"Anyone who knows diesel engines knows I am correct. Just because a leak does not make it to the floor does not mean it is not wet at the point of leaking. "
I agree with that. What I was trying to point out to the OP, was that he needs to look carefully for leaks. One would think that a leak that could cause 5MPG difference in fuel economy would be easy to spot. I would have thought it would leave obvious spots on the garage floor, for example. In my case it did not. The last little 2" stub of the daisy chained fabric covered fuel return line on the injectors was the culprit. You're right, it was wet in the area immediate area on the engine. I even knew it was slightly wet. However these hoses are known to weep and you would think it would amount to much at all. I was gonna fix it when I had some time, but never thought it would lower my MPG by 5MPG. I had measured the MPG many times and it was stuck at about 20-21. Soon as I replaced that little stub hose, it went back to
25-26.
You think that was caused by air being sucked in, not fuel leaking out. But I don;'t think so. One day when I was having the car parked in the rain, I left it running for 5 mins until the attendant drove it away. When he did, I could see a pretty large oil sheen on the wet pavement. That return line is only pressurized when the car is running. So, what was happening was the leak was only active while the car was driven. Put it in the garage and shut it off and not even enough fuel made it to the ground to leave a mark. I think some of it likely evaporated off the engine before it made it to the ground. Even looking at it with the engine idling, there was no evidence that there was a steady stream coming out, it just appeared slightly wet. Yet, going down the road with the engine at speed, it was likely leaking a lot more than I ever thought.
So, my main point was, you need to look carefully for leaks, because there may not be as large amount of fluid around as you might expect.