Volvo Penta will not rev above 1500

[DougAnnapolis]

Our highly reliable 2007 Volvo Penta, 40 HP, 4 cylinder, stalled while motoring at 2100 RPM. Could not restart. I changed primary filter - no luck. Towboat brought us home. After changing secondary filter and bleeding the fuel system 3 times, I could get it start, but runs rough. Even a full throttle it cycles from 500 RPM to 1500, then drops back to 500. Will run for 15-20 minutes and then stalls.

Inspecting old filters the primary filter shows a few small black spots but 95 % of filter element is clean. I use BioBore, and FTTP each time I refill. Last big fill-up was in Solomons island and we motored with no incident for 47 miles.

Any suggestions?

[Lepke]

Sounds like an air leak in the fuel system. It's easier to suck air than fuel. Look for a wear spot in the fuel piping, loose fitting, filter gasket not tight. Also could be an in tank strainer is plugged.
Air leaks and plugged lines both can limit rpm and cause intermittent run problems. Often debris problems are relieved when the engine stops and pump suction goes away. The debris falls away and the when the engine starts is gathered up again. An air leak doesn't drip fuel when small. As it gets bigger, fuel will leak when engine is stopped.

[jamhass]

Do you have an emergency fuel shutoff? Similar thing happened to me, the shutoff had gotten partly pulled by accident.

[DougR]

Remove the fuel hose which runs from the primary Racor filter to the engine at the hose end which connects to the lift pump. Then connect an outboard tank or other container with diesel fuel directly to the lift pump. Run the engine off the small tank and see what happens. This will eliminate any potential problems with the boats fuel system. Be sure all the hose clamps on the engine mounted fuel hoses are tight.

A clear hose could be placed on the fuel return line barb to check for fuel flow there. ie. bad lift pump = little or no return fuel flow

The other thing that causes erratic RPM could be a broken spring in the governor linkage, but this is the court of last resort as its under the engine timing cover.

DougR

[a64pilot]

May sound stupid, but check oil level, if it's high that could point to a bad / failing lift pump, besides checking oil level is easy and free.

[belizesailor]

Good suggestions so far. Vaccum leaks can be a bitch to isloate. Any fuel system work recently.

Restricted air flow could be another issue. Check filter and intake.

Throttle linkage working OK? Disconnect it and work throttle by hand to check.

Clogged/failed injector(s).

Govenor problems. Not likey, but would investigate if all others fail.

Is exhaust normal?

Ah, and just remembered...a clogged exhaust elbow can cause exactly your symptoms. Remove and inspect.

[DougAnnapolis]

thank you for all the suggestions. After checking for air leaks (none) diesel mechanic cracked open one injector and noticed insufficient fuel to cylinder. The fuel pump diaphragm had ruptured after 1050 hours. Replaced fuel pump, engine is now much quieter and we get more power at lower RPMs. Mechanic says 1000 hours on the volvo penta is about normal for fuel pump to fail.

[belizesailor]

Wow, that seems low. My Volvo MD2020s have about 4,500 hours and the lift pump diaphrams are still in good shape (just cracked them open last year and looked, diesel lab tech inspected them too and agreed).

[a64pilot]

Carry an electric pump as a spare, easy to plumb in a hurry and they are far less expense I bet than a spare mechanical Volvo pump.