Unexplained loss of revolutions on my Vetus m3 22hp diesel.

[themannsindorse]

P { margin-bottom: 0.21cm;* Motor will run normally for maybe an hour or so and then the revolutions (typically 2000 RPM) start to drop off slightly then recover. On two occasions now the revs dropped right away and motor actually stalled. The motor restarts quite normally (thankfully as I otherwise would have to have called the RNLI) and ran properly for the remainder of the trip. What may be the cause? When the revs start to drop, the motor sounds quite normal..no roughness or miss firing cylinder etc. Yesterday after an hours motoring I was motoring up to my mooring at tick-over and the same thing happened again. Again no roughness but just a gradual decline in revolutions over a period of four to five seconds. I have had this intermittent problem for some time now and last winter changed all the fuel filters etc. Someone suggested that the fuel tank could not vent and a partial vacuum was being created. Yesterday I loosened the cap and still have the problem. Anyway why is it happening now and not over the last three years.
So..I started to think about water in fuel etc and so today did the removing water from the filter thing and we shall see if that works. I could see no obvious water but did notice a hiss of escaping air (in or out I am not sure) as I unscrewed the water trap on the secondary filter.
Apart from any ideas you guys might have on the cause of this, I do have two specific questions about the Vetus:


I will admit to have forgotten to bleed the fuel system when I changed the filters and so could this be the culprit? Does air in the system match my symptoms?


My normally excellent manual on my motor was vague on bleeding the lines. It just states:
“ The fuel system is self bleeding. Prime the system with the manual pump to purge” It makes no mention of unscrewing the two obvious bleed nipples on top of both filters. So I unscrewed the two nipples one at a time and did the pumping. I managed to get neat fuel out of the secondary filter but nothing from the primary filter however hard I tried. Is there a procedure I should have followed?


I would be grateful for your advice


Peter
Poole
Dorset
UK

[ShaktisBoy]

Several possibilities come to mind. 1st yes air in the system will sometimes cause your symptoms. I had identical symptoms last summer and the culprit turned out to be a failing seal on a racor filter allowing a tiny bubble of air in every minute or so. After a while enough air was in the fuel system to cause reduction in rpm and every so often a stall. I have also seen the inner liner of the fuel lines themselves collapse causing fuel starvation. Last idea is a failing lift pump not being able to keep up with the fuel demand for a given RPM.

[DeepFrz]

The "hiss of air" as you opened the filter drain is indicative of a partial vacuum in the system. It certainly sounds to me like you have a blockage somewhere upstream from the lift pump. As you loosened the tank fill cap (I presume before the instance) that would indicate to me that the partial blockage is in the fuel line between the bottom of the pickup tube and lift pump.

[Stu Jackson]

In addition, a friend of mine had this same issue although his engine wouldn't restart. Crud in the pickup tube inside the tank, or the dreaded OEM stupid screen there. Blow back through the fuel fill connection on the primary filter. Check your vent. Check all connections. Air hiss? That's a new one! Good luck.

[ShaktisBoy]

Air hiss I missed that. Definitely pull the fuel line and see if it has collapsed, it will be obvious if it has. It will look like a collapsed drinking straw inside the rubber fuel line. If all looks good the blow back idea is a good one.

[a64pilot]

I had exactly the same symptoms, I had what looked like tofu stuck in my pickup tube, blowing back through the tube will put the blockage back in the tank, remove the tube and blow it out so the blockage doesn't just go back into the tank.

[themannsindorse]

Hi All. Replaced all the pipes and new filters. Also cleaned out tank and fresh fuel. Result..motor AGAIN falters after a hours running ....****! THIS WAS THE CLUE however. It turned out that the lift pump begins to fail once fuel in the tank had warmed up vie the return pipe. Why I am not sure but it did. One question though. On replacing the pump I found a 2mm shim between the pump flange and the Vetus m32 motor flange. What's its purpose.. anyone know? Is it Vetus's way of making pump fit other motors or by removing it, would it have made the little pokey thing from the motor pushed further into pump housing to make an old unit pump harder? Anyone know? Thanks. Peter