Saturday morning, out of the marina, engine running sweetly. A mile down the channel it starts hunting, revs rising and falling on their own. We ease the throttle, it picks up, and thirty seconds later it dies completely. The starter spins it over happily and it fires for a few seconds before dying again.
So what happened? It’s a common scenario: The fuel had been sitting in the tank since September, we’d had a lumpy passage home the week before, and the filters had never been changed since we bought the boat.
Most of us are never going to rebuild an engine, and there’s no need to. But narrowing a fault down to one system saves an enormous amount of grief, and quite often enables us to get home under our own power.
What a diesel needs to run
A petrol engine has an ignition system, with plugs and coils and timing, all of which can fail. A diesel has none of it. Squeeze air hard and it gets hot enough to light the fuel on its own, which is why a diesel will happily keep running with the ignition key in our pocket.
So the list of things it needs is short. Clean fuel arriving at the injectors, air to burn it with, compression to light it, and somewhere for all that heat to go.
Compression wears out slowly, over thousands of hours, so it rarely goes from fine to broken between one weekend and the next. Air is seldom in short supply in an engine bay. That leaves fuel and cooling, and between them they account for nearly every engine that stops when we didn’t want it to.
It turns over but won’t fire
The starter is spinning, so the batteries and the starting circuit are doing their job. Fuel is the place to look.
Start with the easiest questions. Is there fuel in the tank, and is the gauge telling the truth? Boat fuel gauges are usually fitted to an oddly shaped tank, so the needle can be somewhat erratic. If we don’t know this ones quirks, how much can we really trust it? Then check the stop control, because plenty of engines have a manual stop lever or pull cable that can easily get left out, or pulled by accident.
After that, air in the fuel lines. This one is most common after changing a filter or running the tank very low. A diesel injection pump works by squeezing fuel to very high pressure. Liquid barely compresses, so the pressure builds and the injector fires. Air compresses beautifully, so the pump squashes the bubble and the injectors never see the pressure they need. Get one bubble in the line and the engine will not run until we’ve pushed it out, which is what bleeding is.
It starts, then dies
The engine fires, runs for anything from a few seconds to a few minutes, then fades and stops. This one can be particularly tricky if we had time to untie the mooring lines before it died! It’s using the fuel already sitting in the filter and the lines, then finding it can’t pull any more through.
So the restriction must be upstream of the injectors. A blocked primary filter, a blocked tank pick-up, or a soft old fuel hose collapsing flat under suction.
Where does the blockage come from? Two things, usually working together. Water gets into the tank through condensation in the airspace above the fuel over winter and through a leaking deck filler. Then diesel bug, a microbial growth that lives right at the boundary between the water and the diesel, feeds on the fuel and turns into a dark sludge.
All of that sits still on the bottom of the tank for months while the boat is on the mooring. Then we have one properly lumpy passage, the whole lot gets stirred into suspension, and the next few hours of running draws it straight into the filters. Which is why the engine that behaved impeccably all summer chooses the trip home from the roughest weekend of the year to give up.
If there’s a water-separating pre-filter with a clear bowl, look at it. Water is colourless and sits underneath the diesel, so a layer in the bottom of the bowl is water we need to drain. Fuel gone dark, cloudy, or stringy is contaminated.
No water coming out of the exhaust
Stop the engine as soon as it is safe to do so. That doesn’t mean back in the marina, it means as soon as we aren’t 20m away from a rock.
Most marine diesels pull seawater in through a seacock and pump it through a heat exchanger to take the heat out of the engine’s own coolant. That seawater is then injected into the exhaust, where it cools the gases enough for them to travel down a rubber hose. With no water going in, the hose is carrying dry exhaust gas at a temperature it was never designed for. Hoses burn through, and then we have exhaust fumes and carbon monoxide inside the boat, which is a much bigger problem than a stopped engine.
With the engine off, work along the water’s path. Is the seacock open? Is the strainer full of weed or a plastic bag? If both are clear, the impeller is next.
An impeller is a small rubber pump with flexible vanes, and it relies on the water flowing through it for lubrication. Run it dry for a few minutes and the vanes shred. If we pull one out and vanes are missing, go and find them, because they’ve travelled downstream and will be blocking tubes in the heat exchanger.
The temperature alarm
The alarm is a late warning. By the time the sensor is unhappy, the engine has already been running hotter than it should for a while, so it’s a stop-and-look, not a keep-an-eye-on-it.
This could be a blockage like we already described, but it could also be low coolant levels on the fresh water side, and a heat exchanger that’s furred up over the years and no longer moves heat the way it did.
There’s one clue that gets overlooked. On a lot of marine diesels the raw water pump and the alternator are driven off the same belt. So if the battery warning light comes on at the same time as the temperature, we’re probably looking at a belt that’s slipped or snapped, not two separate faults. Black rubber dust around the pulleys is the same story a bit earlier.
The oil pressure alarm
Stop the engine straight away.
Oil pressure is what holds the moving parts apart on a thin film of oil. Lose the pressure and they run metal on metal, and the damage happens in seconds. It might turn out to be a faulty sender rather than a genuine loss of pressure, but that’s a call to make with the engine stopped.
Then check the oil level, and check the bilge under the engine. Oil where it shouldn’t be tells us where it went.
It runs, but won’t make revs
Before we blame the engine, look astern. A rope round the prop or a season’s growth on the blades means the engine is being asked to push harder than it can, and that shows up as heavy black smoke because there’s more fuel going in than the available air can burn.
If the prop is clear, this is another fuel restriction, but a partial one. A half-blocked filter will pass plenty of fuel to idle sweetly on the mooring and nowhere near enough to hold cruising revs once underway. An engine that’s fine at tickover and gutless under load is one of the more useful symptoms we get.
Three jobs to learn on a calm day
None of this is much help if we’ve never had the covers off. Three jobs turn most of the faults above from a tow into an hour’s work, and all three are far easier to learn alongside on a Sunday afternoon than on our knees in a seaway with a torch in our teeth.
Bleeding the fuel system. Find the bleed points on our own engine and learn the order they go in, because it varies between manufacturers and the manual is the only thing that knows.
Changing the impeller. Getting the pump cover off is usually the hard part, so check that the tools on board reach the screws before we need them to.
Changing the fuel filters. If the boat has primary water separator filters, these are usually quick and easy to change, and often don’t even require tools (other than some paper towel and a pair of gloves to keep the diesel away). Do one ourselves at the start of the season, before we go anywhere.
What to carry
We don’t need a complete spare engine, just a few bits in a dry locker. The ones that don’t fit our engine are useless, so buy by part number from the manual.
- Spare impeller, plus the cover gasket or O-ring that goes with it
- Spare primary and secondary fuel filters, with their seals
- Spare alternator and water pump belt, or belts if there’s more than one
- Engine oil and gearbox oil, and something to catch the old stuff in
- A few hose clips and a length of self-amalgamating tape for a weeping hose
- The engine manual, printed, on board, and not only on a phone
- A filter wrench and the specific spanners that reach the awkward fittings
- Clean rags, absorbent pads, and gloves
- A head torch that lives in the engine locker
Alongside the spares, get into the habit of a look round the engine before every start. Plenty of people use WOBBLE for this: Water, Oil, Belts, Bilge, Leaks, and Electrics. Two minutes with the cover off finds a loose belt, a weeping hose, or oil in the bilge while we’re still tied up and the chandlery is open.
Questions we get asked
How often should we change the fuel filters? Follow the interval in the engine manual as a baseline, then change them early after a winter lay-up or a rough passage, because both stir up whatever has settled in the tank.
Does a biocide fix diesel bug? It kills the growth, but the dead material stays in the tank as sludge and can still block filters. A badly contaminated tank usually needs draining and cleaning as well.
The engine is perfect on the mooring, so why does it only fail at sea? Running at load draws far more fuel through the filters than idling does, and a passage stirs up sediment that sits undisturbed alongside.
Do we need the RYA Diesel Engine Course? It’s a one-day course covering the main systems and how to fix the things that don’t need a workshop. A sensible investment for anyone taking charge of a boat.
Can we get home without the engine? On a sailing boat, usually yes, and it’s a good reason to keep the sailing skills sharp. Think about anchoring, or picking up a mooring rather than going alongside though. Also, call the harbour ahead and let them know your situation. They might send someone out in a RIB just in case.
