enthusiasts Group Archives Forums General General why would a valve cover crack?

my dad took the gl to get an oil change and only after the oil change (might've been a few days) he noticed a nasty smell. pops the hood the and the valve cover on the zetec and cracked around the oil spiget(hope thats even the right word). if i'm reading the invoice correctly its says they put in 4.5 quarts (i have to doublecheck what the zetec takes) but in general what could cause this crack (in any engine). lack of oil, too much oil? the shop is very reputable and has been servicing the car for years now. we arent going to bring it to their attention because unfortunately i dont think they would say they might've caused it. there's no way they would've missed this when they poured the oil in and there's no way they would've let us drive the car away without bringing it to our attention. they just arent that kind of shop. so i'm going to attempt to change the cover myself. trying to find a used one. are zetecs all the same from contour, cougar, mystique? i'll order a new gasket. and the current bolts can be reused, right? going to try to change the timing belt too if thats a diy job. my dad has been checking the oil and adding any if needed but but has been driving it to work for over a week now, maybe 2 miles one way. he probably shouldnt but has no choice.