Originally Posted by C lectric
Couple things I wondered about.
You tested the tank vents by blowing air. That may not show a problem. If the hoses are kinked shut , under air pressure they would open up and show good. yet not be. If they are collapsing blowing air will not give an answer.. Just be sure as that is exactly what happend to me years ago, they collapsed.
I think in another thread you said you have vacuum guages on the filters now. Telltales types? Or did you think to read them on the monitor? What do/did they read when you started having trouble last time?
Dip tubes too close to the bottom? If as the fuel weight drops maybe the tank bottom is bulging up and partly blocking the diptubes. Try cutting the dip tube ends on about a 45o - 50o angle. Blunt the sharp end. This creates a larger suction surface area resulting in less velocity [I've forgotten the specific term] right at the entry but also if the tank bottom is the problem the cut will prevent blockage.
Or put a 90o fitting on to prevent this if you can get it in.
If this is the case when the engine quits then the tank may drop from the dip tube to quickly for you to see it on your monitor unless you were watching it at the time. The telltale guages would be better for this.
Did I see that you have rigged a fuel pessure guage. That should help at the next occurence to see if pressure drops and filter vacuum remains low.
You've been fighting this for a long time. Great persistence..
I've been fighting this problem since I bought the boat 3.5 years ago.
Could be the vent lines collapsing but examining them makes me think that is not the issue. They look fine and I did blow air out the aux tank vent line disconnecting it from the tank and using my compressor to blow out air. This was about 2 years ago. Also when I blew air into the tanks through the vent holes on the side of the boat the other day, the tanks filled with air and then blew out when I removed the compressor hose. If they collapsed then I would think air would remain in the tank. An easy check would be to remove the tank fill cap next time this happens. I will do that just to make sure venting is not the issue next time this happens. But I would think a clogged vent would cause problems with fuel feed when the tanks are full as well as half full.
I ran the engines off the aux tank the other day at the dock and only just above idle. There is only about 15 gallons in the tank or about 2.5" of fuel from the bottom. The engines ran fine so if the dip tubes are too close to the bottom then perhaps it would have shown. A better test of course is to run at load at the dock. I can't really run them on the water with fuel this low because as the bow rises with only that small amount of fuel in the tank it may move to the back and uncover the dip tubes. So I will run it at load, about 1200 rpm at the dock. Anymore and my dock buddies won't be buddies anymore.
I have vacuum gauges on the racors but not tell tale types. I also have vacuum gauges just after the next set of filters, both are monitored by a camera viewed on my plotter. This is a recent setup and I have not had this failure since I installed the camera's. I also have a fuel pressure gauge installed on the port engine also monitored by a camera. This was recommended by Tony Athens on boatdiesel.
xsbank; the dip tubes go to the bottom of the tank. These Cummins engines engines don't normally need an inline fuel pump. Incidently the return lines also go to the bottom of the tank so fuel is not splashed or aerated by the return fuel.
Twistedtree; Tony Athens on boatdiesel highly recommends a multi-stage filtration system which I installed right after purchase, each filter monitored by a vacuum gauge. It just came to me but when the PO brought the boat over to the haul out facility both engines quit on the way over, and yup he had very little fuel in all the tanks. At the time we blamed it on old fuel. However changing the filiters at that time did not fix the problems, the engine had some work done on them not related to fuel. So this may have been a problem before purchase, and not caused by adding the additional filters and monkeying with the fuel system by adding a squeeze bulb priming system and changing the fuel tank selector valves, something in the back of my mind that I always considered.
Engine fuel intake is near the bottom of the engine and slightly below the filters-2ft. The tanks are the same level as the engines. I replaced the seals on both racors 1.5 years ago. If air is getting into the system at the filter the rpm decay would not be linked to the amount of fuel in the tank.
You asked: As an experiment, what about running with the racor element removed? I have bypassed everything except the on engine filter and still got the decay.
RickB, to get air bubbles out of solution (not due to an air leak) wouldn't the vacuum have to be far greater than the vacuum the pump could produce? Just trying to wrap my head around this.
One thing to think about. When we went over to the Bahamas, about half way across the rpm decay occurred on the stbd engine and I notice high vacuum. I changed the racor element and switched to the full saddle tank. Rpm came back to normal. 20 minutes later the exact same thing happened to the port engine, however this time I did not change the filter element, just switched tanks and rpm came back.
Later on that same trip rpm decay again feeding from the saddle tanks with fuel about half full. I noticed high vacuum on the FS1000 filter and changed it, rpm came back to normal for 15 minutes and the rpm decay re-occured. Upon retruning home, I cut open the FS1000 filter and posted pix of it along with the racor element on boatdiesel. Replies were that both filters looked new, not clogged and would not cause the high vacuum readings I was getting. So if I could figure out what caused these high vacuum readings, I think I would solve this problem.
I did notice the other day when running the engine at near idle from fuel from the almost empty aux tank some bubbles in the racor bowl that seemed to go away when I increased rpm.