One of the most common situations at our workshop goes like this: a driver walks in, says the car runs poorly on gas, and wants us to fix the LPG installation. The trouble is that in a large number of cases the fault is not in the gas at all, it is in the engine. And sometimes it works the other way around, drivers spend months chasing engine faults when the LPG system is the actual culprit. Telling one from the other is the first and most important step in diagnosing any LPG-converted car.
If you are wondering whether what you are experiencing is even a fault or just a normal difference in how engines behave on gas versus petrol (sound, slight power variation), that is a separate topic we covered in our guide to operating differences between petrol and LPG. Here, we are strictly talking about faults and how to correctly attribute them.
The Petrol Switch Test
This is the foundation of everything. Switch the car to petrol and drive it long enough to reproduce the conditions under which you noticed the problem. Five minutes is not always enough. If the fault only appears once the engine is warm, drive on petrol until the engine reaches operating temperature. If it only shows up under hard acceleration, test that same acceleration on petrol. If it happens on a cold start, start the car the next morning and let it run on petrol from the very beginning.
The key is to test under the same conditions where the problem appears. If the problem disappears the moment you switch to petrol, the fault is in the LPG system. If it persists on petrol, the engine is at fault and gas has nothing to do with it. This single test eliminates at least half of the wrong assumptions that drivers bring into the shop.
Faults That Are Exclusively on the LPG Side
When the petrol test shows the engine running cleanly, the problem is somewhere in the gas system. Here is where we look first.
Vapour-phase and liquid-phase filters are the first item to check. Gas filters trap impurities from the fuel. When they clog, flow drops and the engine does not get enough fuel on gas. Replacing filters is straightforward and inexpensive, yet it resolves a surprisingly large number of complaints about poor running.
The reducer (vaporiser) converts liquid LPG into gas. When the diaphragm inside weakens or freezes in cold conditions, pressure becomes unstable. The car may jerk, stall, or struggle to switch over. The reducer requires regular LPG system servicing that includes diaphragm and filter replacement.
LPG injectors clog or start passing gas unevenly over time. Individual cylinders receive too much or too little fuel, causing jerking, rough running, and increased consumption. Cleaning or replacing the injectors restores normal operation.
Calibration and mapping are the software side. When maps drift from optimal values, the mixture goes off and the engine runs poorly on gas even though the hardware is perfectly fine. A recalibration with diagnostic software usually resolves it.
Delayed or failed switchover occurs when the temperature sensor on the reducer malfunctions or switchover parameters are incorrectly set. The car stays on petrol longer than it should, or never switches at all.
Gas level and sensor issues arise when the sensor inside the tank gives incorrect readings. The car behaves as if the tank is empty when it is not, or vice versa, and the LPG controller may switch back to petrol as a safety measure.
For a deeper look at every situation where a car runs poorly on gas specifically, see our guide to poor LPG running.
Engine Faults That Drivers Wrongly Blame on LPG
This is probably the most important section of this article. There is an entire category of engine faults that are amplified on gas, leading drivers to conclude that LPG is at fault when the same problem would exist without a gas installation at all.
Spark plugs and ignition coils are the classic example. LPG burns more slowly than petrol and demands a stronger, more stable spark. When a plug starts to weaken or a coil loses output, the engine can compensate on petrol because petrol ignites more easily. On gas, that margin vanishes, a cylinder starts misfiring, and the driver assumes gas is the problem. In reality, the plug is worn and needs replacing regardless of fuel type.
Compression and valves are another frequent cause. An engine with low compression or valves that do not seal properly runs adequately on petrol, but on gas the difference becomes dramatic. On older cylinder heads, valve seats can deteriorate, especially on the exhaust side where LPG raises operating temperatures. But the root cause is mechanical, not gas-related.
Lambda sensor and catalytic converter affect mixture regulation on both fuels. A faulty lambda sensor sends incorrect data and the engine ECU miscorrects the fuel quantity. This shows up on petrol and gas alike, but on gas it becomes noticeable sooner because the system is more sensitive to deviations.
Intake leaks and false air directly upset the fuel-to-air ratio. A crack in the intake manifold, a damaged hose, or a faulty valve lets in unmetered air. On gas, this is felt more strongly because the LPG system does not have the same compensating mechanisms as petrol injection.
Overlap, When Both Systems Are at Fault
Sometimes the situation is more layered. An engine with borderline spark plugs runs acceptably on petrol. An LPG system that is slightly out of calibration works acceptably on its own. But put a less-than-perfect engine together with a slightly drifted gas system, and the combined result is noticeably rough running.
These are the cases where the petrol switch test alone can mislead you. The car runs poorly on gas, you switch to petrol and it runs "well enough", so you conclude gas is the problem. In truth, both need attention. There is no point calibrating the LPG system on an engine that does not have healthy ignition or adequate compression. Fix the engine first, then the gas system.
How We Diagnose at Our Workshop
When a driver comes in with a "runs poorly on gas" complaint, our process is systematic. First we test the car on petrol under the same conditions where the fault appears. Then we read fault codes from both systems, the engine ECU and the LPG controller. We watch live sensor data while the engine runs on petrol, then while it runs on gas, and compare. Specifically, we track lambda values, injector pulse widths, temperatures, reducer pressure, and injector operation.
This kind of vehicle diagnostics gives a clear answer about where the fault lies. It might be the engine only, the gas only, or both. Only once that is established do we move to the repair. Any other approach is guesswork, and guesswork means replacing parts that are not faulty and wasting time.
If you are experiencing any of the symptoms described above, do not guess on your own. Bring the car in for an inspection at our LPG service centre, or contact us to arrange an appointment.