Fuel Vacuum Question

tcoop · 09-26-2023, 10:47 AM

Let me start out with I have not had any fuel issues to date. I just need an education.

I have a Racor dual filter system that has a vacuum gauge in the center. I also have a vacuum gauge by the helm that I think is also connected but I haven't confirmed that. The engine is a Cummins 5.9 BT. I have not seen either gauge read any vacuum pressure.

What should the vacuum gauge read under normal operation? What might cause the gauge to not register any vacuum but continue to run well?

Thanks
Tim

**slowgoesit** · 09-26-2023, 10:53 AM

We have dual racor filters, each with a vacuum gauge. It is rare for us to have either vacuum gauge read anything other than 0 psi while running. If it comes off of zero, we replace the filters. Probably a little too "proactive", but it works for us.

sunchaser · 09-26-2023, 10:59 AM

Ours behave the same as yours Scott, no vacuum ever.

ofer · 09-26-2023, 11:29 AM

I think if your fuel is above your engine fuel intake, it basically siphons the fuel into it there for creating no vacuum.

DDW · 09-26-2023, 11:44 AM

I've never seen a significant vacuum reading on the trawler, but on the sailboat with the tanks below the engine and the fuel filters well above both, when working well it will read 2-3" of vacuum. If something gets plugged, 6-7" and then the engine quits running. That point probably depends on how good the engine lift pump is.

tcoop · 09-26-2023, 03:22 PM

So if the gauges don't show any vacuum on a regular basis. What do folks use as a guide to change filters?

Comodave · 09-26-2023, 03:27 PM

I believe Racor recommends every year, but that may be in order to sell more filters. I have gone years without charging them without any issues.

BDofMSP · 09-26-2023, 05:40 PM

And I change my Racor filters every spring, but just because it's relatively cheap and it's simple, not because I think they need it. OTOH my vacuum gauges on the Racors DO show values, but they do not get very high, even at the end of the year.

Comodave · 09-26-2023, 05:44 PM

If your fuel is relatively clean in the tanks the filters will last a long time. If the fuel is dirty, not so much…

DDW · 09-26-2023, 06:19 PM

It is claimed that the water blocking ability of the filters declines with calendar time, and that is why you should not just change filters regularly, but change with more freshly manufactured ones. Hard to know how much of that is marketing, the water blocking ability of them is wrapped up in hydrophobic coatings, so it is plausible. Old mechanical injection systems are somewhat tolerance of water, newer very high pressure common rail with piezo injectors, not so much.

RickyD · 09-26-2023, 06:48 PM

On my system I read a vacuum with the engines off. My filters start to clog before I even get it to the red zone on the Parker vac gauges on the filters. As mentioned in other posts, you can shut down the fuel supply and watch the vac gauges to see then then engine starts to stumble. On an old boat I had vac gauges in the fly bridge and you can expect a vacuum loss with distance.

tcoop · 09-27-2023, 08:56 AM

Thanks everyone for the great insights. I won't worry about the vacuum being at zero. I will check today but I think the tanks are at least partially above the filter. I do have the filters on a maintenance list but after going through 1900 gallons this summer with the same filter I was suspicious something was wrong.

Portage_Bay · 09-27-2023, 10:13 AM

I doubt there is anything wrong. To add my $0.02 to this thread consider total fuel flow rate, burn + return. In other words how much fuel are you running through the filters? 1900 gallons isn't a lot of fuel through the filters.

I used to run a boat with a Cat 379 main engine. It burned 10 GPH, return was I'm guessing 20 GPH. Meaning it moved 30 GPH through the filters. In a week that was 5040 gallons.

I ran 2 micron elements in the Racor 900s because changing the engine block mounted filers was b!tch. Realistically done only dock side. So not something I wanted to risk having to do underway in rough weather.

When the tanks were pressed up the level was above the Racors and the fuel pump. When the tanks were low it was the opposite. I didn't notice any appreciable difference in the vacuum readings in either state.

With one exception I never saw the vacuum readings rise much. That was when I took on a load of very dirty fuel and headed out into rough weather. Then the duplex set up saved my bacon. That and a lot of spare elements aboard.

Quote:

Originally Posted by tcoop

Thanks everyone for the great insights. I won't worry about the vacuum being at zero. I will check today but I think the tanks are at least partially above the filter. I do have the filters on a maintenance list but after going through 1900 gallons this summer with the same filter I was suspicious something was wrong.

Jeff F · 09-27-2023, 02:19 PM

I have the same unit as the OP, and on mine the guage doesn't have a drag indicator.

Vacuum is going to be highly dependant on fuel flow rate. There is little to be gained in checking when there is no load on the engine.

My strategy is to run up to maximum cruise speed for an hour or so every 100 hours or so. When I'm doing that I do a series of checks - engine and exhaust temps, vacuum, leaks, etc. I'll also switch between filters to test each one individually. This is also a good test of the impeller and cooling system.

My normal cruise is at about 25% of that load, so my thought is that issues are going to show up during my test rather than out of the blue.

Delfin · 09-28-2023, 10:21 AM

What the gauge is measuring is how much suction is required to pull the fuel through the filters. Zero suction means no blockages in the filter, so that's the ideal. The fact that the gauge measures some, or increasing suction doesn't mean the filter HAS to be changed. It just means the fuel pump is pulling harder to get the fuel required for combustion, and this can mean the filters are clogging, or that the pump is trying to pull more fuel through the filter than the flow rating of the filter, whether that flow is needed for combustion or not. The excess is simply returned to the tank, and since different engines pull return different amounts, you can't make a generalized conclusion about what value of suction above zero is ok, or not ok. The only time you really know the filters are clogging is when rpms drop when running. Otherwise, you might run happily for a year without issue, even though the gauge shows some back pressure.

Jay N · 09-28-2023, 12:38 PM

Further to Delfin's comment: "The only time you really know the filters are clogging is when rpms drop when running".

With clean/new filters, run your engine under load at cruise speed, partially close off the fuel supply valve prior to the filters/vacuum gauge. Listen for rpm reduction/rough running and watch the vacuum gauge. The vacuum gauge reading will tell you when the engine is struggling to pull enough fuel. Open the valve, monitor.

For WESTERLY with a Cummins 220B, 10" of vacuum is max. 49 year old steel tanks with half-bowl sumps drained monthly, and clean added fuel.

When the vacuum gauge reads 3" vacuum at cruise rpm's (equals 6" vacuum at full load rpms), or every 4 years, switch to a clean filter. Over the past 26 years) only once had to go to a new filter based on vacuum (and that was a 2 year interval).

Fleming · 09-28-2023, 03:15 PM

My understanding is that most engines will run with up to 7" on the vacuum gauge, and the filter should be changed at 3".

DDW · 09-29-2023, 07:29 PM

Quote:

Originally Posted by Delfin

What the gauge is measuring is how much suction is required to pull the fuel through the filters.

But also, the hydrostatic head between the gage and tank or engine. On my sailboat, with the filters about 5' above the tank and engine, it will read 3" vacuum with the engine shut off which is about right for a 5' column of diesel. The engine doesn't see this as the pressure is regained on the fall back to the engine. But it is a real vacuum at the filters.

Lepke · 09-30-2023, 11:21 PM

Racor filter elements last much longer with clear fuel than most yacht owners realize. But it's cheap insurance to change early.
I change mine at 7"hg.That is usually near 500 hours. With 2 Detroits that cycle 70+ gallons an hour, that's about 35,000 gallons thru the filter. But I'm careful where I buy my fuel and I use a good fuel conditioner so I don't have organisms growing in my tank. And I run 2 micron filters so the fuel stays clean. Also clean fuel makes the injectors last longer between rebuilds. I haven't changed an injector in 12 years.

tcoop · 10-01-2023, 10:45 AM

Well this is the education I was looking for, thanks everyone. I think I will try limiting the fuel supply to see if I get a vacuum reading on the gauges sometime in the future.

My thought was, if I am monitoring a gauge I want it to read something, that's why I thought something wasn't right initially. I was also expecting that changes were needed much more frequently than they are.