Would this cause high fuel vacuum

[Unclematt]

I had the fuel system upgraded over the winter. The boat had 1/2 copper fuel line and was replaced with 3/8 hose. There is a new Racor dual filter set up too. The problem is I now have high fuel system vacuum. If I remove the elements there is no vacuum at all. I have 30 micron filters in now with 12hg. Could the smaller hose be causing this issue? Any thoughts would be great.

 

[Comodave]

What engine? I had 3/8” hose on our SP225 Lehmans and didn’t have any high vacuum issues.

 

[OldDan1943]

Why did "someone" reduce the diameter of the hose?
I would have thought "they" would have used a 1/2 inch hose.
The volume passing through the smaller hose will be less w/o drawing a suction.
There are math formulas that will prove that. How much less, I cant even remember the formula.
Compare the volume of a 1/2 inch tube, of a specified length, compared to the volume of a 3/8 tube of the same length.

 

[Comodave]

1/2” fuel hose isn’t as prevalent as 5/16” or 3/8”. Depends on what the horsepower of the engine is. 3/8” is probably fine for most trawlers.

 

[OldDan1943]

1/2” fuel hose isn’t as prevalent as 5/16” or 3/8”. Depends on what the horsepower of the engine is. 3/8” is probably fine for most trawlers.

If we can eliminate the "probably" then start looking for kinks or compressed parts in the hose.

 

[KnotYet]

I had the fuel system upgraded over the winter. The boat had 1/2 copper fuel line and was replaced with 3/8 hose. There is a new Racor dual filter set up too. The problem is I now have high fuel system vacuum. If I remove the elements there is no vacuum at all. I have 30 micron filters in now with 12hg. Could the smaller hose be causing this issue? Any thoughts would be great.

Make sure your vacuum gauge is correct.
Assuming it is, 12" Hg may not be excessive. It's the equivalent of 6 psi.
Interesting that without filters there is no vacuum. That points to the filters being the restriction.

 

[High Wire]

Resistances add up so filter element plus smaller hose results in high vacuum. Sounds like you are right on the flow limit for what is installed. Going back to 1/2" can only help. Make sure ALL the fittings are 1/2" ID as well. If there are hose barbs or elbows, they reduce the already smaller 3/8" ID to something less.

 

[Lepke]

Copper tubing is measured at the outside diameter, hose on the inside. Considering the wall thickness of tubing you lost about 1/8" of inside diameter going to hose. Shouldn't effect it that much.
I use 3/8" tubing for Detroits and 900 Racor serving both engines and generators. Detroits alone pump 70gph. With a new element my vacuum reads zero.

 

[Unclematt]

The gauge is in the red zone at 15 psi not hg. The reason hose size was reduced was due to the size of the chase 1/2 hose would not fit. He said 3/8 would work fine. If it were the hose diameter would I get 0 when filters are removed? The engine is a 6BT 5.9 Cummins 210 hp. My next step is to replace the hose with 1/2.

 

[Comodave]

I think there is something else going on. My 225s ran fine with 3/8”. I would check the hose and look for kinks or other restrictions.

 

[STB]

3/8" hose is fine for those engines, as long as it isn't pinched or obstructed.

If you have a powerful vacuum with the filter element in and no vacuum when you remove it, I'd guess the problem lies with the filter or installation.

Bad fuel can clog a filter fast. Get some water in the tank, get some growth, stir it up, and a clogged filter, you get. Same if you add biocide to a tank with growth.

A well maintained charter fishing boat here was losing power and falling out of plane. The changed fuel filters and it seemed to go away and then came right back. They changed again -- and they could see the filth on the filter. There was nothing complicated -- just bad fuel that isndirtying it up until it is gone. Or, well, gone enough, "Dilution is the solution to pollution".

Ultimately, if there isn't a problem except when that filter is in...that device looks really suspect and the lines seem demonstrated good.

 

[SteveK]

Before you change the hose, care to replace the filters and take a comparison reading to eliminate filters as cause.

 

[Unclematt]

I did the comparison. It had 2 micron in it from new which I changed out to 10 micron which gave me slightly less vacuum. I took the filters out and had 0 so I put in 30 micron and I am still in the red. It is a simple system. Fuel tanks through new hose to a new Racor to new hose to the new lift pump. I have tons of flow. I am not sure what else to do. Keep the thoughts coming. Thanks

 

[OldDan1943]

I did the comparison. It had 2 micron in it from new which I changed out to 10 micron which gave me slightly less vacuum. I took the filters out and had 0 so I put in 30 micron and I am still in the red. It is a simple system. Fuel tanks through new hose to a new Racor to new hose to the new lift pump. I have tons of flow. I am not sure what else to do. Keep the thoughts coming. Thanks

Do you have a duel Racor filter? When you move the lever to the other filter, do you get the same readings?

Personally, I would discuss it with the people who did the work. They have not accomplished what you paid for.

 

[Gdavid]

Are you certain that your vent lines are clear? Try removing a fuel cap and if your vacuum decreases, you have a plugged up or possibly kinked vent.

 

[Unclematt]

Yes dual Racor and yes same reading primary, secondary and both. I checked for vacuum in the tank there was none. No kinked lines as far as I can tell everything is good except the vacuum reading. The guy that did the work is to overwhelmed with work to be of any help.

 

[fgarriso]

If the vent trick doesn't work remove the fuel hose at the tank for just a moment and see if the vacuum drops. if so you have a plugged fuel pickup in the tank.

 

[smitty477]

The gauge is in the red zone at 15 psi not hg. The reason hose size was reduced was due to the size of the chase 1/2 hose would not fit. He said 3/8 would work fine. If it were the hose diameter would I get 0 when filters are removed? The engine is a 6BT 5.9 Cummins 210 hp. My next step is to replace the hose with 1/2.

A few thoughts...
-If there is no room for larger hose in the chase how will you go to 1/2?
-If there is that limited room in the chase what are the chances that the 3/8th hose is compressed/kinked in the chase?
-Can you temporarily bypass this chase and run a 3/8th hose to the filters where you can see it and its not compromised to see if the vac drops?
-Are all the connectors and fittings ("T"'s, 90's, connections ,etc) all known to have a full port opening inside not limiting flow rates?

 

[catalinajack]

I think, but not certain, that the previous posters have missed an important fact that needs to be verified although Lepke did make a start. Tubing is measured as outside diameter while hose is measured as inside diameter. No one asked the OP whether the original copper tubing is truly 1/2 ID tubing. Many folks, myself included, have made this error. On my boat the copper fuel lines are 1/2 OD making the inside more like 3/8. My guess is that is what the OP has. If so, his problem lies elsewhere.

If his copper tubing is indeed 1/2 inside, that would make the tubing 5/8 by industry standards. Wouldn't that be very unusual?

 

[Gdavid]

3/8" ID hose should be plenty for a cummins 210. It sounds like he is encountering this issue at idle so something else is up here. I cannot imagine how it is possible, but perhaps the filter isn't fitting into the racor housing correctly or the racor just needs to be thoroughly cleaned out.

I would be tempted to run a 3/8" hose from the bottom of a jerry can full of fresh diesel to the inlet side of the primary filter housing and see what happens.

 

[catalinajack]

Another thought - could the problem lie within the Racor housing itself? Although it would be a pain, bypassing the filter housing with the vacuum gauge rigged in-line would be telling. Is my thinking flawed? The OP did zero said he gets no vacuum with no filter in the housing. I didn't miss the fact that the dual Racor is new.

 

[OldDan1943]

I would be tempted to run a 3/8" hose from the bottom of a jerry can full of fresh diesel to the inlet side of the primary filter housing and see what happens.

I like that idea....

 

[rgano]

I run 3/8" ID fuel hose for my 315 horse Yanmar pulling 21 gallons an hour (12 gallons returned) through 2-micron Racors, and my Racor vacuum reads zero on the remote gauge at the helm. That's a bunch more flow that that Cummins. So, I am liking the clogged line theory espoused here, and I fully support the idea of pulling the input hose off the Racors and sticking a new short piece of 3/8" hose into a can of fuel as a troubleshooting measure. I had a friend who fought and fought a fuel starvation issue for a long time (I even had to tow him home once) until he opened up his fuel tank and found a piece of rubber from God knows where plugging the fuel outlet.

 

[SteveK]

Fuel hose sizes

Are we all talking about the same size.
I mean a 3/8" ID has 0.55" OD
1/2" ID has 3/4" OD
Look at 5/16" ID has 1/2" OD

What is the OD of the chase

 

[Comodave]

It could be that the fuel hose has delaminated inside and partially plugging the hose. Even new hose could be bad. Run a temporary hose outside the chase and see what happens with it. Have you tried a different gauge?

 

[Unclematt]

I called Racors tech guy today. He claims it is the length of the run (25') using 3/8 ID that creates the vacuum. He says I must up the size to eliminate the problem. I will keep you posted on what I do and how it worked.

 

[Larry M]

Here’s my experience with high vacuum. I re-plumbed a fuel system for a friend to fix some air leaks. When we got done we had high vacuum. It took us a while but we found I had nocked loose a lot of crap that was restricting the flow at the ball check valve. That may not be the problem but I thought I’d throw it out there.

 

[BeeJay]

The common method to join a rigid tube is with a fitting that doesn’t restrict the diameter. On the other hand a hose typically requires a barb style fitting that further reduces the diameter and hence the area available for flow. As already mentioned, to flow the same amount of fluid requires either higher vacuum (if you are pulling the fluid) or more pressure if you are pushing. In your application I think that singularity looking at reduced hose diameter is not taking into account the overall picture. In saying that I completely understand the reasoning for trying a smaller diameter hose, don’t you just hate trying to route stuff on a boat whilst hanging upside down in a confined space using only Braille - then intently listening to see if you can guess where that important what not you just dropped ended up.

 

[BeeJay]

Maybe more accurate to say “resulting in higher vacuum” in place of “requiring higher vacuum”

 

[RT Firefly]

Greetings,
Mr. U. Re: your post #26. . ?The Racor guy has probably hit upon the source of your high vacuum.


During my career I built scores of glass vacuum systems and I've observed that it isn't only the size of the constriction it's also the length.