fuel starvation challenge

[damienk]

alright. If anyone can solve this one, I will be immensely grateful. My A4 dies shortly after starting (suddenly, like if the ignition was turned off) and it is fuel related. To make it simple:

- if I disconnect the fuel line from the tank and hold it in the air and keep it filled with gas, it runs perfect and forever.
- if I connect it to the tank, or if I LOWER the fuel line , it dies (it dies within seconds when lowered at the level of the fuel tank).

I would say fuel pump of course, but voila: I tried it with 2 different new fuel pumps, no difference. I tried everythingween: The fuel pump is new. New fuel filter. Rebuilt carb. New fuel lines. Even a new freaking fuel tank (when I realized it ran when disconnected but not when connected, I assumed it was the fuel tank, until I noticed - today - that how high I hold the fuel line matters).

I don't know what else to do. I am desperate. There is no leak in the fuel lines. No fuel leak anywhere. The pump is new, and works (if I disconnect between the carb and the pump and start the engine it fills a cup of gas in no time). The pump is 1.5 to 4 PSI, which I understand is what is required. And it is not like I relocated everything recently, everything is where it used to be.
I have no idea what to do next. I am pissed at all the parts I bought for no reason, and I am clueless. Any suggestion, apart from hiring someone to hold the fuel line up in the air and fill it as it goes?

[ndutton]

I chose to reply in this thread because it has more information than the other in the troubleshooting section.

Sounds to me like you have a fuel system leak but not the kind that drips fuel. I bet you're sucking air through a clamp, fitting or seal somewhere (we've heard of fuel filter seals that cause this before). The entire fuel system is under negative pressure up to the fuel pump and positive pressure thereafter. A leak ahead of the pump that allows air into the system will stop the fuel flow.

Raising the fuel line and pouring in fuel changes the gradient ahead of the pump from negative to positive pressure and the problem temporarily disappears.

The only other thing that comes to mind is a stopped up tank vent. As a test try running the engine with the fuel fill cap removed. That will provide good venting for sure. If the engine dies again we can forget the vent as a possibility.

[BunnyPlanet169]

Two questions -

Are you reading a fuel pressure gage, or is your pump (s) rated at 1.5 - 4 psi?

Pump test done with normal fuel line position, normal tank, and you only took the hose off the carb inlet barb, no other changes?

PS - internal hose obstruction? There was a weird post on here a while back where someone found a piece of plastic inside their fuel line. Troubleshooting nightmare 'cause it's so random. It could be that moving the fuel line is the issue, not the coincident change in elevation.

[damienk]

Thanks for the suggestions.

BunnyPlanet: the pump is rated 1.5 to 4 PSI. I don't own a fuel pressure gauge.
Therefore, I haven't done any pump test. With everything in normal position, the pump spits a healthy cup of gas when starting the engine (hose disconnected at carb) . With everything connected in normal position, it doesn't work. With hose disconnected at tank level in normal position, it doesn't work. With hose disconnected at tank level and raised above normal position (above tank level by 2 inches at least), it works flawlessly.

Also, no internal hose obstruction. (hoses are new, tested, and I also checked each hose separately multiple times, by blowing, sucking, and testing the flow of gas through each of them.

Ndutton: fuel tank vent is fine, I tested it, and also removed it (cap open) to rule it out. A leak was my next guess too, but...where? I checked all clamps (all double clamped), all hoses, the fuel filter housing is the same as before and I checked it multiple times. No bubbles anywhere, no fuel leak...

I was thinking that maybe the pump isn't powerful enough, and that my carb is finicky with the PSI? Could it be low pressure? What does 1.5 to 4 PSI (fuel pump range) mean? That the pump delivers 1.5 when it is having a bad day, 4 on good ones? What makes it be at one end or the other of this range?

[ndutton]

Take care to avoid assuming everything is good when something clearly isn't.

How about a test with an outboard fuel tank and single hose from it to the fuel pump, tank positioned at the same level as the main tank (eliminates all fuel system components up to the pump)?

Engine runs fine - add components back into the system one at a time, testing after each addition.

Engine quits as before - get after the pump and carb.

Hey wait. Do you have an oil pressure safety switch for the fuel pump power? How many terminals on the switch? Have you bypassed it by placing a jumper wire across the terminals?

[joe_db]

I went absolutely nuts tracking down a few fuel issues.
Lesson 1 is YOU NEED GAUGES!
I ended up with a fuel pressure gauge and two vacuum gauges - one on the input side of my filter and one on the output side.
My issue turned out to be debris in the tank that would clog the pickup tube, cause fuel starvation, and fall back off when the suction dissipated. Drove me nuts until I figured that out
Lesson 2 came awhile later - liquid filled gauges are very nice because the needles don't jump around. What I didn't know was they change readings with temperature if you just use them as received. You need to mount them with the little rubber stoppers facing up and cut the ends of the stoppers so the gauge is no longer air tight. THEN they read correctly after they get hot.

As for your issue, I can't say for sure yet but I believe an air leak has been mentioned. That might be it. I would get a gauge or two and get some measurements. Fuel pumps are sold with pressure ranges, not a fixed number. The output will vary with the load on the pump, voltage, and maybe temperature. 1.5 to 4 PSI is the correct range for an A4. My pump sits rock-steady at about 3.8 - 3.9 PSI.

[Marian Claire]

What is getting me is the "dies suddenly" and "dies within seconds" part. Post #1. There should be fuel in the line, filter, pump, carb bowl.
Even if you turn off a petcock the engine still runs for awhile right?
What is the relative elevation of the filter to the fuel level in the tank? Just thinking back to my filter at the highest point that slowly filled with air, due to a leak, and led to shutdown.
Reaching but. Could misaligned floats play into this. Floating to high/sticking and stopping the flow. Only allowing fuel to flow when the "extra" head, negative to positive, pressure is added by raising the line as mentioned in post 2. ??????
Dan S/V Marian Claire

[sastanley]

I agree that a pressure gauge would help. I have a cheapy from Amazon made by Mr. Gasket (#1561)..it was $13. I installed mine just before the carb..The engine will run even around only 1 PSI of fuel pressure, but a gauge at the carb will confirm or eliminate a fuel delivery problem forever. My mechanical pump delivers right around 2.75-3 PSI.

Neil, good thought with the OPSS!

[hanleyclifford]

If you have a fuel filter/separator make sure you have not reversed the input and output connections. When spinning the cartridge on I like to apply a thin coat of machine oil to the gasket to ensure a tight seal.

[gregsails]

Is it possible to pressure test the fuel system from say the pump inlet and back?

[67c&ccorv]

alright.

I would say fuel pump of course, but voila: I tried it with 2 different new fuel pumps, no difference. I tried everythingween: The fuel pump is new. New fuel filter. Rebuilt carb. New fuel lines. Even a new freaking fuel tank (when I realized it ran when disconnected but not when connected, I assumed it was the fuel tank, until I noticed - today - that how high I hold the fuel line matters).

What kind of tank?

[ndutton]

Sure, you can test but that's what the gauges the guys are talking about will do on the system in situ. Ahead of the pump = vacuum gauge, after the pump = pressure gauge.

[Rbyham]

I look forward to hearing what resolves this. Nothing comes to ind here except that you will be happy once it is solved because you now have a completely new fuel system. Having bought a 48 year old boat with original A4, one thing I did right was pull all fuel components out and replace including tank. I have enjoyed 18 months now of zero issues in a boat and engine of that age and which had been in storage 8 years when I purchased. The day is coming soon when you will be glad you handled it this way...

[BunnyPlanet169]

Thanks for the suggestions.

BunnyPlanet: the pump is rated 1.5 to 4 PSI. I don't own a fuel pressure gauge.
Therefore, I haven't done any pump test. With everything in normal position, the pump spits a healthy cup of gas when starting the engine (hose disconnected at carb) . With everything connected in normal position, it doesn't work.

Damien - can you elaborate on this please? Everything normal, the pump spits gas.... For how long? Like, can you fill half a liter? Spits, or steady pulsing stream? Are you sure the pump is plumbed correctly? It has a distinct inlet and exit. Most of these small pumps are rated approx 30 gallons per hour, so you should get a quart milk jug every 30 seconds.

If you have a oil pressure safety switch (USCG required with an electrical pump) you can bypass it temporarily and then just turn on the ignition for 15 seconds to test.

Last weird thought - does moving the hose around for testing brush against other stuff? Like a wiring bundle, or the wiring to the OPSS? I've had situations where one plausible solution was in fact the physical interaction with the actual, unrelated, problem.

[JOHN COOKSON]

The fuel pump is weak or not working. When it is gravity fed everything is fine. When the fuel pump has to pump the fuel it is to weak to do so.
How much electricity are you getting to the fuel pump?
Try running a jumper wire from the boats battery to the fuel pump, bypassing the boats wiring & OPSS, and see what happens.

TRUE GRIT

This assumes the tank, filters, and fuel lines are not the problem.