Hi, I have a 1990 Sedan Deville I'm presently getting about 18mpg on the highway instead of the 23 I used to. My engine also seems to be struggiling to get gas to it. Sometimes it doesn't get any gas to it at while I am going up hill and it will die out. I was told it was the valves and I need to get a valve job done. I was wondering if could be anything else? Also does any body no an average price for a valve job? Or how much it would cost to replace a V8 4.5L engine? It would be greatly appreciately for a response.