Air in fuel – cavitation vs restriction and entrained air

[Dan]

I’m experiencing air in the fuel (a lot) and the loss of performance that goes with it, its clearly visible in the clear tube I have installed pre lift pump.

Prior to stripping everything out of the fuel delivery system to troubleshoot I had 1 HG of restriction after the filters, vac gauge installed on outlet side of filter.

Testing to date has has seen me strip everything in the fuel delivery/return on both engines to.

Supply
5/8 SS pickup from tank, 5/8 elbow top of tank, 15 feet of 1/2 hose to 1/2 joiner, 3 feet of clear hose to lift pump.There is no 90’s ,no filters, no water seps, no shutoff valves – its a straight shot from top of tank to the lift pump, hose/fittings and double clamps all brand new. Suction tube is a weld in fitting (no strainer on pickup confirmed ) and was blown out with air.

Return
3/8 hose from engine to 5/8 elbow top of tank, fuel sprays out from the top of the tank.THERE IS NO DIP TUBE

This video perfectly represents what I am seeing in the clear hose and perfectly duplicates my vessel fuel system install/environment and end result. (apart from the powered lift pump)

There’s many boats out there without lift pumps/air separating systems performing perfectly.

Question is – how do I overcome this problem?

Link – forum wont let me post direct links

add this https
to this ://www.youtube.com/watch?v=hWeQUDyDEnY

Topic

[Clark Leighs]

The clear vinyl tubing should be a heavy wall to prevent a collapse.
However with the aeration I would not expect that to be a problem.
No guarantee of course.

[Dan]

Thanks Clark,
The lack of a return dip tube is likely the main cause of the problem given the high return rates of the P7100 fuel pump and the small capacity/shallow tank size, the results at the end of the test indicate that, the fuel had 55 minutes to settle and purge air.

Installing a return dip tube is easier said than done, supply dip tube is an issue as it cant be removed

To test the theory of supply dip tube failure and/or return fuel aeration in one go I’m going to

Remove the return and dump the fuel into the other tank on one engine, remove the level sensor fittings on both tanks and get a visual of whats going on in there.

and

Put clear hose on the supply side at tank and repeat test.

Something I thought of after the test was the clear hose attached to the lift pump may have collapsed under vacuum under load, it didn’t at the dock test, will confirm under load.

“What do you have for a filter?”

Nothing,
After extensive testing with the Racor’s and fuel valve in place I finally gave up and bypassed everything and ran a new fuel line direct from tank to lift pump. no 90’ fittings just straight fuel hose—- Tank -15 foot of 1/2’ fuel line to 1/2 joiner’ to 3 feet of clear hose to lift pump.

[Clark Leighs]

Your suggestion of using the level sensor hole to drop a hose in may be needed. Make a cover plate using the same pattern as the sensor plate.
An elbow, in a tapped hole could create a solid connection for a dip hose. If the problems appears to be resolved then I would suspect the diptube.
If you are sure you have exhausted most other air sources it may be worthwhile.

It is not common for a diptube to develop a pin hole but nor is it unheard of.

What do you have for a filter? Some Racors may have a check valve in them that can be installed incorrectly which can cause air entry problems. THe actual valve has hard to read installation direction embossing which is often missed.

I’m sort of wondering if the tank top, no dip tube, return fuel is foaming the fuel.
Diesel is bad for foaming. If the return line is close to the p/u outlet maybe the foam does not have enough time to dissipate.

Over some running time too much fuel is foamed making its way into the supply. Maybe made worse by rough seas.

If you are positive there are no air leaks from poor hoses, connections, filter sealing, and the air gets worse as the tank level drops and so on then maybe experiment with a dip tube.

One

[Dan]

Clark,
Thanks for your response
I cant confirm the pickup tube is ok, I put a camera down in there through the sender unit hole on both tanks, they looked good, it’s unlikely but plausible that both of them are compromised, they are weld in fittings so I cant remove them to get a visual or pressure test. I pulled both elbows off the top of the tanks, both where sealed well,I cleaned them up and reinstalled them with sealant.

The only test for the pickups I can think of is to remove the sender units and drop the hose into the fuel, it will be difficult to duplicate normal operating conditions that way, the air in fuel issue has plagued this boat for years,It’s always performed far better with full fuel/calm weather, the engines can only ever hit 3000 in that scenario.

I may have multiple problems
Return fuel spraying out from the top of the tank (aeration and foam)
Fuel height vs lift pump height
Tank capacity vs returned fuel, if my numbers are correct aerated fuel is recycling through the tank multiple times an hour.
Agitation in the tanks from tank slosh.

All of the above or combined could be the issue, testing that theory from a cold start at the dock

There is no visible air at the lift pump up to 2000 RPM

Testing from idle to 3200 in neutral after a 5 minute run @2800 (WOT)

Visible air in systems at idle 30 seconds after run
Lots of air visible from approx. 1500 and above
Less air below 1500 after a short time
Then after a short period of time less air visible at idle.

Idled home through harbor 25 minutes/shutdown 30 minutes

Very small amounts of air at idle
Very small amounts of air visible up to 2000 RPM

[Clark Leighs]

Any chance the pick up tube has a perforation in it? Pressure tested it? I don’t mean the “blown out”.

Are you positive the elbow at the top of the dip tube has a good seal of the threads.

[Author]

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