FAQ: Fuel Management

How much fuel will my engine use?

This is a very common question and it helps to know a bit about typical engines. A two-stroke outboard motor is the heaviest user of fuel and the figures in examples we give will always be the worst possible case. The new four stroke and EFI technologies will give you significant reductions in fuel consumption.

Worst Case Two Stroke Fuel Consumption Formula

Divide the horsepower by 10. (This gives you the number of US gallons per hour at full load.)

Convert US gallons per hour to litres by multiplying by 4.

Example: A 100 horsepower engine will use approximately 10 US gallons or 40 litres per hour flow at maximum revs.

As a rough guide an inboard four-stroke engine will use about 60% of the typical outboard engine, for example, a 100 horsepower engine will use 6 US gallons or 24 litres per hour, once again at maximum revs, and this is a worst-case situation.

NB: These are typical values for a planing hull and a typical boat with almost any petrol outboard engine will be reaching a maximum speed of approximately 30 knots unless it has an exceptionally large engine for the size of the boat.

Will the DIESEL 3200 work with my diesel engine?

Navman's diesel system has been designed to be installed and operate on most diesel engines in the 100 to 400 horsepower range. However, the key specification is the fuel flow rate through the engine (note: this is not the consumption rate).

For the system to work accurately, the flow in the supply pipe to the engine must be less than 400 litres (105.7 gal(US)) per hour.

The flow in the return pipe back to the tank must be more than 25 litres (6.6 gal(US)) per hour.

Can I use two instruments to display fuel data from a single fuel flow transducer?

Yes, if you have two FUEL3100 or DIESEL3200 instruments then fuel data is shared by these instruments over Navbus.

Why does my fuel computer give LOW or erratic readings?

  1. This is usually due to dirt or contamination in the fuel line clogging the transducer if it is the older style screw apart transducer you may open the transducer and clean the internal gauze filter. If you have the current one piece flow transducer then it will have to be returned to your distributor for replacement.


  2. Another cause of erratic readings is the way the newer EFI engines operate which is by delivering fuel to the engine from a fuel reservoir. This reservoir (or bowl) is refilled from the main fuel tank only when the bowl level is getting low. This produces a high flow rate through the fuel transducer when the bowl is filling and little or no flow rate when the bowl is not filling. Although the fuel flow transducer can accurately follow these changes, it can be difficult to display the fuel flow rate due to the rapid variation. To overcome this the fuel computers all have a flow filter setting in the menu, which acts as an averaging or smoothing factor on the readings. Usually a carburetted engine will require a setting of 10-15 seconds whilst an EFI engine will require a longer setting. Remember that the flow filter setting does not affect the accuracy of the fuel used or the fuel remaining, but does affect the display of fuel flow. Always use the lowest filter setting that gives a smooth flow reading.


  3. Another cause can be excessive running at trolling speeds with smaller HP engines. Currently as a guide we feel that NAVMAN fuel products work most effectively in engines of at least 50 HP .If you use an engine of less HP than this and spend prolonged periods at low revolutions then accuracy may be a problem as low fuel flow rates are the most difficult to measure accurately.

Why does my fuel computer give HIGH or erratic readings?

  1. This problem is almost always caused by air getting into the fuel suction line before the flow transducer. Check all of your fittings up to the filter for signs of air leaks. The metal clamps, if over tightened distort the flow transducer tails and allow air ingress. This air then accelerates the turbine, which consequently becomes erratic and over reads.


  2. Check that the fuel line is clean, i.e. that the transducer is not clogged up with dirt, and check that the turbine is spinning freely. This can be done by very gently blowing into the turbine and listening for a whirring sound. This must be done carefully because the turbines are made to spin in petrol, which acts like a lubricant. If the turbine is spun too fast in air the bearings can be damaged.


  3. Each flow sensor is generally accurate to +/- 10% out of the box and as each fuel transducer will have slightly different characteristics then onboard calibration may be required. We have found that by methodically going through the manual much calibration related problems are solved. There is a good section on calibration in each user manual.

Why does my fuel computer display flow whilst the engine is switched off?

In choppy seas fuel can surge back and forwards through the flow transducer. This back and forth motion causes the flow transducer to measure flow readings, which affects the accuracy. The solution is normally to place a fuel resistant one-way valve between the flow transducer and the tank.

This problem can also be a symptom of inadequate baffles in the fuel tank allowing fuel to surge in the tank causing pressurised surge in the fuel lines. If you have this problem on an 2100 Series F41 then just wire the unit into the ignition so that when the engine is off the gauge is off.

Should Fuel 3100 be wired so that it always has power or should it be switch on/off with the ignition switch?

Always wire your Fuel Computer into your boat's ignition so that when the engine is turned on, the Fuel Computer is recording the fuel used. If an accessory switch is used and you forget to turn the unit on then the Fuel Computer will not have recorded the fuel used and will be inaccurate. If you are unsure how to do this, contact a qualified marine electrician.

Will the fuel flow transducer obstruct the fuel supply to my engine?

The current one-piece flow transducer is rated at a maximum flow rate of 130 litres per hour (32 US gallons per hour). In modern terms this is a relatively large amount of fuel especially with the advent of far more economical engines. In cases where users suspect the flow transducer of being the cause of a fuel supply problem then it is generally found that the design of the fuel delivery system is marginal to start with, in other words adding the flow transducer may be the straw that breaks the camels back but it is not the cause of the problem.

The critical measurement in any outboard engine fuel system is the total backpressure at full throttle. This is measured in 'Inches of Mercury'. The absolute maximum acceptable limit amongst most manufacturers is 7.0 inches of mercury and ideally should be far less (Always consult your outboard manufacturer for detailed recommendations). Our fuel transducer creates a maximum backpressure of 1.0 inch of mercury backpressure so even if your manufacturer specified a maximum of 4.0 inches backpressure then you have plenty of margin for the fuel delivery system components.