Please help! Engine stalls at idle - fuel line obstruction??

[jkujawa]

Okay guys - I'm stumped!! I posted this on another forum and got some great info (much thanks to those who already posted) but I want to get this darn thing fixed tomorrow so I need all the help I can get!

Twin engine boat. Starboard engine runs fine at high speed, but will only idle for 5 or 10 minutes before sputtering to a stop. I swap the fuel lines (port to stbd, stbd to port) and the stbd engine runs fine, but then the port won't idle.

I replace the primer bulbs, replace filters and replace starboard fuel line from the fuel tank to the Racor. Whichever engine's racor filter I connect this starboard pickup line to exhibits the stalling. I've swapped the line 5 or 6 times and either engine will run fine when it is connected to the port fuel pickup line from the tank.

The guys on the other forum suggested a fuel pump, which I agree really makes the most sense. But I don't think I was clear that the problem is only on whichever engines is connected to the starboard fuel line.

So, since either engine will run fine when connected to the port pickup line, I assume both engines, fuel filters, primer bulbs, fuel flow senders, and all lines in between are fine. I also assume the port pickup and fuel line to the Racors is fine. Since I have replaced the fuel hose between the stbd fuel pickup and the Racors I have to assume that that is fine. I have inspected the tank and there does not appear to be any debris obstructing the fuel pickup in let in the tank (inspecting through the fuel sender port). I have to assume that there is an obstruction in the fuel pickup in the fuel tank, or the small aluminum shutoff valve/nipple on top of the tank.

I have blown compressed air backwards through the pickup, and even backflushed it with gasoline with a primer bulb. I have tried to feed a small piece of wire into the nipple and through the shutoff valve, but can not get anything past the 90 degree elbow down into the tank. I don't want to try to unscrew any of the fittings because if the small aluminum fitting cracks it will be a huge project to fix (the deck will have to come off to repair it, as right now I am accessing all of this through a small inspection plate in the deck)

I'm stumped! Any suggestions??

[Chaps]

Sounds more like an air leak than an obstruction. When the engine dies how do you get it to restart - pump the primer bulb? If it was an obstruction the problem would affect high speed operation more than low speed. I theorize a small air leak because during high speed the significant amounts of fuel moving through the system would cover up the problem but at low speed the fuel would have an opportunity to drain back to the tank due to the leak providing a vacuum break in the system. It could be a crack in the filter housing or the fuel flow sensor from an overtightened fitting for instance. It's possible that such a leak wouldn't necessarily allow gas to drip out but if it does it would likely do it just at the moment you intentionally shut the motor down after a high speed run.

[jkujawa]

Chaps - 5/29/2006 6:07 AM

Sounds more like an air leak than an obstruction.

Very interesting...
Any suggestions on a way to check for that?

Yes, to answer your question I need to pump the primer bulb to get it to run for any length of time. It will usually start immediately after stalling, but will only run 5-10 seconds if I don't first prime the bulb.

Thanks.
John

[jethro1]

Yep, it could be a small air leak as Chaps mentioned. I was thinking a fuel pump earlier when I did not pick up on the fact that the problem follows the fuel supply system. Maybe it is a vacuum leak that lets air into the system when the engine pump is applying a small amount of vacuum but when you are at power the increase vacuum closes the leaking hole. Who really knows, this is just speculation at this point.

To check for a vacuum leak get a small hand held vacuum pump. Sears and most auto parts stores have them. Good thing to have for many reasons. Close off the fuel system at the outlet from the tank. Remove the fuel line where it connects to the motor and drain it of as much fuel as you can. Use the pump to apply a slight vacuum, say 3 - 5 inches of mercury. Watch the vacuum gauge to see if the vacuum will hold. If so, all is well. If not, you have a vacuum leak that needs to be fixed.

Now, if you don't find an indication of a vacuum leak there still could be a problem with the fuel pickup tube. It might have a crack in it as well. The only way to determine its condition is to remove it for an inspection.

One final quick and dirty check before you do much more. Remove and replace the hose(s) from the Racor to the engine (both sides of the bulb). This is a shot in the dark but fuel hose is not that expensive and it will be an easy and quick thing to do.

[Chaps]

Without taking anything else apart unhook the fuel connector at the engine. Make sure the fuel tank fill cap is tight. Apply a tiny bit of air pressure to the fuel tank vent say by slipping a short hose over the vent and blowing hard into it or if you can get the vent hose off easily blow into it. A regulated source of air, say 2-3 psi if you have it is easier but use your lungs if you don't. Anyway, gas will leak from your problem spot.

[lakensea]

Pull the pick-up out of the fuel tank - you may have a crack near the top.

[Mumblerone]

What brand of motors? Haveyou been burning more oil in one than the other. The greater oil use would also imply and air leak in that line/motor. I also assume your anti-siphone valves are not sticking at idle? They are integral to the right angle fitting at the fuel tank.