Fuel pressure leak down questions

[dburns]

Upon reaching the 1/4 mark in the fuel tank, the fuel pressure gauge would shake and gyrate between 0-4 psi until it ranged down to 0 and after a while the engine would die of fuel starvation. Until that point, the gauge would read 3-3.8 psi and was generally steady. Adding gas to the tank alleviated the symptoms. The bottom of the tank is roughly parallel to the top of the block. The fuel pump is mounted on the bulkhead with approximately 15 inches of head to the bottom of the pickup.

Although I had a theory or two, I decided to follow a diagnostic process based on what I've gleaned from this forum rather than getting my exercise by jumping to conclusions. Following are the steps and findings:

• Visually checked vent and pressurized with fuel fill cap removed--no obstruction
• Dipped tank with Gasoila Water Finding Paste on dipstick--gas only
• Drained Fram primary filter/water separator--gas only
• Removed primary filter element and inspected--appeared to be fine, but replaced primary filter and polishing filter with no joy
• Removed 10 year old Facet pump with 800+ hours and disassembled--check valve appeared fine, filter clean, cap gasket dry rotted, replaced with new Facet pump and no joy
• Removed, cleaned carb and checked float and needle valve, reinstalled with new gaskets on bowl and flange--no joy
• Checked fuel line, fittings and installed new clamps starting at the carb--found tiny amount of fuel residue on shut off valve at tank. Replaced with new valve. Joy!

Currently the engine runs smoothly with less than 1/4 tank of fuel, fuel pressure steady at 3.4-3.8 psi. Upon shut down, fuel pressure drops to 1.2 psi in about a minute. After two hours the fuel pressure is .8 or so, and after four hours it has leaked down to 0 psi.

While I believe the fuel pump was getting weak, I also believe an air leak at the shut off valve caused the pump to overwork, introduced cavitation, and was most problematic when the fuel was lower in the tank because the pressure assist from the effect of gravity on a fuller tank is diminished.

So much for background. Here're the questions: Is the leak down I'm experiencing typical? If not, what could I have missed?

Thanks!

[ndutton]

I think you have a crack or pinhole in the in-tank fuel pickup tube right at the 1/4 tank level.

edit:
Sorry, it seems we didn't actually answer your question. I would not expect the carburetor float valve and fuel pump check valve to hold pressure very long so as for whether or not your fuel pressure leak down is normal, I'd say it is. This is offered without any real testing, only opinion. I still think the pickup tube needs checking.

[edwardc]

, what could I have missed?

Thanks!

A pinhole near the bottom of the pickup tube in the fuel tank could produce these symptoms. As long as it's submerged, everything works fine. Pull and check the pickup tube.

Edit: Neil types faster than I do!

[ndutton]

Edit: Neil types faster than I do!

Wow, sad. I type with two fingers. It shows we are both thinking the same thing and that it's not mentioned in Del's list elevates the confidence that this is it.

[thatch]

This scenerio reminds me of something that happened to me many years ago involving a rather large rented generator. With about 1/4 left on the fuel gauge it died. Adding fuel solved the problem which prompted the question to the owner, "what's going on here?". His reply was that he had shortened the pick-up tube about 2" to get it away from the rust in the bottom of the "unserviceable" tank. So, my question for you is, is your pick-up removable so that you can check on Ed's and Neil's diagnosis ?
Tom

[dburns]

A pinhole leak in the pickup sounds logical.

The pickup tube is mounted in a plate that also contains the fill tube. The tank is located directly beneath the cockpit floor and will have to be removed to pull the tube for inspection. It's a 25 gal cylindrical monel tank. When I first got the boat 12 years ago I pulled the tank for cleaning and pressure testing. Pulling it was a couple of hours and reinstalling was an all day job.

Is there a way to confirm the presence of a pinhole leak in the pickup tube before removing the tank?

[ndutton]

Your boat, your choice but before spending hours on tank removal I'd consider an access plate in the cockpit sole. They come in a variety of sizes and colors, even stainless. I can't think of a way to confirm a tube leak other than visual inspection.

[joe_db]

I think pinhole leak in fill tube + leak in valve = too much air for the pump.
As for the leak-down, this is exactly why I shut my pump off and let the engine run until it quits when I am done for the day. When I know I will need the engine again, I just let it run until the fuel pressure drops to zero, which is only about 10 seconds or so.

[edwardc]

Is there a way to confirm the presence of a pinhole leak in the pickup tube before removing the tank?

I don't know if this would work as I've never tried it, but it might be worth a shot.

• Run the tank down until it produces symptoms.
• Temporarily insert a piece of clear fuel hose ahead of the pump and as close to the tank as possible.
• Run the pump and observe the clear hose looking for air bubbles

If there's enough air coming through for the pump to lose pressure, it should be enough to see.

A quick Google for "clear fuel hose" turned up this

https://www.amazon.com/Motion-Pro-12-0003-Clear-Fuel/dp/B000UKM4GI

[dburns]

Great suggestion! I'll try about anything before resorting to cutting an access port or removing the tank. I can easily attach clear hose to the pickup fitting on the tank and run it to the primary filter. Observing fuel flow at that point will be easy as well. I've now got about 18 gal of gas to burn before I can try. At my usual rate that's 25+ hours of run time.

After July 4th the plan is to sail outside from Topsail to Cape Lookout and then on to Beaufort, Oriental and then Ocracoke. Then on to Little Washington before heading back to Topsail. There will be plenty of motoring along the way, and with the right stuff on board, once I hit the 1/4 tank mark I'll give it a go.

[JOHN COOKSON]

Try This Before You Cut

When the engine seems to run out of fuel blow on a short piece of hose back into the tank with your mouth. If you don't hear bubbles in the tank something fishy is going on in the tank.

TRUE GRIT

[joe_db]

Discovered an oddity today - my fuel pressure comes back from 0 after sitting a day. I don't know if this is new or not because I only recently have a fuel pressure gauge I can see from the helm, but I ran the fuel out and the engine quit one day and the next day it was up to 0.5 PSI The engine actually started and ran for a couple of minutes with the fuel pump off, so it was not a gauge artifact. I am not sure if this is heat causing gas to expand or what, but I might turn the valve off next time and see if that changes things.

[dburns]

Diagnosis/resolution

Man have I learned a lot.

First the diagnosis: air leaks. The shutoff valve that was leaking air was removed, but the problem persisted. As I've experienced too many times, there is seldom a single failure resulting in a problem on my engine. I found the pickup tube fitting at the top of the tank was a little loose. The fitting couldn't be turned 360 deg due to interference from the fill hose, so it was loosened as much as possible, Permatex Ultra Black sealant applied and it was tightened as much as possible. Then a coat of sealant applied around the base. That seemed to do the trick with the near full tank but testing was needed.

I installed clear fuel line at the tank and at the Facet pump inlet. With the fuel delivery system intact, running the engine resulted in no bubbles at the tank end of the fuel line. However, the flow was negligible and at times appeared non-existent unless the line was slightly elevated. At the pump end, fuel seldom completely filled the 12" clear line, but a slight flow could be detected. The line on the tank end sagged a bit and never completely filled--there was always what appeared to be an air bubble in it, but obviously there was some fuel making its way to the pump. Fuel pressure taken just before the carb stayed around 3 psi.

Side note: My pump has always run hot, around 125 deg in 90 deg air temp. I re-read the post here re: hot pumps by Roadnsky. Mine always quickly heated and until recently ran with no problem.

To get the fuel level down to the point where I originally experienced the recent problem, around 1/4 tank, the line was removed from the carb and placed in a 5 gal jug. The pump was jumped to avoid putting current through the coil and immediately the fuel line completely filled, no bubbles. The pump was no longer hot, it was quite cool, about the temp of the gas in the tank. After emptying the tank to less than 1/4 full, still no bubbles. Blowing in the line resulted in bubbles in the tank--thankfully no tank removal needed to examine the pickup tube. Line reconnected to carb, engine started. No bubbles, but the fuel flow lessened to the point observed before. And the pump heated again. Interestingly enough, an occasional bubble would come from the pump back up the line at idle. Fuel pressure still at 3 psi.

Just for fun, I reinstalled the faulty valve. Bubbles galore throughout the line at full flow. The line was reconnected to the carb and there were NO bubbles, and the previously observed negligible flow appeared. After just a few minutes at idle, the fuel pressure dropped to zero and the engine stopped. Faulty valve removed, full flow, no bubbles. Line connected to carb, engine runs great.

Putting the clear fuel line on both ends was most instructive. It's too bad clear fuel line isn't made such that it is USCG approved. The short sections opened a window to the operation of the fuel delivery system that was extremely helpful.

Here's what I learned from the experience/confirmed from reading posts here:

• The pump only pulls fuel from the tank as needed/allowed, therefore the line doesn't stay completely full, unless it is pulling at or near capacity
• Flow impacts Facet pump temperature
• Unless at full flow, bubbles from air leaks may not appear since the demand from the carb is so little
• This forum is awesome.

Thanks folks--Happy July 4th.

[roadnsky]

Side note: My pump has always run hot, around 125 deg in 90 deg air temp. I re-read the post here re: hot pumps by Roadnsky. Mine always quickly heated and until recently ran with no problem.

FWIW... to this day my pump still runs "hot" in warm/hot weather.
Today in 101° air temp the Facet temp was 120°
Note that it's been years now and the pump is still running strong.

[edwardc]

Great troubleshooting Del. Nice and methodical, changing one thing at a time.

Glad you didn't have to go through the pain of removing the tank!