Engine Stalled?

[Alasdair]

I was out on my yacht motoring along for 20 mins when the engine (yanmar 3QM30h) began to loose power, unresponsive to throttle, then stalled. Luckily I was not in a seaway, no wind or drift to contend with. I suspected fuel starvation as the problem. Both the 70 Litre tanks were 75% full, I was drawing from the port tank with fuel return going to starboard. Engine would turn over normaly on starter so not siezed. I had a look at the primary filter and found it nearly dry. Switched over to starboard tank with fuel return to still to starboard. Bled the engine's fuel system and engine started easily, no more problems for the remainder of the day. Back on the mooring rechecked port tank, plenty of fuel, switched back to port tank and ran the engine under load for 30 min with no stalling.
Thoughts - Having the fuel return going to opposite tank set up a pressure vacuum in the port tank thus defeating the fuel pumps effort to lift fuel? Is this possible considering each tank has a breather pipe?
Gunk in the tank temperarily blocked the fuel pick up line? It was in dead calm that I was motoring so no fuel sloshing about, recent fuel samples drawn from bottom of each tank appeared clean?
Any advice would be welcome.

[SkiprJohn]

Possibly a loose bleed screw on the filter let some air in the system? Tank pickup tube blocked by something temporarily? There are just so many ways it could happen. Thankfully you got it running again and a trace through your fuel system and tightening hose clamps and bleed screws will take care of the problem.
kind regards,

[Sandero]

With an empty filter this sounds like a air leak... Engine won't run with an air block even with a full tank. I learned this earlier this summer when I thought I ran out of fuel for the first time in 27 yrs... but it made no sense as I thought I had at least 10 gals in the tank... I did. Turned out to be an air filled filter! The culprit was a loose connection to the filter which allowed a tiny drip of diesel... unnoticed over time... but air to get in and that stalled the engine.

Look for a leak! On the low pressure side before the injector pump it's hard to find.

[Alasdair]

An air leak is possible. I recently installed a secondary filter (Cav type) as boat did not have one. Perhaps I did not bleed properly or something is not tight enough.

[bill good]

I think CAV filter must be after fuel pump. (Don't ask me how I know this)

Bill

ps I just added an electric pump before the CAV filter & a plus is easy to bleed.