I once had a call from an owner who was up the river, having a picnic. The engine had run perfectly on the way up. When they turned the key to come home, it would not start.
The fault turned out to be a connector on the main engine fuse. There was nothing wrong with the engine.
That is the pattern more often than not. When a small diesel refuses to start, the cause is usually something simple, somewhere between the battery and the engine or between the fuel tank and the engine. You do not need to be a mechanic to find it. You need to check things in the right order.
Stopped and Secure Is the Best Place for This to Happen
When an engine stops under way, the priorities are different, and I have covered that in When the Engine Stops: What Really Happens Next. This blog is about the quieter moment: you are alongside, on a mooring, at anchor or pulled into the bank, and the engine simply will not go.
That is not an emergency. You are not moving and nobody needs to hurry. Rushing is the one thing likely to make it worse.
If you decide not to go out, that is a sound decision. I would much rather an engine let an owner down alongside than halfway home.
First, Listen to What It Is Doing
The textbook says a diesel needs fuel, air and compression to run. On the water, it is more useful to listen to what happens when you turn the key, because that tells you which half of the problem to look at.
Silence, a single click or a slow, laboured turn points to the electrics. The engine is not getting enough power to spin.
If it turns over at a healthy speed but will not catch, the electrics are doing their job. The problem is fuel, or on a cold day, preheat.
Follow the Power From the Battery to the Engine
What I tell owners is to start at the battery and work towards the engine. Start at the engine and work backwards, and you are more likely to walk straight past the fault.
In order:
- The battery switch. Is it on? It is the simplest fault and one of the most common, often because someone turned it off at the end of the last trip and forgot.
- Which battery. Most Plymouth Pilots have a single battery, but some have two, particularly the Pilot 24'. If there are two, check the switch is set to one that is charged.
- The battery terminals. Are they tight and clean? A terminal can look fine and still be loose enough to stop a starter motor.
- The main engine fuse and its connectors. Check the fuse, then the connections either side of it. On the picnic call, the fuse was fine. The connector was not.
- The key switch and wiring to the starter. Look for anything loose, chafed or green with corrosion.
These engines do not have a neutral start switch, so the position of the throttle is not what is stopping it turning over.
If everything is sound up to the starter and it still will not turn, the fault is likely to be the starter motor or its solenoid. That is where I would stop and call someone.
Preheat Is the Step People Skip
All modern diesels, and some older ones, have a preheat stage. Heater plugs warm the combustion chamber so the fuel will ignite on compression.
On a warm afternoon an engine will often start without it, which is how people get out of the habit. On a cold morning the same engine will turn over happily and refuse to fire.
If it is spinning but not catching, give it the full preheat time the engine handbook asks for, then try again.
Then Follow the Fuel From the Tank to the Engine
If the engine turns over well and preheat has been done, apply the same rule to the fuel: start at the tank and work towards the engine.
- Is there fuel in the tank? Running out of fuel is one of the most avoidable reasons anyone ends up needing help. Look, rather than trusting memory. A diesel that has run dry will need bleeding before it will run again, even once the tank is filled.
- Is the fuel valve open? On most Pilots it sits just before the water separator. It is easily left closed after a service, or knocked by someone rummaging about in the engine bay.
- What is in the separator bowl? Pilot fuel tanks generally have no drain, so the separator is where water shows up and where it is drained off. Water or dark sediment in the bowl means the fuel is not clean.
- Is there air in the system? This is the one that catches people out. It usually follows a service or a filter change, or comes from a small leak in the fuel line. The engine may turn and not catch, or start briefly and then die.
Air has to be bled out before the engine will run properly. It is a job most owners can learn to do themselves, and the engine handbook shows where the bleed points are. If you have never done it, alongside is the place to learn. How a service is finished off has a lot to do with whether you meet this problem, which is one reason the routine in What Does Plymouth Pilot Maintenance Actually Look Like? matters.
If the engine will not start on the first outing of the season, Laying Up for Winter covers what should have been done before it was left to sit.
Why Holding the Key Down Is Not the Answer
When an engine will not start, the instinct is to keep trying. A few attempts do no harm. Long, repeated cranking without the engine firing is another matter, for two reasons.
The first is the battery. Every attempt drains it, and a flat battery turns a fuel problem into two problems.
The second is less well known. On a raw-water-cooled engine, the engine pumps seawater into the exhaust as it turns, and exhaust pressure normally pushes it out. When the engine is cranking without firing, there is no exhaust pressure. The water can build up in the exhaust and, with enough cranking, find its way back into the engine, and water in an engine is an expensive repair.
This is not a risk from two or three attempts. It comes from long, repeated cranking, usually after running out of fuel or while bleeding the system. If you expect to crank for a while, the usual practice is to close the raw water seacock until the engine fires. A Pilot typically has just one seacock, so there is no hunting around for it. It has to be opened again as soon as the engine is running.
Knowing When to Stop
Most of the time, working through the power and then the fuel will find the fault. How Reliable Are Small Motor Boats? explains why the engine itself is so rarely to blame.
If it does not, stop. You have ruled out the simple causes, which is useful to whoever comes to help, and you still have a charged battery and a dry engine.
Where you are matters too. Alongside your own berth, an engine that will not start is a nuisance. Up a river, a long way from your mooring, with the tide or the daylight running out, it is a different situation. My years on lifeboat crew taught me that embarrassment is the worst reason to wait. Asking for help early costs nothing, and if the engine starts in the meantime, you simply let them know.
The Short Version
When you turn the key and nothing happens, it is rarely the engine.
The owner up the river had nothing wrong with the engine at all. What stopped them was one small connector between the battery and the engine.
Battery to engine first, then tank to engine. The answer is usually somewhere along the way.