Air in Fuel System Puzzle

[Dockhead]

I'm on the last leg of my annual UK to Baltic and back cruise, and Murphy has really taken a chunk out of my behind.

In the middle of the North Sea, the bearings failed on my alternator. My crew was nervous, but I assured them confidently -- we have a heavy duty diesel generator on board which can supply all our power. The alternator and diesel generator back each other up. The diesel generator is extremely reliable; nothing to worry about."

Well, what should happen but -- you guessed it -- the diesel generator suddenly stops. So now both systems are out and I don't have any way to get power into my batteries.

I futzed around with the generator, concentrating on the fuel system, tightened everything up, redid the leads to the electric fuel pump, checked for leaks and loose connections, and finally got it running again by bypassing the primary filter and connecting it to the main engine's fuel supply hose. I decided the primary filter must be clogged. I was supposed to replace it before leaving, but received the wrong part, so the old filter was overdue for a change. I adapted the wrong filter, hooked it up, ran the generator, then cleaned up and got ready to leave.

And it died again.

I noticed that there was air in the secondary filter (which has a clear bowl). How could air get in there? It is between the lift pump and injection pump and should be supplied with pressurized fuel. If there is no suction there, how in the world can there be air??

I bled it, fuel came out the first (of three) bleed screws, the secondary filter bowl filled up, and the generator ran again.

Again I got cleaned up and ready to leave and again -- the generator stopped.

Again there was a lot of air in the filter bowl. But this time I couldn't bleed it -- air doesn't come out. Strangely I can get some fuel out of the second and third bleed screws, but not the first one.


What the blazes is going on? I simply can't understand it. The air must come from somewhere, but where? Maybe the fuel pump is intermittent? I replaced it last year. If the lift pump stops, then perhaps the injection pump will draw air from somewhere.

From where I don't know -- maybe there is a lot of residual air in different places in the system, but it doesn't matter when the lift pump is running. There are zero fuel leaks between the lift pump and injection pump, so I can't imagine that there is an air leak there -- anyway that whole section should be under pressure.

My next shot will be to hot wire the lift pump, but I would sure be grateful if someone has experienced something like this and has any kind of insight.


The generator is a Kohler 6.5 EFOZ with a three-cylinder Yanmar.

[Kenomac]

I had the same issue last season. Does the same fuel line or fuel filter (as in Racor), "T" and service both the generator and engine? If so, your engine fuel lines are sucking the fuel out of the generator's fuel system.

Don't you have two alternators on your Yanmar? One to charge the batteries and a smaller one to charge the engine starter batteries?

Another strong possibility is old and cracked fuel lines allowing air to be sucked in. We replaced all the engine and generator fuel lines when this happened last season. How old are your fuel lines?

BTW: I just reread your post, you can't hook up both your engine and the generator to the same fuel supply, the engine will suck all the fuel out of the generators system and vice versa. Each engine needs to have it's own fuel supply back to the fuel tank and independent filter system.

You might wanna consider adding some solar to the mix next season.

[Wotname]

Originally Posted by Dockhead

.........
My next shot will be to hot wire the lift pump, but I would sure be grateful if someone has experienced something like this and has any kind of insight.


The generator is a Kohler 6.5 EFOZ with a three-cylinder Yanmar.

Can you jury rig a gravity feed either into the lift pump or better straight into the IP???

[Kenomac]

BTW: I just reread your post, you can't hook up both your engine and the generator to the same fuel supply, the engine will suck all the fuel out of the generator system and vice versa. Each engine needs to have it's own fuel supply back to the fuel tank and an independent filter system.

Another possibility is the fuel sensor safety switch on the generator.

[transmitterdan]

Sometimes the dip tube in the fuel tank gets a pinhole well above the bottom. When the tank fuel supply reaches that level then air can get into the fuel supply. This wont affect a 4JH3 style main engine too much as they are self bleeding. But i think a 3 cylinder small block may not be so forgiving.

If you have two fuel tanks try switching to the other one, if the other tank is low on fuel consider transferring fuel from the one you are using to the empty one.

[Blue Stocking]

Not directly related to your fuel problem, but at the origin of your problem, i.e. failure of the alternator bearing, and it's knock-on effect for continuing the trip.
Blue has an 80 amp alt. for start batt. on the belt for water pump">raw water pump, driven by crankshaft. If this bearing fails I lose cooling system.
135 amp is driven on it's own belt off separate crank pulley.
When custom building the saddle-type mount for the 135A, on a W46 Westerbeke, I built that unit so that either alt. could quickly be swapped to the other's place. Have used this feature once.
BTW, can also dump ocean into bilge, use bilge pump thru dedicated filter to supply raw water. Thankfully, tested at dock, but never needed in haste.

Agree with the comments re: 2 engines off 1 supply causing starving.

[Iaangus]

Are you sure the air bubbles you see in the clear bowl weren't there before you started having trouble? I had a similar issue with a diesel engine (it seemed starved for fuel) and on close examination there were air bubbles in the fuel water separator.
I tightened, retightened and tightened again every fitting between the tank and strainer and never could get rid of the bubbles. I eventually found the problem to be a gummed up valve on the injection pump probably due to water in fuel. My point is the bubbles may not indicate a problem.
are you sure your tank is vented? Is there suction if you remove the fuel tank cap while engine is running for a while? Also try blowing air back in to the tank from lift pump or strainer. Good luck

[Snore]

Completely remove and inspect each hose clamp. I have gone bonkers looking for a air leak only to find the crimp that holds the clamp screw had failed. The resulting uneven clamping allowed air in.

[Dockhead]

So I hot wired the fuel pump, and it didn't solve anything.

What is weird is that fuel comes out of bleed screws 2 and 3, but not 1 (the one which allows the filter bowl to fill with fuel).

Now what.

I have already switched the fuel intake over from the generator's primary filter to the main engine's one, so that eliminates the pickup in the tank and the primary filter. The generator ran for a while off the main engine filter, but then when I hooked it up to its own filter it also ran. Then stopped and wouldn't run either way.

I think I've eliminated the fuel pickup and primary filter.

Why would nothing come out of the #1 bleed screw? Doesn't make sense to me at all.

The air in the system I was referring to is not bubbles -- it's air in the filter bowl. I don't understand where it comes from, but maybe it's just sucked through after the hoses have been off.

Next steps as far as I can see:

1. Take apart the secondary filter housing and examine it minutely for clogs or malfunctions.

2. Hot wire the safety relays (or just disconnect temporarily the stop solenoid).

3. Minutely examine the hoses and clamps.


Anything else?

[Kenomac]

Did you try what I suggested? If the engine fuel lines are still hooked up along with the generator, air will be sucked into the generator fuel lines via the engine lines or air will be sucked out of the generator lines.

When was the last time you changed all your fuel lines?

[mikecambrai]

The problem is most likely to be in the suction side somewhere. I had this intermittently with my generator and after many days of looking I found that one of the small jubilees holding on a plastic pipe was not round but elliptical thus allowing a small amount of air into the system.
Therefore I recommend checking every joint prior to the lift pump.
Best of luck chasing this frustrating problem.

[sailcrazy]

FWIW..We had an elusive air leak in our Diesel Genset, and after....a lot....of looking we found 2 things-even though it has been installed and working since 1989! The secondary (eng-mounted) Racor fuel filter had a right-angle fitting on the fuel inlet....and it had loosened just a little. And the big culprit was the vent plug on top of that Racor filter....it had cracked just a little, and that allowed air to leak in during ops, and just a touch of wetness when it was shut down. Little "Nit-Noys" made a big difference!

[sainted]

OK. If I understand this situation right, you're getting air in the secondary fuel filter bowl and when you open bleed screw one (to fill the bowl) nothing comes out.

The reason that the secondary bowl is getting air in it is that the injection pump is sucking diesel out of it but no diesel is coming in.

Look for a clog in the hose that runs from the primary filter to the secondary filter (or in any case the hose that feeds the secondary filter). Hoses can fail internally. The rubber can flake off or swell closed.

We had a similar problem on a gen set on a sport fish. Generator would run for a while, then quit. Turned out the problem was a partially clogged METAL line between the secondary filter and the injection pump.

Diagnostic path is as follows: Pressurize the fuel system with the lift pump. Disconnect every fitting one at a time starting with the output of the lift pump. When you get to the fitting that has no diesel coming out of it, you'll know where the problem is.

[DeepFrz]

Are you positive the electric pump is actually pumping fuel?

[SecondBase]

Another useful set of specifications put forth by the EPA describes the life expectancy of fuel system components as 10 years for boats and 5 years for PWC's. If you're going to disassemble and reassemble maybe replace the hoses and bulbs(if you have them) at the same time.