Fuel Problems
Fuel related outboard problems are the most common.
Fuel related outboard running problems often lead to serious and
expensive engine damage.
Some fuel related problems include:
- No Petrol
- Old Petrol
- Water in Petrol
- Foreign Matter in Petrol
- Air Leaks into the Fuel System
- Restrictions to the Fuel Supply
- Dirty Injectors
- Low Fuel Pressure
- Idle Air Control Valve
- Flooded Combustion Chamber
- Flooded Carburetter
No Petrol:
Running out of petrol would probably be the very first thing to check. This is more of an operator error.
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Old Petrol:
Liquid petrol does not burn. It is the petrol vapour that burns.
The outboard fuel system has to meter the correct amount of petrol for the desired
engine speed, mix it with the correct amount of air and atomise it into tiny
droplets in order for the petrol to vaporise by the time it is compressed in the combustion
chamber. This happens 50 times every second on a two stroke outboard operating at 3000 rpm.
Petrol starts to lose its ability to quickly vaporise as soon as it is made. The petrol
will last a reasonable time and is quite satisfactory if you are going boating regularly
but problems occur when the fuel sits for any extended period. Some manufacturers recommend
not using petrol that is more than two weeks old
(Reference 1) .
Petrol that is over two months old should be considered suspect if you are having outboard running problems.
Only the very freshest petrol of the manufacturer recommended octane rating should be used on high
performance outboards.
The fuel inside the carburettor float bowl is vented to atmosphere.
As this fuel sits for extended periods, the lighter spirits evaporate
from the fuel and leave a thick, oil like substance and eventually this
will develop into a hard varnish. There are many treatments available for
this to simply add to the fuel but with only very limited success.
If petrol varnish has developed inside the carburettor, it will
mean the carburettor will need to be removed, stripped andsoaked
overnight in a carbon cleaner.Then it will need to be thoroughly cleaned and fuel
flow tests performed to ensure the carburettor is clear.
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Water in Petrol:
The fact that water doesn’t burn is not the only reason that
water causes running problems with your outboard. Water has a higher
viscosity than petrol and it blocks the small jets and orifices inside
the carburettor which may stop the outboard from running properly. The water
can mix with the petrol, oil and air inside the carburettor and create lumps of
foreign matter that can cause blockages in the emulsion tube inside the carburettor.
Sometimes the water is separate from the petrol and will sit at the bottom of the
carburettor bowl and can be easily drained from the carburettors. Sometimes the
water will mix with the fuel, making the fuel appear cloudy.
Water separating fuel filters are essential. These will remove most water but don’t
work very well to remove water that has homogenised into the fuel. When choosing which
water separating fuel filter, ensure to select one with a sight bowl at the bottom of the
filter. This will allow you to identify that the filter is collecting water and allow you
to drain the water from the bowl before the entire fuel filter fills with water.
Fuel tank vents on remote fuel tanks should be protected from rain or water by placing
a cover over them such as a piece of ply or a rubber mat.
Fuel tank vents of built in fuel tanks should not be exposed to splashing while under way
or from the water sloshing over them while at anchor.
Fuel filler caps should be positioned so they repel water when it rains.
Water will normally settle under the petrol.
Water can be removed from carburettors by removing the bung at the bottom of each
carburettor and pumping good quality fuel through with the primer bulb.
Make sure to clean up all the fuel and dispose of appropriately and keep ignition sources
away. Turn the battery off before working with the fuel system.
This may get the outboard running again, but the carburettors will still need to be stripped
and properly cleaned.
If water is inside the carburettor, it will mean the carburettor will need to be removed,
stripped, thoroughly cleaned and fuel flow tests performed to ensure the carburettor is clear.
On EFI outboards the water can get trapped in the vapour separator which is the large metal
container often located behind or near the intake manifold.
At the bottom of the vapour separator there is a drain bung connected to a little flexible
hose. Open the drain bung with a little screwdriver and pump good quality fuel through the
vapour separator using the primer bulb. The little hose can be moved to direct the fuel and
water into a container.
Make sure to clean up all the fuel and dispose of appropriately and keep ignition sources
away. Turn the battery off before working with the fuel system.
This may get the outboard running again but the vapour separator will still need to be
stripped and properly cleaned.
If water is inside the vapour separator, it will mean the vapour separator will need to be
removed, stripped and thoroughly cleaned.
Failure to properly clean the vapour separator can cause serious and very expensive damage
to EFI components.
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Foreign Matter in Petrol:
Foreign matter blocks the small jets and orifices inside the carburettor.
Unfiltered air travels through airpassages inside the carburettor and not just through the carburettor
throat. Over a period of time, small dust and salt particles find their way into the
carburettor to create restrictions and blockages. Even very small particles that pass through the fuel
filter with the petrol can bond with each other and create restrictions and blockages.
Ensure there is a 10 micron fuel filter fitted to your fuel supply and that the
filter is clean and renewed regularly.
If foreign matter is inside the carburettor, it will mean the carburettor
will need to be removed, stripped, thoroughly cleaned and fuel flow tests
performed to ensure the carburettor is clear.
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Air Leaks Into the Fuel System:
Air in the fuel supply lines may cause leaning out and possible
serious and expensive damage such as piston failure. The outboard
is designed to run on a specific ratio of petrol and air. The more
air in this ratio means the more oxygen there is to burn and the harder
and faster the engine will run up to a point when the piston will start to melt.
The engineers who developed the outboard have determined the best ratio of petrol and
air for the best balance of power, emmisions, reliability and longevity.
Air can get into the fuel supply system from leaking gaskets, hoses, fuel filters or fuel connectors.
If there is fuel seepage around a fuel connection it may indicate an area where air is getting into the
fuel supply. This can be inspected on the outlet side of the primer bulb by squeezing the bulb and
inspecting the components but not on the vacuum side.
Fuel air leak tests should be performed on the fuel system of the vessel while
the outboard is in the water under normal operating conditions and the vessel operated
at all throttle settings, including wide open throttle.
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Restrictions to the Fuel Supply:
Restrictions in the fuel supply will cause leaning out and possible piston failure.
This is very similar to having an air leak with the ratio of petrol and air being incorrect.
There is no extra air getting in, but the restricted petrol causes the ratio of air to fuel to be
incorrect and a lean condition results. The more air in this ratio means the more oxygen there
is to burn and the harder and faster the engine will run up to a point when the piston will start to
melt.
Restricted fuel supply can be caused by blocked fuel filters, failed fuel hoses and incorrectly
set up fuel supply system.
Fuel vacuum tests should be performed on the fuel system of the vessel while
the outboard is in the water under normal operating conditions and the vessel
operated at all throttle settings including wide open throttle. This is usually done simultaneously with the air
leak tests.
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Dirty Injectors:
On electronic fuel injected (EFI) outboards the high pressure fuel pump
supplies petrol to the electronically controlled fuel injectors at a certain pressure.
A computer is provided with information from a series of sensors and the computer sends a
signal to the injectors to open as required. The injector sprays the pressurised petrol
into the intake manifold in a fine atomised pattern.
Sometimes the tips of the injectors get dirty and this causes some of the petrol to
not be atomised properly. The poorly atomised petrol does not get the time to vaporise
and therefore will not burn thoroughly. The injectors will have to be removed, cleaned
and tested.
Outboard computer diagnostic software is required to carry
out individual testing of injectors. The injectors may have to be removed for
spray pattern and leak testing.
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Low Fuel Pressure:
Fuel pressure is normally supplied to the outboard by a primer bulb
before the outboard is started and then by a low pressure diaphragm
type fuel pump while the engine is running.
Some boats are fitted with electric low pressure fuel pumps but this is not common.
The diaphragms and check valves in the low pressure fuel pump often fail.
If the fuel pump has failed, the primer bulb can be constantly squeezed in order
to provide the outboard with fuel while running.
Low quality primer bulbs are often the cause of not being able to provide the outboard
with enough fuel pressure for starting.
Most of the outboard manufacturers have good quality primer bulbs that can be relied
on. In particular, the BRP (Johnson / Evinrude) primer bulbs are a very good quality
long lasting product.
I have never seen a good quality aftermarket primer bulb that lasts a reasonable amount of time.
Primer bulbs should be fitted in a vertical orientation with the arrows facing up. This
will allow gravity to help the internal check valves open and close properly which will
assist in pumping up the fuel.
The primer bulb should be able to be pumped up hard.
On EFI outboards the high pressure fuel pump has fine fuel filters that protect the
pump and injectors from foreign matter. If this filter gets blocked, fuel pressure
will be reduced and the injectors will not inject properly.
Fuel pressure gauges are required to test for blocked high pressure fuel filter and fuel
pump output.
Special techniques are required to remove and replace most high pressure fuel filters.
The high pressure fuel pump wiring is protected by a fuse, sometimes located at the front of
the outboard in a series of fuse holders.
Check the condition of the fuse if you suspect the high pressure fuel pump is not operating.
It is best to familiarise yourself with this procedure before departure.
Check the size and amperage of the fuses on your outboard and put some spares in the boat.
The fuses are commonly mini blade type fuses.
Always replace the fuse with the same amperage rating and never use wire or alfoil for example
as this can and has caused fires.
Outboard computer diagnostic software is required to test operation of most high pressure fuel pumps.
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Idle Air Control Valve:
The throttle valves on an EFI outboard are closed at idle and an idle air control
valve allows air into the intake manifold at start up and during idle conditions.
If this valve fails, the outboard may not start or may idle too fast. Sometimes
the outboard may start by manually opening the throttle valve slightly.
Outboard computer diagnostic software is required to test the idle air control valve.
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Flooded Combustion Chamber:
An outboard engine requires a precise mixture of fuel and air to
start. If there is too much air or too much fuel the engine will not start.
During cold starting, the fuel and air mixture needs to be a lot richer
than normal, meaning there is a higher ratio of fuel to air. Generally,
this is achieved by activating the choke shutter and restricting the air
flow which makes the mixture richer, or by injecting extra fuel into the
intake air flow.
Electronic Fuel Injected (EFI) outboards have an electronic control module
(ECM), sensors and an idle air control (IAC) valve that controls the richer
fuel and air mixture required for cold starting.
Once the outboard has started and warmed up slightly, there is no need for
extra choking or priming if the outboard is running properly.
If the outboard has been choked or primed for too long or been used when it
is already warmed up it may cause the combustion chamber to become flooded.
What to do if you have a flooded combustion chamber
A flooded combustion chamber needs the ratio of fuel and air to be leaned
out, meaning it requires more air in the combustion chamber.
If your outboard controls allow, put the throttle into the wide open
position in neutral.
If your outboard does not allow full throttle in neutral, you will need
to open the throttle shutter manually. Beware of rotating parts of the
engine and make sure the outboard is in neutral.
Opening the throttle fully will allow more air to get into the combustion
chamber and make the fuel and air mixture less rich.
Some 4 stroke outboards have an accelerator pump that pushes air into the
float chamber to help with acceleration. Moving the throttle when the engine
is not running can cause flooding. Don’t move it over and over again.
Operate the starter while in the full throttle neutral position.
The outboard will fire a couple of shots and then more and more until the
outboard starts in a flurry of smoke as the excess fuel is burned.
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Flooded Carburetter
The carburettor utilises a float that floats on top of the fuel in the
float chamber. The float is connected to an inlet needle and seat that
shuts off the fuel.
As the primer bulb is squeezed, fuel enters through the inlet needle and seat,
and fills the float chamber. The float lifts up and pushes the needle into the
seat and closes off the fuel when it gets to a height determined by the float
level adjustment.
EFI outboards have a vapour separator that is very similar to the carburettor
float chamber and can have similar flooding faults.
If the float is cracked it can allow fuel to get into the float and it will not
float up and close off the fuel.
If the float is worn out of adjustment it will not close off the fuel at the correct
height and allow fuel to flood out of the carburettor.
If the inlet needle is worn it will not close off the fuel properly and allow fuel to
flood out of the carburettor.
If there is foreign matter on the tip of the needle or the seat, it will not close off
the fuel and allow fuel to flood out of the carburettor.
If the needle gets stuck in the seat, it will not close off the fuel and allow fuel to
flood out of the carburettor.
What to do if fuel is flooding out of the carburetter
Be sure to turn off the battery, remove all ignition sources and dispose of the
fuel responsibly when performing these tasks.
If there is foreign matter stopping the needle from sealing in the seat, this can be flushed
out by removing the drain bung at the bottom of the carburettor or vapour separator and squeezing
the primer bulb to flush out the foreign matter.
If the needle is stuck in the seat, it can be loosened by tapping the carburettor body or vapour
separator with the plastic end of a screwdriver.
The carburettor or vapour separator should be stripped and thoroughly inspected by an experienced
outboard mechanic if it is flooding.
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- Contact Cairns Outboard Service for your outboard service requirements.