My 1992 300E has been having an intermittent problem starting up. Once running there's no notieceable problem. I believe it's a fuel delivery issue. What's common on these cars -- are there check valves that I need to worry about? The last incidence was on a rainy night it took a good 15 minutes of turning over before it fire up. Sometimes it takes less time to turn over. But it's beginning to happen more and more often. Ideas/suggestions?