Fuel restriction and RPM loss.

[Dan]

I have been troubleshooting a loss of WOT RPM’s on both engines under load, using a borescope I followed the supply lines across the top of the tanks to where they exit the aft compartment thru the bulkhead into the ER.
Both supply lines are kinked as they enter the engine room thru the bulkhead, and appear to have flattened out on the bend over the years.
There is no access the area -I’m creating a work plan to replace and reroute them,it won’t be easy…..

Add to the equation in my Frankenstein fuel system it would appear I’m woefully under spec’s (fuel line size – right angle fittings – run length)

My concerns

Delivery
The only reroute option would see the delivery hose rise up 7 – 9 inches above the tank with a run length of 18 to 20 feet with two, maybe three 90 degree elbows in the fittings including the fuel filter and engine fittings.

Return
Completely rerouting the return lines will add a lot of length and complexity to the work, I can see an option using 3/8 — straight run of about six feet — no fuel coolers, one elbow.

Top of tank is approximately 12’ lower than the top of valve covers ,bottom of tank level with the top of the sump.

The Racor’s are mounted below the height of the fuel pump, with two feet hose connecting to the lift pump.

Ive read Tony’s article on fuel line sizes and restriction.
The existing setup is using 3/8 hose and fittings
Proposed change to 5/8 delivery – 3/8 return
Engines are 4BTA’s.

Thought’s?

Topic

[Dan]

Easy or not, from all you have posted ( especially your last post) , would lead me to believe you need to somehow get a FULL 1/2″ ID line (minimum ID) all the way to the bottom of your tank

Tony, to clarify
1/2’ fittings = 27/64 ID
1/2’ hose = 65/128 ID

In your above equation are you allowing for ID loss on the fittings or are you saying 1/2′ minimum all the way?My latest plan if I can get it all in there would see the hose not rising above top of tank using

Tank to Racor
1 x 1/2’ 90 (top of tank) to 1/2’ valve to 1/2’ hose barb
2 x 1/2’ sweeping elbows
1 x straight 7/8 to 1/2 hose barb to Racor
15 to 17 ft of 1/2’ hose

Racor to lift pump
1 x 90 1/2’ elbow hose barb
1 x Straight 1/2 hose barb
2 ft 1/2 hose

[Dan]

Thanks Tony, I will make it happen.
I’m concerned about priming with the new hose run being up so high, the squeeze bulb solution would be perfect however there is just nowhere practical to install it, I’m looking into my options.

Rob,
Pickup and return tubes are accessible through deck-plates, I will pull all to confirm sizing and check for blockages. Florida Marine Tanks list two options for pickup tubes on their website (1/2 and 3/8) both my pickups and returns look to be 1/2’ reduced down to 3/8 for the engine and gen.

With the returns I’m reading conflicting info as to whether they should terminate at the top of the tank or be terminate below the lowest fuel level.Seaboards preference is clear here but why, is it to reduce foaming?

Some say it assists with cooling if terminated at the top and reduce back pressure.

[Rob Schepis]

Can you access the top of the tanks / pickup tubes ?

[Tony Athens]

Easy or not, from all you have posted ( especially your last post) , would lead me to believe you need to somehow get a FULL 1/2″ ID line (minimum ID) all the way to the bottom of your tank and then get that to the input of your first filter or primary.. .. ……………………………… In the mean time, I’d remove the fuel fill lid/cap–Then remove the fuel line from the primary filter input and back blow the line using high pressure air ( like 50-100 PSI) and listen in the tank…. Back blow for about 60 seconds.. See if that helps temporally..

[Dan]

Mark,
I don’t think there is any point, it would be very time consuming and I’m 100% confident both my supply and return lines are bent 90’ at the bulkhead, the tank pickup tube/elbow and valve would be coming out anyway to reorientate it in the right direction, eliminate one elbow and to confirm pickup tube diameter.

I cant get verniers in there to measure the ID diameter of the tank elbow (it looks to be 3/4 possibly 5/8) to a reducing elbow (looks to be 1/2’) to the valve (1/2?) to another fitting that I thought may have been a non return valve and finally the 3/8 hose nipple.
I removed the hose and push a long zip tie up through the multiple fittings up to the first elbow, it was all clear.

Prior to fitting the vac gauges I closed off the valve 1/4 turn to test the reaction, the result was the engine struggled to reach 1500 RPM and hunted,1/2 closed it was close to shutting down.

[Mark Tripi]

The reason I would try at the tank, is to see if you have a restriction there or not. Check either side of the valve. If the numbers are lower there you would know where to look. I am not talking about blowing it out, just seeing if that is where the problem lies.

[Dan]

Tony – Thanks for the input

The gauge is attached directly to the outlet side of the filter with one of these (see pic)

Just so you know, you can do all your checking at the dock in NEUTRAL at the dock ………………Your total fuel flow is the same under load or in neutral with the Bosch P pump/fuel system you have.

I didn’t know that! and it explains why I didn’t see much vacuum increase under load.

1 user thanked author for this post.

Dan Sullivan

[Tony Athens]

Those are very high numbers–Like about 2X where they should be……… Exactly where do you have the gauge installed?

Just so you know, you can do all your checking at the dock in NEUTRAL at the dock ………………Your total fuel flow is the same under load or in neutral with the Bosch P pump/fuel system you have..

I’d use 2400 in neutral for doing the trouble shooting or finding where the excess restriction is.. Line, fittings, pick-up etc or a combo of the above……………………………………2400 In neutral should really be at 3″ hg or less measured at the inlet on the fuel pump. ……………………..Your on-engine filter is under pressure ( after the Pump)

[Dan]

Mark,I considered that! however it’s probably futile and will be difficult to do given access, at this stage if I where to break into the fuel connections I would blow the line out back through the pickup with air and eliminate that as a potential issue.

When I got such high vacuum readings at idle/1000 RPM’s I expected to see the gauges in the red under load/high RPM’s,but there wasn’t a lot of difference.I’m wondering if under high RPM’s the fuel lines are porous or split in the suspect area allowing air into the fuel and distorting the vacuum readings,if my theory is right that would show up as less vacuum on the gauges and delivering a restricted supply mixed with air, the fuel lines are 21 years old, with a sharp bend in them, potentially crushed, potentially undersized, I just cant access them to confirm.

My only option for replacement runs the fuel lines ( about 20 feet) level with the top of the engines and backdown to the Racors located at the front of the engines (racors are below crankshaft height)

Determining the fuel line sizing for that length and height above the engine i’m unsure of, or if its even an option and would appreciate any input on that.

[Mark Tripi]

I wonder Dan, if you could pin it down by placing the vac gauge at the fuel valve before the hose fitting, to see if there is any restriction before getting into the 3/8 line?

[Dan]

Pic

Having issues with pics

[Dan]

Update

I don’t want to write a book here ,but without supplying all known history/data I cant expect any input.

I received and fitted the vacuum gauges (drag-point) to the outlet side of the racors, both engines are performing the same and giving very similar results, upon starting the engines both immediately showed 4 HG at idle (750 RPM’s) and 4.5 HG at 1000 RPM’s (no load)

At 2400 (under load) 5 HG.
At 2600 (under load) 7.5 HG
2600 RPM’s is currently full throttle (under load) both fuel pumps at the stops, fuel solenoids are fully open.
WOT (NO LOAD) 3200
Up to 2600 RPM’s boat is performing faultlessly, effortlessly jumping on to plane, running clean with RPM’s matching the boats historical hull speed.
Boost under load at 2600 is 22.5 PSI
Fuel pressure at on engine filter still unknown.

Aftercoolers,turbo’s, boost hoses, fuel condition, aneroid, fuel filters, hull/running gear condition and air supply are fine, I recently ran two test runs with a digital tach and confirmed readings to dash tach’s.

History
WOT (NO LOAD) 3200
With these props both engines have turned 3030-3050 RPM’s full fuel, water and loaded with people, current WOT (loaded) 2600 RPM’s – lightship (low fuel/water, one person)

My boat was originally gas, hence the 3/8’s fuel lines, the factory installed fuel lines for this boat with 4BTA’s is 5/8’s.
Engines where installed new in 2006,I purchased it with 27 hours runtime, currently at 1730 hours.
Both engine’s fire up instantly cold.

I can’t be certain of the crushed/constricted fuel lines coming through the bulkhead as I’m looking at the backside of them with an extended borescope and can’t get the camera end close to the area,attached is a pic (not my boat) of the other side of what I can see ,this is the factory setup with 5/8 lines (mine are 3/8) on my boat there is a generator in a soundshield with what looks to be about one inch of clearance to the bulkhead, removing the gen for inspection would require open heart surgery on the boat….

I can at this point only assume the lines have deteriorated over time/use and collapsed on the sharp bend or/and blocked pickups, rerouting/replacing the fuel lines is going to be a huge task and I want to cover all bases before I dig into this,the only additional test I can do is run them both on a small tank,I would have to do both at the same time as the engine’s performance are heavily reliant on both engines performing equally in this boat, easier said than done (tight)

I welcome any input in case Iv’e missed something.

[Dan]

Thanks for the response, I am waiting on delivery of gauges and will not be changing anything until I have all the data/info required to guide me on a solution.

Am I on the right path with 5/8 delivery 3/8 return as described above if I have to replace the fuel lines?

[Tony Athens]

Have you put a vacuum gauge just before the engine, or a pressure gauge on the ON-Engine fuel filter?

[Author]

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