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Fuel Flow

You cannot prove the integrity of the whole fuel delivery system by simply measuring the fuel pressure alone. This is one of the most common mistakes made in diagnosing the fuel system.

Fuel pumps produce fuel flow not pressure. Restricting the flow produced by the fuel pump creates the pressure in the fuel system. This component "FUEL PRESSURE REGULATOR" is the only item controlling the pressure in the EFI fuel system.

The greater the restriction to flow, the greater the pressure produced, up to a point. When the restriction finally causes the flow to cease, no further gain in pressure can be achieved.
This technique is physically applied for the pressure demand test of the fuel pump by closing the supply line with the ball valve unit.

Another common mistake made when diagnosing the fuel delivery system is that just because there is sufficient flow and pressure, the system is fault free.

What is paramount? It is the quality of fuel flow. Bubbles of air, if system is at fault or fuel vapour (gas cushions) suspended in the fuel flow cannot be seen when the fuel is under pressure if you do not have special designed flow meters. Imagine a bottle of soft drink. We know the bubbles are in there, but we cannot see them until we remove the cap and release the pressure. Once the pressure is gone, any gas suspended in the liquid instantly becomes visible due to gas expansion in the atmosphere.

The same applies to fuel delivery systems. If air or fuel vapour is suspended in the fuel flow, they cannot be seen when the fuel is under pressure. (Assumingly to operators which do not have the INTER-JECT equipment.) The Inter-Ject equipment with specially designed flow tubes and indicators will allow visible detection of air and fuel expansion. Due to vapour cushions in the fuel system, an erratic and unstable movement in the fuel flow indicators signalling these problems to the operator.

The air and or fuel vapour does however have a profound effect on how the engine performs as any air or fuel vapour suspended in the fuel flow reduces the density and its spray stability of the fuel, which in turn affects the air/fuel ratio by making it lean due to frequently intermittent air or gas spits into the manifold. The result is poor engine performance due to a fault in the fuel delivery system even though the fuel delivery system shows good pressure and flow when tested the conventional method. The injector flow meter is temporally put on an air counter presser cushion whilst engine is running via the system flow meter. This technique is used to find any fault in fuel or in the electronic components by pulling the wire connections on and off to read the finer synchronisation effectiveness eg. Oxygen sensor, idle stabilisation, spark advance, knock sensor, injector leaks, system leaks, and the list goes on.
To find more about this | follow this link. |

Fuel quality and type is another aspect of fuel delivery system diagnostics that is ignored on any service.

Petrol, which is uncontaminated, appears crystal clear when viewed though a transparent container. Each classification of fuel has a dye added to it for easy visual identification, (purple, regular unleaded; light yellow, premium unleaded; light red, leaded and so on) however, each of these classes of fuel are easily seen through.

What happens to the fuel which is crystal clear and the internal fuel system is contaminated with moisture, resin and rust? DO NOT over look this.

INTER-JECTRON with colour diagnostic fault indicators which changes the chemical in the fuel into a number of colours, will tell you the particular contamination in the fuel system, plus it suspends all the contamination and eliminates it in the normal combustion process. The chemical is incredible, so powerful it kills any acid or any microscopic organism in fuel as well. | See example |

We have established that there is a list of criteria necessary to be tested to completely prove the integrity of the fuel delivery system.

1. Fuel flow
2. The quality of the fuel flow
3. Fuel pressure
4. The ability of the system to hold pressure for a period of time when the system is at rest. But be careful it is not so, on some European make and models as they have other enhancements in-build into the fuel pressure regulators regulator. Those loose pressure instantly to "0" soon the engine is off.

Poor quality fuel due to contamination can change the flash point of the fuel into a negative manner in a lean fuel mixture which causes an increase of spark voltage in the ignition system and that can arc inside the distributor cap and from the rotor button to fire into cylinders which are on the intake stroke causing detonations.

The fuel pump on EFI engines is a specific type of pump. They are known as constant or fixed displacement pumps. They function as the name suggests. For each revolution of the pump a fixed volume of fuel is delivered. The faster the pump rotates the more fuel is delivered. The pump must be able to provide 3 times of demand pressure to sustain constant regulated system pressure. The quantity and quality of the electrical supply govern the speed of the pump and the fuel flow. Some systems have current limiting devices such as resistors placed in the pump's electrical circuit or ECU controlled.

Other systems such as found on the Nissan Pintara, electronically dither the ground circuit to the fuel pump when the engine is idling, (throttle position switch in closed position for 20 seconds after engine start, or 20 seconds after engine is on idle) This has the effect of limiting Fuel Flow when not required due to enhanced idle stabilisation.

The available electrical current to the pump when the engine is idling, causing the flow to drop. As soon as the engine is above idle, the ECU ceases to dither the ground circuit and pulls the negative side of the pump fully to ground. The increase in available electrical current causes the fuel flow to increase. (Read on why this above enhancement is required by | following this link | )

Because a fixed displacement pump is not supposed to skid or slip on the fluid being pumped, it is theoretically possible to actually stall the pump by totally restricting the pump outlet. This is a potentially lethal situation when pumping petrol. As greater loads are placed on the pump (greater flow restriction) the electrical current requirements also increase. If there was no fuse in the fuel pump circuit the current draw would increase proportionally to the load on the pump and an explosion can result if the fuel tank is near empty, due to high vapour ( the highly volatile gas pocket above the fuel pump) and the fuel swirl pot not submersing the pump completely to cool it.

Fortunately, all pumps have a pressure relief valve build into the pump to release pressure by directing the fuel flow internally back to the pump inlet. If the fuel pump fuse blows when testing a pump by flow restriction, you should be suspicious of a faulty pressure relief valve or a wrong low amp fuse has been fitted into the circuit. Test the pump again, this time by routing the electrical current travelling through the fuse to the fused current measuring circuit on your multi-meter. This time monitoring the current draw of the pump on the meter. If the current drawn exceeds the fused rating of the pump circuit, the pump is faulty and should be replaced.

The current draw test is an excellent method to determine the integrity of a fuel pump. When the system is at low load levels (engine idling only) current draw should be minimal. If the current draw is high at low pump loads levels, the pump and or circuit must be deemed suspect, but on the other hand a blocked fuel filter, fuel regulator or charcoal evaporative system, or a blocked strainer sock can throw you well and truly off track as it will cause high current draw also.

The general rule of thumb for very good flow is approximately 120 l/h or 2 litres per minute for large multipoint fuel injection and 60-70 l/h or 1 litre per minute for centre point or throttle body fuel injection.

The variants to this rule are systems that regulate current flow to the fuel pump when the engine is idling. These systems have a fuel flow of approximately 60 L/h and 90 -120 L/h at other times. Low fuel flow are also caused by restrictions in the fuel filter, wrong fittings, and fuel density if the fuel tank if near empty and very hot fuel is flowing through the flow meters signalling aeration's. The lack of fuel flow will cause the engine to be fuel deficient when at high speeds and or under heavy load. Be aware of those hot summer days.

It is therefore essential that the fuel system has the correct flow rate. Systems that have low fuel flow could have blocked fuel filters. A faulty pump electrical supplies and/or is an early warning of imminent pump failure.

Fuel vaporisation is normal in all fuel delivery systems that have a fuel pressure regulator. When fuel moves from the hot environment of the fuel rail and passes through the fuel pressure regulators orifice that causes a fuel friction, heating and expanding the fuel evaporative chemicals added by refineries, even more so with a combination of liquid expansions and vapour, only visible as expanded gas bubbles after the pressure regulator. If a temporally transparent hose is joined into the return line you can monitor the variations in differed pressure setting. This phenomenon is easily understood, as it is the basic principle of how air conditioning functions.

The problem is that if the fuel vapour is not condensed back into liquid form prior to being drawn into the fuel pump intake, the density of the fuel flow will be compromised and cause engine performance problems. Different manufacturers attack the problem by employing different methods. Whichever method is used is irrelevant.

What is essential is that all traces of fuel vapour be eliminated from any fuel entering the pump intake otherwise engine performance will be compromised.

Fuel vapour can also be formed by a restriction in the pump intake. A blocked pre-pump filter screen or a stalled primary pump that is fitted in the system prior to the main high volume pump could cause this. Whenever the pressure on fluid is lowered (ie. air pressure) the boiling point of that fluid is also lowered. Fuel is drawn into a pump by the action of the pump reducing the pressure in the fuel intake area. Atmospheric air pressure acting on the head of the fuel level in the tank forces the fuel into the pump. If the pump requires more fuel than what can be passed through pre-pump filter screen or primary pump, the pressure on the fuel being drawn into the main pump will be substantially reduced and the pump has fuel/vapour cavitations.

If the restriction to fuel flow entering the main pump is great enough, the pump will cavitate and fuel vapour will be introduced into the fuel flow causing engine performance problems. Fuel vapour causes fuel pumps to produce noise. Quite often the cause of a noisy fuel pump is fuel vapour entering the pump intake, not some mechanical anomaly with the fuel pump. The presence of fuel vapour in the fuel flow can be detected by using the Inter-Ject LB291/2F diagnostic equipment.

By using your LB291/2F equipment and observing the flow indicators in the flow tubes they should be smooth and steady. Because the presence of fuel vapour changes the density of the fuel, the spinning flow indicators are so sensitive registering the slightest changes in the fuel. The flow indicators will act erratically when fuel vapour is present.

The presence of fuel vapour in the fuel delivery system is to be avoided at all costs. It will cause high engine temperatures because of lean running, hot starting problems, lack of engine performance, detonation, rough engine idle and compromised exhaust emission levels. It will also cause damage to other components, like ignition cross fire, air flow meter and map sensor damage, and engine seal blow out, oil and head gasket leaks due to an overheated engine.

Fuel delivery systems are generic systems found on all internal combustion engines. The fact that more often than not, they are not fully tested is quite amazing, considering their impact on engine performance.

The fact that fuel delivery systems are generic means that once you understand how they function on one vehicle, you understand how they function on all vehicles.

With your Inter-Ject equipment and more importantly the INTER-JECTRON and DIESEL CARE & CURE FUEL ADDITIVES an ordinary tune will be a SUPER-TUNE. Your clients will love the end result.      | Order Form |