Runs perfectly on petrol, but the moment it switches to LPG it starts jerking, losing power, or burning noticeably more fuel than before. This is the most common reason LPG drivers bring their car to the workshop. The problem almost always has a specific cause, and the diagnostic order in practice looks roughly the same every time.
LPG Phase Filters as the First Suspect
The LPG system has two filters: one for the liquid phase (before the reducer) and one for the gas phase (after the reducer, before the injectors). Both catch impurities from the fuel, and LPG quality at filling stations varies. When the filters clog, gas flow drops, the mixture runs lean, and the engine responds with jerking, power loss, and an unstable idle.
The reason filters always top the list is simple: they are the most common culprit, and replacing them is quick. Filters should be changed every 10,000-15,000 km, but many drivers neglect them and drive 40,000-50,000 km on the same set. In those cases, a filter replacement alone completely solves the problem. More on service intervals and what a routine LPG inspection covers can be found in the LPG service advice.
LPG Injectors: Dirty, Out of Calibration, or Worn
If the filters are in good shape, the next suspects are the LPG injectors. They perform millions of open-close cycles, and over time deposits build up, sealing elements wear down, and springs weaken. The result is imprecise gas dosing, so one or more cylinders get too much or too little fuel.
The symptoms resemble clogged filters but differ in one detail: with injectors, the problem is usually uneven. One cylinder runs worse than the rest, so the engine has a characteristic uneven jerk rather than a uniform power loss. Diagnostics reveal a difference in injection times between injectors, which immediately narrows the problem down.
Injectors are typically serviced with ultrasonic cleaning at 80,000-100,000 km, and a full replacement comes due at 150,000-200,000 km, depending on the system type and maintenance quality. In many cases, cleaning is enough to restore them to normal operation.
The Reducer (Vaporiser) and Why It Matters for Power
The reducer is the component that converts liquid LPG into gas before the injectors send it to the engine. For it to work correctly, it needs the right operating pressure, a healthy diaphragm, and enough heat from the engine coolant.
Reducer problems produce a specific pattern: the car runs poorly on LPG until the engine warms up (especially in winter), loses power under load, or switches back to petrol on its own. If the diaphragm is damaged or the spring has weakened, the outlet pressure fluctuates and the mixture is never stable. On larger-displacement engines (1.8, 2.0 and above), a common issue is an undersized reducer that simply cannot supply enough gas for a more powerful engine, particularly at full throttle.
Reducer health is checked by measuring the outlet pressure under load. If the pressure is out of spec, the repair usually involves replacing the diaphragm and seal kit, which brings the reducer back to factory performance.
Spark Plugs and Coils: the Petrol Side That Ruins LPG
This is the cause drivers rarely guess on their own, yet it is extremely common in practice. Worn spark plugs or weak ignition coils affect LPG operation far more than they affect petrol operation. The reason is physical: LPG requires a stronger spark to ignite the mixture than petrol does. When a spark plug is worn or a coil has weakened, the spark is strong enough for petrol but not strong enough for LPG.
The result is a car that runs perfectly (or nearly so) on petrol, but jerks, misfires, and consumes more on LPG. Many drivers assume the problem lies in the LPG system and search for faults there, when in fact replacing the spark plugs and checking the coils would fix things immediately. More on when and how to choose spark plugs can be found in the spark plug advice.
This is also why LPG map recalibration is always performed after any petrol-side service (plug replacement, coil replacement, intake cleaning). Old maps tuned to the old engine state no longer match the new state, so the car jerks on LPG even though the petrol side is now better than before.
Hoses, Fittings, and the Petrol-to-LPG Switchover
Two less common but important causes deserve attention. A leak in the LPG hoses or fittings can be small and hard to notice, yet enough to upset system pressure and cause unstable running. The smell of gas in the engine bay or around the tank is a clear signal for an urgent check.
The other symptom drivers describe is jerking or hesitation during the switchover from petrol to LPG. On a properly working system, the transition is smooth and nearly unnoticeable. If the car stumbles, briefly stalls, or jerks during the switch, the problem may lie in the switchover timing (the coolant temperature at which the ECU activates LPG), in a pressure mismatch, or in communication between the LPG and petrol ECUs.
How to Approach the Problem: the Diagnostic Order
When a driver comes in with the symptom "runs well on petrol, poorly on LPG," the workshop check follows this order:
- When was the last LPG service performed? If more than 15,000 km or a year has passed, filters and a basic recalibration are the first step.
- What is the condition of the spark plugs and coils? If they are old or mismatched, they are replaced before even looking for a fault in the LPG system.
- LPG system diagnostics: checking injectors (flow rates, injection times), the reducer (pressure under load), and mapping.
- Visual inspection of hoses, fittings, and electrical connectors.
In the majority of cases, the problem is resolved in the first two steps. Clogged filters or worn spark plugs account for more than half of all visits with this symptom. If you are not sure where to start, book an appointment and we will check the entire LPG system on the spot.