Question: My boat is equipped with a vacuum gauge between the two fuel filters on my diesel cruiser. My buddies keep telling me to check this frequently because it will tell me whether my engines are suffering from fuel starvation due to plugged up fuel filters. The trouble is I really don’t have any idea what the gauge is supposed to read and how to tell if anything is wrong. Help! I’ve sent in a picture of the set-up on my boat. You can see the gauge I’m talking about in the center.
Answer: Well you’re certainly not alone when it comes to confusion on this issue. The truth is, the actual normal readings are going to vary depending on your boat’s fuel system design. Things like fuel temperature and filter micron rating as well as actual location in the system will cause variations in the reading that is acceptable on a given installation. The gauge is reading inches of mercury or vacuum in “inches of Hg” and it’s very sensitive. Any change from the norm means big things are happening in your fuel system. Specifically, the filters are becoming plugged and are indeed creating a fuel starvation situation that can at best cause a loss of engine power and in the worst case cause catastrophic engine failure.
Here’s the way to find out your normal setting. Take a reading immediately after your fuel filters have been replaced and the engines are running as they should. Write down this benchmark value in your engine maintenance log. You have one right? What you will be looking for is an increase of 3-4 inches of vacuum as a maximum before new filters are needed.