Buying a car with LPG: what to check besides the gas equipment itself

A car with LPG equipment gets inspected mostly from one angle: is the system itself sound, does it leak, is the paperwork in order. That is correct, and it is half the job.

The other half is what the gas did to the engine over its service life. And that is where the difference lies between a good purchase and a cylinder head rebuild.

How OBD reads gas system operation is covered in its own article. This one is about the consequences for the engine and the inspection order.

The main point: valves and seats

The most important item, and the one almost nobody checks.

Why it happens. Gas burns at a higher temperature and lacks petrol’s lubricating properties. Exhaust valves and seats run hotter and without the film liquid fuel provided.

The consequence: accelerated seat wear, reduced valve clearance, and in advanced cases a burnt valve.

What that means for a buyer: an engine running on gas needs regular valve clearance checks, and on engines without hydraulic lifters that is critical. Question for the seller: when were the clearances last adjusted, and is there paperwork.

What diagnostics shows: directly, nothing. Indirectly, misfires P0300–P0304 and rough idle. The search order is in misfire troubleshooting.

What to check on site: a cold start and idle smoothness. Compression separately at a workshop — on this kind of car that is not a wasted expense.

The catalyst

The second part that suffers.

A poorly tuned gas system produces a lean or rich mixture and raised exhaust temperature. The catalyst operates in a regime it was not designed for.

What to look at: code P0420, the downstream oxygen sensor signal, the catalyst monitor.

Do not rush to conclusions: P0420 has several causes, and the catalyst is not the first of them. The verification order is in the P0420 guide.

Spark plugs

Gas requires a different heat range and a smaller gap. On gas, plugs live shorter lives and need replacing more often.

A sign of a careless owner: plugs fitted “as for petrol” with standard replacement intervals. That leads directly to misfires and to stress on the coils.

Fuel trims and the changeover

The most informative thing you can check in ten minutes.

What to look at on petrol: long-term trim should sit within ±5 %. A steady offset means the system is set up wrongly or there is a vacuum leak. How to read them is in STFT and LTFT.

What to look at on gas: warm the car, wait for the changeover to gas, and read the trims again. A sharp shift at changeover indicates poor calibration.

The petrol → gas transition should be imperceptible. Hesitation, a jolt, or a stall at changeover is grounds to negotiate or walk away.

What to check in the equipment itself

Briefly, because this is the obvious part:

1. The tank’s expiry date. It is limited, and after expiry the tank must be replaced or recertified. The date is stamped on the tank.
2. Installation paperwork and a registration entry where the law requires one.
3. Leaks — smell in the boot and around the reducer.
4. Hose and mounting condition.
5. Date of the last service on the reducer and filters.

The inspection order

1. Cold start on petrol. Listen to the valvetrain and general smoothness.
2. Read all codes, pending and permanent included.
3. Trims on petrol — at idle and at 2500 RPM.
4. Warm up and wait for the changeover to gas.
5. Trims on gas — compare against the petrol figures.
6. Per-cylinder misfire counters.
7. Readiness monitors — all not ready means the memory was just cleared.
8. Test drive on gas and on petrol separately.
9. The date on the tank and the paperwork.

The general protocol for any used car is in its own article.

What this means for the price

The honest arithmetic.

The upside: the fuel saving is real, and over high mileage it is substantial.

The downside: a car with LPG costs less than an equivalent without it — and not only because of demand. The market prices in the risk to valves and catalyst.

Practical conclusion: if the seller produces records of regular valve clearance adjustment and system servicing, this is a normal purchase. If there is no paperwork and “we never did anything, it just drives”, budget for cylinder head work.

In short

• The main risk is not the equipment but the valves and seats: gas burns hotter and without lubrication.
• Question one for the seller is when the valve clearances were adjusted and whether records exist.
• The catalyst suffers from poor calibration; P0420 is a familiar guest here.
• Compare trims on petrol against trims on gas — a sharp shift means poor setup.
• The tank has a limited service date; check it before the deal, not after.

Download the app — codes, fuel trims, and misfire counters are read with an ELM327 adapter over Bluetooth SPP on Android.