Vacuum Gauge Pressure

[sailscubasurf]

I've purchased a vacuum gauge to install on my Raycor 500 primary fuel filter for my Perkins 4 - 108. I've just replaced both primary and secondary filters and would like to know what kind of vacuum pressure I should expect with the clean filters and what vacuum pressure would indicate i need to change filters and/or the filters are clogged.

[a64pilot]

Mine is marked in inches of mercury I think, it says Hg. It runs usually about 5.
It is a Racor Gauge.
I believe height above fuel tank and any possible restrictions like length of tubing and number of elbows etc can change the normal reading.
Mine is marked so that it goes from a yellow marking to red at 10.
My normal may not be your normal of course but would not expect it to run in the red with clean filters.

[Horizon Watcher]

Another factor to be aware of with a vacuum guage reading, is the depth of the fuel in the tank. The lower the fuel level in the tank, the higher the reading will be compared to the full tank reading. This differential is more pronounced if your tanks are deep in the keel, as opposed to level with the engine.

[a64pilot]

I am interested though on hearing what others Vacuum reading is, to see if mine is typical.
Also a data point as you say is how far below the gauge is the tank? Mine is about three feet to the top of the tank.

[GordMay]

“... Analyzing and troubleshooting a fuel system with the vacuum gauge may seem intuitive, and for the most part it is. If the vacuum increases, the filter needs to be changed.
For the vacuum gauge’s readings to be meaningful, you must first establish a baseline with clean filter elements.
If, with clean elements the vacuum is 1″ or 2″ of mercury, this essentially becomes your “zero set point,” and it should be recorded on a tag or label adjacent to the gauge.
Anything over that is a result of clogging or a fault.
If the zero set point is high, say above 4″ of mercury (most vacuum gauges are calibrated in inches of mercury, anything over about 7″ is considered high, although most engine manufacturers set their own limits), the system has other resistance-to-flow problems that should be addressed.
If no notation was made of the vacuum with clean elements with the engine under heavy load, calling on the vacuum gauge to help troubleshoot an operating problem later will be ineffective.
If the system always ran at 7″ of mercury (not good), then this reading may lead to chasing faults that don’t exist, or they may exist elsewhere in the system. High vacuum, by the way, can lead to reduced fuel lift pump life as well as cavitation erosion within the injection pump ...”

➥ https://www.proboat.com/2012/04/the-vacuum-gauge-tool/

[CSY Man]

I had a vacuum gauge for 10 years on a Racor 500 feeding a Perkins 4-108.
The gauge never left the green arch. I called Racor customer service and asked how often I should change filters if the gauge stayed in the green all the time.
They said never..
That being said, I always filtered my fuel before filling the tank and also opened the tank every 5 years and cleaned it good with paper towels and windex on the inside and used a stuff called Pri-D to keep algae from growing. Used 2 micron Racor.
The gauge paid for itself..

[Jammer]

You should see the static vacuum when the engine has been running and is then stopped. This is due to the weight of fuel in the lines. As noted upthread, it will be slightly higher as the fuel tanks approach empty, than it is when they are full.

What then is of greatest interest is the difference in vacuum between that reading and the reading when the engine is operating at load. With clean filters the difference should be negligible, less than 1". Any appreciable increase means its time for new elements.

[Bill O]

I'll agree w/Gord that the "zero" baseline (clean filter) needs to be established. Too establish the clogged vacuum level will be a little more complicated, as each fuel system will be different. Basically it will be an observational experiment on the vacuum level and if you can see your filter element how cruddy it looks. Once you reach your comfort level of when the filter needs to be changed, then mark your vacuum gauge accordingly. If you have a solid cartridge, then you need to wait until the engine isn't running correctly to set the level.

In our current boat, we keep the tanks fairly clean and we typically only see 0.5-1 inches on the Racor 500s. We can easily observe our gauge as it isn't in the engine room as we come up/down the companion way ladder to see if the vacuum is starting to rise. Not having the gauge in the engine room was a modification we made on our last boat and had that one adjacent to the engine panel. When running the motor one always checks the engine panel to make certain the engine is running correctly, so why have the fuel vacuum gauge tucked away in a remote place? To make a long story shorter, we used an automotive vac. gauge that could be remotely mounted (which is also cheaper than the Racor). This way you can observe changes more frequently and address the issues before the motor shuts down as you are entering an inlet in some nasty chop. (Yes, it should be part of rounds check when underway, but easily forgotten.)

Another bonus for a vac. gauge is one can tell if they switched the fuel on properly. We have dual filters so we can switch tanks/filters on the fly if needed and sometimes it happens that you don't open the valves correctly. If you aren't too groggy to look at your vac. gauge, you will notice it going to full vacuum in seconds. Usually you still have enough time to put the valves in the correct configuration before the engine shuts down due to fuel starvation.

Cheers,
Bill O.