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Discussion Starter · #1 ·
I have a 1985 Catalina 30 with the MX25 diesel. The tank does not hold the full amount of fuel it is supposed to. The tank is stamped that is holds 18 gallons. When i measured it, I calculated its volume at 17.5 gallons.I ran out of fuel about 10 miles out of the harbor. I sailed in , in deteriorating weather, but I would have wanted the engine.

The fuel gauge read full when the tank was full and empty when I ran out. When I filled the tank, it topped out, absolutey full at 12 gallons.

What is causing this. Could I have 6 gallons of sludge in the tank? The fuel looks clean after the the filters and the engine runs good.
 

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3 things you need to know:
- Is the tank square? If not have you compensated for the shape?
- The fuel pick up tube - how far off the bottom is it?
- The gauge sender may not be hitting the bottom either.
 

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I believe the fuel pick-up is located above the bottom which accounts for some (maybe 10%?). While I've never run completely out, I've run 2 Catalina's as far as I dared with the gauge on E and never been close to the refill being near tank capacity.
 

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I have a 1985 Catalina 30 with the MX25 diesel. The tank does not hold the full amount of fuel it is supposed to. The tank is stamped that is holds 18 gallons. When i measured it, I calculated its volume at 17.5 gallons.I ran out of fuel about 10 miles out of the harbor. I sailed in , in deteriorating weather, but I would have wanted the engine.

The fuel gauge read full when the tank was full and empty when I ran out. When I filled the tank, it topped out, absolutey full at 12 gallons.

What is causing this. Could I have 6 gallons of sludge in the tank? The fuel looks clean after the the filters and the engine runs good.
"Topped Out" according to the fuel gage or by squirting over-flow from the breather tube?
 

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I have a 1985 Catalina 30 with the MX25 diesel. The tank does not hold the full amount of fuel it is supposed to. The tank is stamped that is holds 18 gallons. When i measured it, I calculated its volume at 17.5 gallons.I ran out of fuel about 10 miles out of the harbor. I sailed in , in deteriorating weather, but I would have wanted the engine.

The fuel gauge read full when the tank was full and empty when I ran out. When I filled the tank, it topped out, absolutey full at 12 gallons.

What is causing this. Could I have 6 gallons of sludge in the tank? The fuel looks clean after the the filters and the engine runs good.

#1
Your pick up is NEVER on the dead bottom of the tank and is usually 1" or more off the bottom. This means you will never get the full rated capacity.

#2 Sailboats heel and slosh back and forth in a sea thus exposing the pick up to air.. As tanks get low it is very easy to suck air. You may not have been out of fuel but may have very likely sucked air especially if motor sailing..

#3 My rule of thumb is to never let my tank get below 1/3 full to prevent sucking air.

#4 You could also have a low spot/trap in your fuel vent thus causing you to "burp" full early....

#5 Don't EVER count on a boats fuel gauge to be accurate, unless you have specifically calibrated it.

#6 To calibrate:
A) Know actual tank volume measure and calculate.

B) Drain tank 100% by removing sender and sucking it 100% dry.

C) Fill in 25% known increments and mark gauge glass

D) Use a Sharpie, paint marker, tape etc. at 25% increments E, 1/4, 1/2, 3/4, F....

E) Now you KNOW the gauge is accurate, but this will still NOT be accurate when heeled....
 

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Discussion Starter · #6 ·
3 things you need to know:
- Is the tank square? If not have you compensated for the shape?
The tank is not square. I did compensate for the shape.
- The fuel pick up tube - how far off the bottom is it?
The pick up tube is rubber and does hit the tank bottom. It curves so it seem to lie flat on the bottom or very close.
- The gauge sender may not be hitting the bottom either.
The tank shape is such that the bottom of pick up tube is lower down than the bottom of the sender.

#2 Sailboats heel and slosh back and forth in a sea thus exposing the pick up to air.. As tanks get low it is very easy to suck air. You may not have been out of fuel but may have very likely sucked air especially if motor sailing..

The tank was empty. When we returned to the dock, I tried to start the engine and got nowhere. After filling, it started up fine.

When we filled it, We did get fuel flowing out of the vent and the gauge showed full. I overfilled it deliberately.
 

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When I built a new fuel tank, I calibrated the gage by filling the tank a gallon at a time and making up a table of gage reading and fuel height height above the bottom measured with a stick. Also measured the pick up tube length compared to tank depth. That way if the gage doesn't appear right you can measure reliably with a dip stick and you know at what level the tube will start seeing fuel.
 

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Discussion Starter · #8 ·
I am fairly confident about my tank capacity being 18 gallons. but holding only 12 usable gallons. So my real question is, do you think I could have 6 gallons of algae, shmootz, dirt, gunk built up inside? And if so, how does one go about cleaning out said gunk?
 

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Glass, loosing six gallons to sludge would be pretty extreme, but frankly I wouldn't consider it impossible if the tank has never been cleaned. But it would be the worst case I have heard of. However since you didn't have a filter problem, motoring in rough weather, I highly doubt this is a sludge problem, my guess is you pick up hose is just shorter than is should be or has a hole somewhere above the bottom... Say at around the 12 gallon mark.
 

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Discussion Starter · #10 ·
Thanks for all your suggestions. I'll have to put some more investigation into it. I'll let you know if I learn anything. Meanwhile, I'll sail based on a 12 gallon range.
 

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It's unlikely you have 6 gallons of sludge. Your pick up unit sits in the bottom and would have been clogged long before now.

You said you tried to start the engine when you were back at the dock and it wouldn't start. So you filled the tank and it started. Is that all you did, fill the tank and start it?
 

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6 gallons short is quite some void, it just doesn't seem possible it could be sludge or some other sort of build up.

I'll be interested to see what you discover...
 

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Discussion Starter · #14 ·
If you ran out of fuel, bleeding the engine would have been required before it would run again. If you just filled up and restarted the engine something else is going on.
Jim
That's something I wondered about. I did not have to bleed the system. When it ran out of fuel, we shut everything down. Since the gauge was on empty and the engine died, I assumed we were out of fuel. Once back at dock and refilled, the engine started right back up (no bleeding the lines). Does one always have to bleed the line after running out of fuel?

My other though is baffles. Could an in-tank baffle be clogged and closing off a third of my tank? Wish I had a way to see inside the tank.
 

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If you had six "gallons" of crud in the tank, the fuel pickup would be buried under it and you'd never have started.

Reminds me of the first time I got gasoline in Canada and didn't realize that was Imperial Gallons...not American Gallons. Any chance that you, ah, fueled in Canada?

The difference between 12 and 18 means there's something very simple but very wrong. I'd be tempted to pull the tank, clean it out, double-check capacity with water while it was out...Look for signs that maybe Bozo cut down the tank at some time in the past?
 

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Yes, you have to bleed the injectors if you get any air in the fuel lines. If your diesel started at no time did you run out of fuel. Your fuel tank was not empty and there were still six gallons in the tank. I think you can forget about the fuel tank being the problem.

Diesels require three things to operate, fuel, compression and air. Fuel therefore appears to be the problem but not because you ran out. Two possible problems are a dirty fuel filter and the electric fuel pump is not working.

I had a similar problem and the fuel pump was intermittently not working.
 

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I wonder what is the relationship between the level of fuel in your tank and your engine. If the tank is above, or partially above, the level of your fuel filter, when it is full or nearly so, your lift pump would have little to do. However, when the fuel level drops below the level of the fuel filter, if the lift pump isn't functioning, or functioning properly, the vacuum created by your injection pump in the fuel lines will not be great enough to suction fuel into you system and your engine will fail as it would with an out-of-fuel starvation situation although you would not draw air into the lines and, accordingly, not need to bleed the lines when the tank was refilled. Likewise, if your fuel filter is a Racor, the back-flow check valve would prevent the fuel line to the injection pump emptying into the fuel filter bowl, but, absent a functioning lift pump, your engine would still die from fuel starvation.

FWIW...
 
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My issue was such that the engine would run all day until we throttled back approaching dock or mooring. That is when the crap that was in the tank would slosh forward and clog the pick up killing the engine. Then after a rest it would fire up just fine until the next slowdown. Tank removal and cleaning was needed.
 

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I had a similar problem on our previous CS. The engine died during a long no wind transit. The fuel gauge read 1/2 tank. Called seatow.
After refilling noticed a small leak on the o ring at one of the fuel filters. When the tank got below 1/2 the filter sucked air.
Jim
 
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