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Old 08-12-2019, 05:38 PM   #1
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Fuel Vacuum Gauge

What would be an ok reading on a fuel vacuum gauge? I did a search here because I know this has been discussed, but no luck. At the end of a 6 hour ride home this weekend my gauge was reading 3.5 to 4 inches vacuum. Filters were changed maybe 20 hours ago, fuel was clean when I changed them. What passes for a normal reading? Engine is a Perkins 4-236.
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Old 08-12-2019, 06:41 PM   #2
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Racor, the fuel filter people, sells a vacuum gauge specifically for this purpose. It is marked with three vacuum ranges: 0-7" is normal, 7-10" is yellow, and 10" and above is red.


See https://www.marinepartssource.com/va...hoCKP4QAvD_BwE


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Old 08-12-2019, 06:46 PM   #3
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What was the reading right after you changed the filter element? I would be concerned about the amount of change. If they read 0 shortly after the change then your reading is showing that something is going on. How bad it is may depend on your system. I would watch it and see what your normal is.
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Old 08-12-2019, 08:09 PM   #4
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How high are your fuel tanks in relation to the filter? If they are below the filter, such as the molded-into-the-hull centerline on our old Hatteras, the vacuum gauge will automatically read a little higher, in our case a couple inches. What RPMs were you running vs max RPMs when you looked at the gauges?
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Old 08-12-2019, 08:12 PM   #5
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Quote:
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How high are your fuel tanks in relation to the filter? If they are below the filter, such as the molded-into-the-hull centerline on our old Hatteras, the vacuum gauge will automatically read a little higher, in our case a couple inches. What RPMs were you running vs max RPMs when you looked at the gauges?
Absolutely. That is one of the reasons you need to learn what your normal is. Check the vacuum after about a half hour after the change and make that reading your baseline. Then watch the increase and change it after maybe 6 to 8Ē rise.
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Old 08-12-2019, 08:22 PM   #6
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Mine sits on 0 for about 50 hours after changing the element.

I change the element when the needle reaches the yellow.

I have Racor 500's with the gauges mounted in the T handle holes.

I use 2 micron filters. Engine is a Lehman 120
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Old 08-12-2019, 08:26 PM   #7
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to those who are interested you can pay $80 for a racor gauge, or pay $9.70 from the Surplus store for the same basic thing correctly marked for fuel filter use.
https://www.surpluscenter.com/Hydrau...y-9-4544-G.axd. I might add this is the same store I get my Engine room fans from. https://www.surpluscenter.com/Electr...er-16-1541.axd
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Old 08-12-2019, 11:14 PM   #8
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A suggestion, spend a few more bucks and get the good gauge that is oil filled, less fluctuations and I noticed more accurate reading. Anything over 7” and my Cummins starts to surge, this in turn might be causing unnecessary load on the lift pump.
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Old 08-13-2019, 06:56 AM   #9
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The tanks are part of the hull, one aft, one midship, one forward. I use the midship tank all of the time, have never used the other 2. The filters are maybe 10’ from the tank, and 2-3’ above it. The RPM on the day I checked was about 1650, which is where I run the engine 90% of the time.
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Old 08-13-2019, 07:09 AM   #10
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Absolutely. That is one of the reasons you need to learn what your normal is. Check the vacuum after about a half hour after the change and make that reading your baseline. Then watch the increase and change it after maybe 6 to 8Ē rise.
Thatís a great idea, I usually just change them, crank the engine to make sure thereís no air in the line, and go to the next project. Seems like it takes awhile to go from 0 to 3.5/4.
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Old 08-13-2019, 08:12 AM   #11
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Just as a point of reference, my fuel tanks are about at the same level as my engines and my vacuum gauges read about 0-1 with clean filters.


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Old 08-13-2019, 09:24 AM   #12
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Just as a point of reference, my fuel tanks are about at the same level as my engines and my vacuum gauges read about 0-1 with clean filters.
Ken
Same for me.
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Old 08-13-2019, 10:36 AM   #13
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This might be a contentious remark so take it as my opinion.

I like a water separator and a screen (strainer) on the suction side of the lift pump and a filter on the discharge side of the lift pump. Rational is that generally, lift pumps push well but don't pull well. The screen provides protection for the lift pump. It is good to be able to quickly and easily see what is getting caught.

There was a good thread recently about fuel plumbing with an electric pump on the side to bleed the system easily. Don't remember if it mentioned vacuum.
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Old 08-13-2019, 10:49 AM   #14
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This might be a contentious remark so take it as my opinion.

I like a water separator and a screen on the suction side of the lift pump and a filter on the discharge side of the lift pump. Rational is that generally, lift pumps push well but don't pull well. The screen provides protection for the lift pump. It is good to be able to quickly and easily see what is getting caught.
This is standard, with the "screen" in most cases being a primary filter like Racor or Fleetguard. After the pump comes the secondary filter. On my old Detroits, there was a pressure gauge after the pump, as well as a pressure switch which triggered the engine hour meter. On the suction side in line with the primaries are the vacuum gauges being discussed here.
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Old 08-13-2019, 12:39 PM   #15
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Clean filters flow easier, but the other thing is volume and suction the pump is drawing, its power. A pump that is drawing greater and harder against any restriction like any filter clean even, is going to create a greater vacuum drop on the supply line than a weaker pump.
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Old 08-14-2019, 07:11 AM   #16
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Also take note that is not only a filter that will cause high vacuum readings. It could be a hose collapsing internally, a plugged tank vent, et al.
Next time take the fuel fill cap off and see if the vacuum goes down.
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Old 08-14-2019, 07:30 AM   #17
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If filter is 3' above fuel level, that alone is worth about 2-3"Hg. SG of HG I think is what, 13 and diesel is 0.9(??). I know this, vac won't go over 30"Hg!!

And make sure your gauge works. Can't tell y'all how many times I had one of those T-handle gauges in my hand while changing filters and the dang thing read like 10"Hg!! Cal fail!!

One trick is if you have a ball valve on your Racor suction, get engine running and close valve. Watch gauge. Should sweep up to at least 20"Hg, then open valve and see the gauge return to normal. Lots of POS gauges out there. Amazing how many flunk this test.

Just gut feel if you read 3" with clean filter I'd change it before it got to 10".
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Old 08-19-2019, 01:42 PM   #18
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And what is your filter body and micron rating of the filter you are using? Not that that makes a lot of difference, but might help more fully understand your system. Is this gauge on the first filter from the tank?

I replace filters at a 5 inch rise in vacuum.

Coming in the pass off a rough day, I once saw this fuel hungry Yanmar of mine running at 10 GPH get a vacuum of 15 inches, and I almost choked because it is normally ZERO. I quickly opened the valve on the standby Racor 500 (both using 2-mic filters). I was situationally aware because my vacuum gauge reading is taken from a tee in the fuel line after both Racors and before the engine-mounted fuel filter, and the gauge is at the helm via a translucent hard plastic line like you use for an ice maker water line. I did this same arrangement for both engines and generator on my trawler - it's about an hour long job including cutting the hole for the gauge.
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Old 08-19-2019, 01:45 PM   #19
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Photos of tee and vac gauge at helm.
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Old 08-19-2019, 02:33 PM   #20
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Like that idea Rich!!. It's on my "to do list" As you may have noticed in my previuos post I've just fitted vacuum gauges on mains and Gens. But the addition of vac's on the main bridge I would see as absolute positive step.
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