Vacuum gauge

[Laundryman]

I have a Volvo d2-40. My Chinese racor type primary filter is positioned higher than my tank. Between the primary and secondary filters I have a vacuum gauge. Once the engine is running it shows a small vacuum . Question, should the vacuum be retained when the engine stops?
will it reduce over time or disappear immediate? If the vacuum disappears, does this point to a small leak please?

 

[Rappey]

I fitted a vacuum gauge but mine is between the primary filter and the lift pump. My fuel tank is in the keel so everything is above the tank.
With new filters the guages went to around 3 whatever its calibrated in. At 10 the filter is "full" and needs changing if it hasn't stopped already.
It stays at 3 when engine is off. It doesn't really move from an engine that's been sat many weeks to running. If it drops off then there must be a leak ?

 

[B27]

There will be two components to the vacuum reading.
The first will be 'static head' due to being at the top of a syphon-like column.
Atmospheric pressure is about 10m of water, which must be 12 or so metres of diesel, so this is gong to be about 0.1 bar per 1.2 m difference between tank fill level and gauge level. So you'd see it with keel tanks, but it would be small on my boat.
The other component is 'resistance' of the fuel flow through the filter, this will be engine speed related and absent when the engine stops. Obviously this gets big when the filter is nearly blocked.

 

[RivalRedwing]

I fitted a vacuum gauge but mine is between the primary filter and the lift pump. My fuel tank is in the keel so everything is above the tank.
With new filters the guages went to around 3 whatever its calibrated in. At 10 the filter is "full" and needs changing if it hasn't stopped already.
It stays at 3 when engine is off. It doesn't really move from an engine that's been sat many weeks to running. If it drops off then there must be a leak ?

units please... I'm having flashbacks to Spinal Tap and 11

 

[Rappey]

units please...

around -3 inHg at rest and not much higher with engine running.

 

[noelex]

This article may be useful:

The Invaluable Fuel Filter Vacuum Gauge – Editorial: My Kingdom for a Set Screw… | Steve D'Antonio Marine Consulting

"Identifying the presence of and locating vacuum leaks can be challenging. One means of determining if you have a vacuum leak is to perform a 'decay test'. This test essentially tests the vacuum-tight integrity of the fuel system between the lift pump and the isolation valve at the tank (all fuel tanks should have isolation valves at supply plumbing). With the engine running, slowly close the supply valve at the tank, while monitoring the filter's vacuum gauge. The needle should rise slowly; when it reaches 10 inches of mercury, shut down the engine. The gauge should hold that vacuum after the engine stops, if it falls you have a vacuum leak somewhere between the lift pump and the tank valve."

 

[Rappey]

For anyone interested in fitting a vacuum gauge all you need is a t-piece to connect into the existing fuel line, some pipe and a guage .
Its great for seeing that there are no air leaks in the fuel system and you have a visual indicator of when your filters are becoming blocked.
Here the guages i bought, glycol filled, stainless housing and cheap.

 

[Roberto]

For anyone interested in fitting a vacuum gauge all you need is a t-piece to connect into the existing fuel line, some pipe and a guage .
Its great for seeing that there are no air leaks in the fuel system and you have a visual indicator of when your filters are becoming blocked.
Here the guages i bought, glycol filled, stainless housing and cheap.

Hello,
I bought the gauge but I have not fitted it yet: I have a CAV filter head and thinking of using one of its free ports, I need to use a hose (the filter is barely visible) and am wondering if the air inside the hose (from the filter to the gauge) might cause the engine to stop. The hose should possibly be routed downwards so it remains filled with diesel when the cartridge is replaced?

 

[RivalRedwing]

Hello,
I bought the gauge but I have not fitted it yet: I have a CAV filter head and thinking of using one of its free ports, I need to use a hose (the filter is barely visible) and am wondering if the air inside the hose (from the filter to the gauge) might cause the engine to stop. The hose should possibly be routed downwards so it remains filled with diesel when the cartridge is replaced?

I did similar. Select the unused outlet port to monitor the filter. I just attached a short length (20cm) of copper pipe between the gauge and the CAV along with a small ball valve to isolate the vacuum unit. The vacuum gauge is above the CAV, no other precautions taken although I probably bled the system via the CAV (can't remember). Started the engine and opened the ball valve to check on vacuum created by the filter, and the engine happily continued running.... I've have encountered no issues subsequently.