A petrol engine that runs badly is usually short of spark, short of the right amount of fuel, or mechanically leaking compression. Owner reports cluster around ignition misfires, direct-injection inlet deposits, fuel pumps that cannot hold pressure under load, and cooling-system faults that end as a head gasket. Diesels dominate the EGR and particulate-filter threads, but petrol engines have both systems too, and they fail for different reasons. This page is the index. The linked pages are the diagnosis, and several of them are the same pages used for diesel.
There is no single league table behind the list. The grouping is the one that workshop guidance and manufacturer system descriptions keep returning to: spark, mixture, air leaks, carbon on engines that no longer wash the inlet valves with fuel, and the cooling system. An engine family can add its own fault, a cam-phaser or a chain, that is not a petrol-wide rule.
Where to start
| What the driver reports | Look here first | Easy to confuse with |
|---|---|---|
| Misfire, flashing emissions lamp, shudder under load | Ignition, then fuelling and compression | A coil blamed for a burnt valve, a vacuum leak, or a head gasket between two cylinders |
| Starts, then starves when the demand rises; worse with a low tank or when hot | Fuel pumps | A blocked filter, a pressure regulator, a sensor reporting a pressure the rail does not have |
| One cylinder lean or rich, fuel smell, poor idle | Injectors | A vacuum leak on that runner, low rail pressure to every cylinder |
| Direct-injection engine, gradual loss of idle quality, misfires, no port-injector to clean the valves | Carbon deposits | A burnt exhaust valve, which is a different part and a different test |
| Temperature gauge high, coolant loss, heater cold, hose stays cold or stays hot | Overheating and coolant | A head gasket or a cracked head pressurising the system |
| White exhaust that does not clear, mayonnaise that is actually oil and coolant, bubbles in the header tank | Head gasket | Short-trip condensation on the oil cap, a leaking EGR cooler where one is fitted |
| Ticking or misfire on one cylinder, air at the tailpipe on a leak-down test | Burnt valves | Wrong cam timing, carbon on an inlet valve, a gasket between cylinders |
| Rough idle, part-load hesitation, EGR flow fault | EGR | A vacuum leak, an ignition misfire, carbon in the intake |
Spark first, then fuel, then compression
A misfire is a cylinder that did not burn its charge. Haynes groups the ordinary causes as worn or wrongly specified plugs, a coil that cannot deliver the spark, a vacuum leak, a weak pump or blocked filter, injectors, carbon on a direct-injection engine, and compression loss from rings, valves or the head gasket. The order matters. Plugs and coils are the common end of that list. A compression fault is the expensive end, and no coil will fix it.
The hardware and the five-gas pattern of an ignition fault are on ignition systems and spark plugs. A misfire that is still there after the spark is moved to another cylinder is no longer an ignition-parts story. Fuel for that cylinder, air that did not go through the meter, and compression are next. Fuelling faults is the shared page for mixture errors on both fuels.
A sustained misfire dumps unburnt fuel into the catalyst. That is a reason to stop using the car, not a separate “exhaust fault” to tune away. The catalyst page is the emissions system on petrol and diesel emissions.
Pumps and injectors
Port injection has one pump doing the work the injectors see, an in-tank electric pump, with a regulator or a controlled pump setting the rail pressure. Direct injection adds a mechanically driven high-pressure pump. Bosch’s current gasoline direct-injection description feeds that pump at up to 6 bar and then raises rail pressure to as much as 350 bar. An early MED 7 system was specified much lower, about 3 bar on the low side and 50 to 100 bar in the rail, so the number on a gauge is only meaningful against the procedure for that engine. Both layouts, and the diesel pumps, are on fuel pump faults.
A pump that is weak usually leans out every cylinder when demand rises. An injector usually shows on one cylinder, as a balance error, a leak, or a spray that has collapsed. Direct-injection tips also coke after shutdown because the residual heat cooks the fuel left in the nozzle. Professional Motor Mechanic describes that path, and the matching one on the inlet valves: the fuel no longer washes the valve, so oil mist from the breather bakes on. Injector diagnosis is on injector faults. Valve deposits are on carbon deposits and cleaning, and they are not the same finding as a burnt exhaust valve.
Cooling and the head gasket
A petrol engine that overheats is often a thermostat stuck closed, a pump that no longer moves coolant, a cap that will not hold pressure, a fan that does not run, a radiator that cannot reject heat, or a low level. Gates’ cooling guidance separates the two thermostat failures: stuck closed and the coolant never reaches the radiator; stuck open and the engine runs cool, uses more fuel, emits more, and the heater stays weak. The shared write-up is engine overheating and coolant faults.
Overheating is also how many head gaskets fail, and a gasket that has already failed will overheat the engine again by pushing combustion gas into the coolant. White smoke that continues after the engine is warm, coolant loss with no external leak, and a combustion-gas test that changes colour are the signs. Emulsion on the oil filler cap is not, by itself, a gasket: short trips leave condensation there. The tests that separate a gasket, a cracked head, burnt valves and worn rings are on head gasket failure.
EGR, filters and boost
Petrol EGR is there to cut NOx and, at part load, fuel use. HELLA’s published maximum rates are lower for a port-injected gasoline engine than for a diesel, and higher again for some direct-injection operation. The valve still sticks, and the vacuum pipes on the older pneumatic valves still leak. Because petrol exhaust is hotter and, on a stoichiometric engine, less sooty, the failure does not look like a diesel intake full of wet soot, but the tests are the same idea: did the valve move, and did the flow change. See EGR faults.
Direct-injection petrol engines make enough particles that later ones carry a particulate filter. It is not a diesel filter with a different badge. Petrol exhaust is hotter, and a close-coupled filter often regenerates without the town-driving crisis the AA describes for diesel cars. The differences, and the diesel case, are on particulate filter faults.
Downsized turbocharged petrol engines add boost leaks, wastegates and diverter valves to the misfire list. A split hose or a wastegate that does not seal leans the story toward “no pull” rather than “one cylinder”. Charger layouts are on forced induction. Charge temperature and why it matters to knock are on charge cooling.
Shared with diesel
Common diesel engine problems links to the same shared pages. Use the petrol column when the engine is spark-ignited:
- Engine overheating and coolant faults
- Head gasket failure
- Fuelling faults
- Fuel pump faults
- Injector faults
- Burnt valves
- EGR faults
- Particulate filter faults
Sources
- Haynes, fault code P0300. The ordinary misfire split: plugs, coils, vacuum leaks, fuel delivery, carbon on direct injection, and compression loss.
- Bosch Mobility, gasoline direct injection. Low-pressure supply up to 6 bar and rail pressure up to 350 bar on current systems.
- Volkswagen SSP 253, Bosch MED 7 direct petrol injection. One early system: about 3 bar, and up to 5.8 bar on a hot start, then 50 to 100 bar in the rail.
- Professional Motor Mechanic, carbon build-up in GDi engines. Inlet valves that fuel no longer washes, and injector-tip deposits after shutdown.
- Gates, engine cooling manual. Thermostat stuck open and stuck closed.
- HELLA, exhaust gas recirculation. Why petrol engines use EGR, and how the rates differ from diesel.
- Petrol and diesel emissions and ignition systems and spark plugs on this wiki.