A fuelling fault is the engine being given the wrong quantity of fuel for the air it has, or being given it at the wrong time. On a petrol engine that is a mixture error, and it ends as a misfire, a rich smell, a failed catalyst or a lean hesitation. On a diesel the quantity is the load itself, so the same class of fault ends as smoke, knock, a long crank or a refusal to pull. The parts that meter the fuel are the pumps and the injectors. This page is the layer above them: how to tell a supply problem from a sensor the controller believed, and from an air problem that only looks like fuel.
Common petrol engine problems and common diesel engine problems both link here.
The question the controller is answering
A port-injected petrol engine estimates the air, then opens the injectors for long enough to match it, and trims that time from the exhaust oxygen sensor once the sensor is warm. A direct-injection petrol engine does the same job from a high-pressure rail. A diesel does not hold a stoichiometric mixture. It measures air so that it knows how much fuel the smoke limit, the torque limit and the boost will allow, and it commands a fuel quantity. EDC15 fuel control is one concrete version of that quantity calculation. Later common-rail controllers are torque-based, but the workshop point is the same: a quantity command is not a measured quantity. Rail pressure, injector opening time and injector condition sit between the command and the fuel in the cylinder.
Anything that lies about the air, the pressure or the temperature will make a plausible command that is still the wrong mass of fuel. Replacing injectors because a trim is high, without asking what the trim is correcting, is the usual wasted repair.
Petrol mixture
Haynes’ misfire split puts fuel delivery beside ignition and compression: a weak pump or a blocked filter cannot hold pressure, and injectors that are blocked or leaking do not deliver the mass the pulse width implied. A lean misfire under load, with the trims driven positive on every cylinder, points at supply: pump, filter, regulator, or a pressure sensor that lets the controller think the rail is full. A lean misfire on one cylinder points at that injector, that plug, or a vacuum leak on that runner. A rich mixture, black smoke on a direct-injection engine under load, and negative trims point at a leaking injector, a pressure regulator that will not return fuel, or a sensor over-reading the air.
Unmetered air is a fuelling fault in the result and an intake fault in the cause. Air that enters after the meter, through a split hose, a leaking manifold gasket or a brake-servo pipe, was never counted. The oxygen sensor sees the lean exhaust and adds fuel until it hits a limit. Smoke-testing the intake, and comparing trims at idle with trims at a higher load, separates a leak from a pump. A leak is worst at idle, when manifold vacuum is highest. A pump that cannot keep up is worst when demand is highest.
On direct injection the inlet valves are no longer washed by fuel. Carbon there upsets the air the cylinder actually gets, which looks like a fuelling or misfire fault. The deposit itself is described on carbon deposits and cleaning. It will not wash off with a fuel additive poured into the tank, because the fuel does not touch that face of the valve.
Cold running is supposed to be richer. A coolant-temperature sensor that reads permanently cold will enrich a hot engine and wipe the plugs. A sensor that reads hot on a cold start takes the enrichment away and the engine cranks without firing. That fault is a sensor, even though the symptom is fuel.
Diesel quantity
Diesel complaints sort by the operating state more reliably than by the noise:
- Long cranking, especially when hot, then a sudden start. Rail pressure is slow to build. Excess injector return, a high-pressure pump that cannot make the cranking pressure, a leaking rail valve, or air in the low-pressure supply all do this. The return comparison and the low-pressure checks are on the injector and pump pages.
- Starts, idles, dies when more fuel is asked for. The low-pressure side often fails this way. A blocked filter, a lift pump that flows enough at idle and not under load, or an air leak on the suction side lets the high-pressure pump cavitate once the volume rises. Transparent low-pressure hose, used in Bosch’s low-pressure test kit, is there so the bubbles can be seen rather than guessed at.
- Rough idle and a cylinder that needs a large quantity correction. That correction is the controller adding or taking away fuel to make the crankshaft accelerate evenly. It is evidence that one cylinder is not contributing. It is not yet proof the injector is the reason. Low compression, a valve that does not seal, and a glow plug on a cold engine move the same number. EDC15 diagnostic data shows the shape of that comparison on one controller family.
- Black smoke and a flat spot at low speed. Fuel without air. Boost leaks, a sticking turbo, EGR that will not shut, and an air-mass meter reading low all let the fuel arrive against less oxygen than the smoke limit assumed. The fuel system may be doing what it was told.
Water in the filter and petrol contamination are supply faults with a different ending. Water corrodes pumps and injectors. Petrol in a diesel tank lowers lubricity and destroys a high-pressure pump. Both are reasons to drain and inspect before a new pump is fitted to the same fuel.
Sensors that fake a fuelling fault
| Reading | If it is wrong |
|---|---|
| Air mass or manifold pressure | The fuel mass is calculated from a false air mass. Diesel EGR quantity moves with it as well. Compare it with a known intake, or with boost pressure measured on a gauge, before condemning injectors. |
| Rail-pressure sensor | The controller closes the pressure loop on a lie. Measured pressure at the rail, with a mechanical gauge and the safety procedure for that system, is the check. High-pressure diesel is not a job for an ordinary petrol gauge. |
| Coolant and air temperature | Cold enrichment, glow time and diesel starting quantity all move. A sensor stuck at one temperature is a common rich or no-start fault. |
| Exhaust oxygen, petrol only | Trims chase a lazy or biased sensor. A sensor that switches, and trims that sit near the centre on a hot idle, are the healthy pattern. Trims hard against a stop are the engine failing to get there, which may be the sensor or the mixture. |
| Crank and cam sync | Injection timing is lost. The engine cranks, fires late, or reports a position fault. This is not a pump fault, even though no fuel appears to arrive at the right moment. |
What links the two fuels
A fault that affects every cylinder at once is usually upstream of the injectors: pump, filter, pressure control, air measurement, or a fuel that is contaminated. A fault that tracks one cylinder is usually that injector, that spark, or that cylinder’s compression. Swapping the suspect injector or coil with a neighbour, where the engine allows it without a coding step that must be repeated, moves the test onto the other cylinder. A common-rail diesel injector is often coded to the controller. Swapping it without dealing with the coding does not produce a clean test. The injector page says what the coding is for.
After a repair, the confirmation is the same shape as the complaint. A petrol engine that was lean under load needs a wide-open-throttle pull with the trims and the rail pressure recorded, not an idle check. A diesel that would not build rail pressure when hot needs a hot restart with the pressure recorded during cranking. Idle, after a new part, hides the fault that only appeared when the system was asked to work.
Sources
- Haynes, fault code P0300. Fuel delivery beside ignition and compression as causes of a petrol misfire.
- EDC15 fuel control and tuning basics and EDC15 diagnostic data and troubleshooting on this wiki. Quantity command, limits, and cylinder balance on one diesel controller family.
- Pico Technology, back-leakage testing. Excess injector return as a cause of lost rail pressure.
- Professional Motor Mechanic, GDi carbon. Direct-injection inlet deposits that upset filling and are not reached by fuel in the tank.
- RAC, loss of power. Injectors that do not deliver the fuel the engine was metered to use.