Air in fuel line after sitting for a few days

[Scuba Too]

I've run into a persistent problem and need some help. If I haven't used the boat for a few days or more air will get into the fuel line. The engine will start just fine but after running for a 30 seconds or so it will stall out. Priming the fuel line with the bulb on the fuel filter housing allows the boat to start and continues to run. I don't have a problem if using the boat over a few days in restarting so it's a slow air leak. I suspect it must be coming from a high point in the fuel system since it's air getting into the fuel line and fuel is not leaking out anywhere. One suspected culprit is the fuel filter/water separator - which is just upstream from the priming bulb. However I've checked the o-rings (t-handle and waist) and they look to be well-seated and in good shape. Other thoughts are the ball valve shut off or the tap into the fuel tank. Has any one else run into this issue? Any suggestions? Thanks!

 

[PanaSeaAh]

Had a similar problem, and found several issues before it was over. Not long after acquiring this boat (R25, 150 HP) found the following:
Fuel line on fuel pump was not tightened correctly. Pretty sure this was a return line banjo bolt, because it did not leak out, but possible air leak in.
Anti-siphon valve on the fuel intake line on top of the fuel tank had a small leak back into tank (not an air leak).
Biggest problem appeared to be that the stem seal nut was not tight on the engine fuel shut-off valve next to the primary filter. Strangely, only leaked air in, no fuel leak out. Probably negative pressure at this point. Would not start if it sat more than two or three days until air was bled out of system. Not sure which of these was the worst, but probably all contributed to the issue.
Replaced the aluminum(?) fuel anti-siphon valve with a brass one, should last longer.
Having told you all the above, I would also note that the engine manual states that the engine needs to be primed if not used for "some period of time", whatever that is. Even after all repaired I may have to prime if boat not used for a week or more, but I do not hear air like before. Different system on yours, but same places to look.

 

[Scuba Too]

Thanks PanaSeaAh (great name BTW)

I appreciate your suggestions. Was the valve stem on the shut off noticeably loose? A mechanic who's familiar with the RT line and Volvos suggested this as well. Mine seems ok but I'll double check it (it's not in a great position to tighten but I'll make more of an effort).

One thing I checked on the boat is that the height of the fuel filter/water separator (FFWS) is definitely at the high point of the fuel circuit. The bottom of the FFWS is about the same level as the top of the fuel tank. Checking the fuel level in this after a few days of absence it seemed that the fuel level was lower than the internal port to the engine, so perhaps it's the FFWS. To test this I've shut off the fuel using the shut off valve with the thought being that the fuel wouldn't run back to the tank. I'll test this over the weekend.

The mechanic also mentioned that there's a fuel return line with a check valve and perhaps that check valve is bad. I've looked at schematics and haven't sorted where this might be yet.

 

[PanaSeaAh]

The fuel shutoff valve stem nut was unthreaded almost all the way, and I could turn it easily with my fingers. On the valve I have, the top nut seats on an o-ring, so it is either all the way down and holding the stem and plug in the valve body, or it is not. I found it because the valve turned very easily compared to its twin on the generator fuel line. It was not really obvious to see. It threads down and stops. Fortunately, good access to that filter and valves. The top half of that filter is above the tank top on mine also. I shut off both fuel valves every time I load the boat, (trailer), but do not do that when in the water. Usually starts right away if within a few days of last run. I do know the fuel will siphon back towards the tank in the fuel feed line if the check valve on top of the tank leaks, so it eventually will find air somewhere. .

 

[Scuba Too]

Update - I've been leaving the fuel shut off when not on the boat as noted below. When I check the fuel filter/water separator it has fuel in it and the engine starts without priming. So it appears to be the anti-siphon valve may be the issue (perhaps in combination with a seal leak in the fuel filter?). I'll try swapping it out and see if that fixes the issue.

 

[PanaSeaAh]

Get a brass/stainless or stainless anti-siphon check, not aluminum. You have probably ruled out the shutoff valve with what you have already tried. Check the old anti-siphon valve to see if has crud in it after removing. If it is aluminum, it is probably just worn. Good news is that you probably have the least expensive of the possible fixes. 🙂

 

[ALICE J]

I had a similar problem on another boat where I would shut down the engine, the next day it would start and run for maybe thirty minutes then shut down with alternating rpm then quit. One time after shutting down the engine I opened the engine hatch, all was very still, but then I heard the sound of bubbles, traced it to the Racor filter, and found the gasket was ill-fitted. I figured out that as the engine cooled, the fuel contracted and sucked air into the filter, finding its way into the injector pump and stalled the engine. Interestingly that air came in, but fuel did not leak out. :roll:

 

[BB marine]

Interestingly that air came in, but fuel did not leak out.

This is a common issue with fuel leaks on the suction side of a pump. Many times a fuel leak in a hose connection or gasket seal will allow air to leak in but fuel can not leak out. The barb fittings are designed to help prevent liquid leaking out. When troubleshooting a air leak in a fuel system in the suction side I would place a clear solvent resistant hose in line on the suction side. I would place this hose at or close to the suction side of the lift pump. Prime and run the engine. Run the engine at different rpm's while testing. Look for air bubbles in the line. If air bubbles are seen then you know you have an issue. The next step is to locate which connection has the problem. Using a process of elimination move upstream of the clear hose and inspect, change fittings or seals at the next location. ( sometimes cutting an inch of hose at the barb fitting and reinstalling with a fresh hose connection is all that is needed or replacing a seal) After making adjustments retest. If air bubbles are still present move to the next connection. Repeat until you get to the tank. I have found many air leaks at barb connections from over tightening hose clamps or using a poorly designed hose clamp.

 

[bkarias]

I had a similar problem on our almost new 31 S. Changing the fuel lines and tightening clamps didn't make a difference. When you shut off the engine, a gurgle was heard near the fuel pump. Vosbury Marine in Annapolis figured it out: the outgoing fuel return check valve (part no. ...928), located at the fuel pump, needed to be replaced (under Volvo warranty)