What Jibes said,
For engines with mechanical lift pumps, the vac gage can go anywhere between the suction side of the lift pump and the filter you are monitoring. On standard RACOR filters (not engine mounted 'guard' filters) the gage would be mounted to the small outlet 'gage tap' or 'bleed tap' on the outlet side of the filter housing/head.
Note - Its best to isolate any gage with a small 'cock valve' and only have the gage 'on' when taking your readings. Gage --> cock valve --> fuel system (gage port on filter housing or 'tee' in the fuel line.)
The 'small' engine mounted filters supplied by the engine manufacturer are not usually designed for 'gross' particulate removal. Theyre there primarily as a 'last chance' filter ... in case the 'upstream' filtration breaches or has a 'break-through'. That 'upstream' filtration used to 'protect' the 'guard' is usually/typically RACOR type filtration @ 30µM (optional) -->
10µM.
Further, the engine mounted filter is usually located between the engine's lift pump and the high pressure injector pump .... and is therefore pressurized. If the 'guard' filter is actually being used as a 'guard' or 'last chance' filter, there is no need to put a gage on them - as usually either they either work or they almost instantly shut down the fuel supply to the engine if there is a 'breakthrough' from an upstream filter ..... no need for a (pressure) gage on the 'guard' filter.
Because of the relatively small surface area of the 'guard' filter; when an upstream 'breach' or 'filter breakthrough' occurs and even if the µM rating of the 'guard' is much larger than the µM rating of the filter that precedes it, - the relatively much higher fuel velocity in the 'guard' - even with much smaller particles than the filters rating - will 'bridge' across the larger 'pores', rapidly plug the 'guard' and quickly shut down the engine. A 'guard' filter either 'works' or or it rapidly plugs ... no gage required.
For the typical recreational boat filter set up:
30µM Racor type 'prefilter' (optional) --- vac gage on the outlet of the filter.
10µM Racor type 'main' filter --- vac. gage on the outlet of the filter.
15-18µM engine mounted 'guard'/last chance filter --- no gage required.
Typical set up: tank --> 30µM (optional) --> 10µM --> lift pump --> 15-18µM engine mounted 'guard' filter --> engine
How to use a vacuum gage to monitor filter condition:
•Initial data set-up with new/clean filters installed.
1. run engine at slightly higher than normal cruising rpm with prop turning (boat tied to a dock, etc.)
2. Very s-l-o-w-l-y close the tank valve until the engine just 'begins' to stumble (begins to lose rpm) - QUICKLY read the vac gage and then quickly OPEN the tank valve. This is the vacuum reading that will correlate to a 'fuel shut down' (insufficient fuel being delivered to) of the engine.
3. Multiply that gage reading value by 0.8
this 'corrected' vacuum reading is your - "
change-the-filter(s)-as-soon-as-possible" value; but, you do have 20% 'reserve' left in that filter to get 'home'. All based on the vacuum developed in the fuel system while you are at 'cruising rpm' and the filters are new/clean.
•Normal operational monitoring:
4. Obviously you have to inspect (and record) the vacuum gage(s) occasionally while at cruising rpm to establish the profile of the filter loading to 'plugging'.
Some will apply an electronic (oil proof) vac gage with an integral 12vdc solenoid switch and adjustable 'set point' (that 'stumble' value 'times' 0.8) to trigger an alarm at a remote location - not cheap!