Diesel start mysteries

By: Scott Fratcher - Marine Engineer/Captain

In this article were going to discuss the three most common engine mysteries and how to solve them. The reason we are calling these engine issues “mysteries” is because our engine seems ready to function, but for some reason it will simply not perform. This can be frustrating and time consuming.

By learning these common engine faults the boat owner can save money and time resulting in more enjoyment on the water.

The mysteries are:
• We push the start button but the engine does not turn over
• A diesel engine that turns over but won’t start
• We leave the dock and our engine won’t reach rpm’s

Lets look at each of these mysteries in turn.

Mystery #1- We push the start button, but the engine does not turn over

Nothing can make a boatie take a quick breath faster than pushing the start button and having no response from the engine. If we listen carefully we may hear a click or grinding noise or maybe a very slow turning engine. Lets look at what each of these sounds means and how to quickly get our engine turning over.

We’ll start with a quick review of how our starter circuit works and then we’ll talk about the common reasons this robust system may suddenly fail. Starting an engine takes a lot of energy for a short period of time. The energy is stored inside the battery within a chemical reaction. When the start button is pushed a small current is passed to an electro magnetic switch (solenoid) that both engages the gears of the starter and closes a large electrical switch capable of passing the 250+ amps it takes to turn the engine over.

Now, lets go back to our first scenario of “push the start button” and we hear a simple “click”. This often means the first step of the start process has taken place. The electro magnetic switch has attempted to engage the starter gear, but no current was passed to the starter. This could be due to an old solenoid having dirty contacts or dirty starter brushes or a seized starter or engine.

The first test to narrow down the possibilities is to turn on the dash lights and watch them carefully as we attempt to start the engine. If the lights dim considerably during a start attempt this is a bad sign. It shows the amperage is reaching the starter windings but the engine is not turning over.

This could mean a seized starter or engine. The seized starter could be from corrosion in the starter that caused the rotor to swell to the point it will no longer move, or possibly the engine has “hydro locked” (See Tradeaboat Aug).

If the gauge lights did not dim, but the starter made a click this means the starter gears are attempting to engage, but the large electrical switch in the starter solenoid did not close, or the starter brushes have worn out and are no longer making electrical contact. This can be considered a good thing as changing a solenoid or starter brushes is within the capabilities of most DIY boaties.

Tip-In this scenario we can often use the plastic handle of a screwdriver and give the solenoid body a good solid “thunk” giving us one or two more starts. This “thunk” works because the vibration loosens just enough dirt or moves the starter brushes enough to allow one or two more electrical contacts.

If the starter made a simple “growling” or “grinding” noise this is most often caused by the small “sprag” clutch inside the starter. Luckily the sprag clutch tend to give us warning by failing over time and by trying to engage the starter a few times we can normally get the engine to start getting us home to make repairs.

If we hear a very slow turning engine this could be a failed battery, shorted starter winding or loose electrical cables. The next test is to check voltage and amp draw at the starter during a start attempt.

If the voltage drops out below 9.5 volts during the start attempt and draws less than 250 amps, then we have a cable or battery problem. We can work our way backwards along the power feed cable looking for electrical lugs to test voltage till we reach good voltage or the battery posts. Clean or tighten the faulty connection and the engine should start.

If we had less than 9.5 volts and over 250 amps then we probably have a starter winding that is beginning to short. This repair is within the capability of most DIY techs, but you will need to buy the windings from a electrical parts warehouse.

If you heard no click, and no light dim during start attempt then suspect the ignition control circuit. The control circuit wire is connected to the spade connection located directly on the solenoid. If the control cable has 12 volts during a start attempt then the problem is in the starter, but if the spade connection shows no voltage then the problem is in the ignition wiring. Look for a safety lockout (IE engine in gear), or bad start button.

Tip-A quick test to check the start system is to cross the main power cable feeding the starter directly to the spade terminal on the solenoid through a short fused wire. If the starter jumps to life we can safely say the problem is in the ignition system wiring.

Mystery #2- A diesel that will turn over, but will not start

This common fault can cause a world of frustration. The engine turns over and over but simply will not “catch”. We’ll take detailed look at why this phenomena becomes a mystery, but fist lets have a quick review of what an engine needs to start.

To start a diesel we need
• Clean fuel
• Air
• Heat

Clean fuel means clean, bubble free diesel at the injectors. Air means a continual circulation of oxygen rich fresh air, and heat comes from compression and/or glow plugs. This combination must all occur at the same time for our engine to start.

Fuel is what most boaties turn to when an engine will not start, but fuel is often the least of our starting problems. Of course we need clean bubble free fuel, but once we have turned the engine over a few times we risk injecting too much fuel that in effect floods our combustion chamber reducing our chances of an engine start.

Another source of complication is the seawater pump. During repeated start attempts the seawater pump is still turning over and filling our exhaust hose muffler/water trap. This means our engine is soon trying to start against backpressure. Backpressure means a restriction on the outlet side of the engine so less air circulation and thus less of a chance of our engine starting. For this reason we close the seawater pump inlet valve during long start attempts. Remember to open the valve soon as the engine starts.

Heat is another often overlooked process in starting a diesel engine. Heat is a result from compression. Compression is often lowest when attempting to start. Heat can be increased by turning the engine over faster, leaving the glow plugs to heat longer, and verifying our valves are set as they should be.

Lets look at the steps to fixing a diesel engine that turns over but will not start.
1. If possible connect a battery charger so we have maximum rpm’s while turning the engine over.
2. Close the seawater intake throuhull (remember to open it as soon as the engine starts).
3. Loosen the injector fuel tube nuts right at the injector. Let a little clean fuel flow. This might dislodge a small amount of trapped air in the injection pump and verifies fuel to the injector.
4. Remove the rubber exhaust hose at the junction where it attaches to the engine. This will prevent the build up of backpressure and often allows the engine to jump to life. Again remember to only run the engine for a few seconds before shutting down and reconnecting the exhaust system.
5. Verify the inlet and exhaust valves have lash. You should be able to turn the engine over by hand and check for free movement (lash) in each valve rocker arm. Tight valves leak compression.
6. Spray a small amount of WD-40 into the air intake while trying to start. WD-40 is a light oil that enters the cylinder pre-mixed with air. It’s a little more flammable than diesel, but not so “explosive” as starting fluid. If the engine will start on WD-40, but shuts down immediately after you stop spraying then the problem is assuredly fuel related.

If these six tricks do not start a stubborn engine the next step is a compression check, but most times when a technician gets called to an obstinate starting engine the problem is solved by one of the six steps above.

Tip-An easy way to start a flooded outboard engine is to give full throttle turn the choke off and give few quick pulls. The full throttle will open the butterfly valve thus reducing the intake manifold vacuum and allow a large amount of air to circulate in the combustion area of the engine thus drying the “flooded” gas. Remember to be ready to quickly twist the throttle back to idle once the engine starts.

Mystery #3- An engine that seems to run fine at the dock, but underway simply won’t push the boat to speed

The engine runs fine at the dock or in neutral, but when we try to get up to speed we can’t reach rpm’s. Maybe we can only reach half speed, but we the defining fact is as we push the throttle forward we seem to get no reaction from the engine.

This is a very common problem and one technicians have become adapt at solving. Still the first time it happens on a boat the drivers often has to take a double look at the slow progress of water passing the hull.

When an engine cannot obtain its rated rpm’s we have to look at the same possibilities as an engine that won’t start. Lets begin with air flow. Look at the exhaust under load. Is it deep black and cloud like? If so the problem is probably air related. Either a clogged air filter, a turbo that is not spooling up or clogged exhaust system.

Black smoke could also mean an overloaded engine. A dirty bottom or barnacle covered prop are classic reasons why an engine will not reach the rated rpm’s. If this is the case we should also see high exhaust gas temperatures.

If the exhaust gas looks clean we begin to suspect to our fuel system or control cable linkage. We may have a classic case of faulty lift pump or semi clogged fuel filter. In this situation we find the injection pump is able to supply almost enough fuel to provide most of our needed horsepower, but not quite enough to get us to speed. In this situation we would expect a clean exhaust, and a cool exhaust gas temperature. It’s not so much a problem except the engine simply does not have the fuel to produce the needed horsepower.

The simple check for fuel flow is to find a place to crack open the diesel fuel return line. Diesel should be flowing back to the tank in a steady stream. If not this is a sure sign the engine is starving for fuel, but not so starved it shuts the engine down, thus the mystery. Check the fuel flow under full load compared to the same rpm’s with the engine in neutral. Any significant change is a sure sign on fuel restriction. To locate the restriction change the filters, check the lift pump, and fuel tank valves.

The control cable linkage can be checked with the engine shut off. Simply give full helm throttle and verify the linkage at the injection pump is fully actuating.

To check for a dirty prop without jumping into the chilly water keep a photo aboard showing the amount of water moved by a clean prop. Use this photo to compare water moved anytime you have a doubt as to prop cleanliness or diesel engine output.

Adapted from "How to make money with boats" by Scott Fratcher