A driver shows up at the workshop, says "I want LPG on this car", and expects us to lift the hood and start drilling. It does not work that way. First we sit down, talk about the car, then the car goes up on the lift for an inspection. Every LPG installation we do at Auto Gas Gaga starts with a detailed pre-installation check, before a single new part comes out of the box. The reason is simple. We do not want to build a new fuel system on top of old problems, and the owner deserves to know exactly what shape the car is in before making the decision.
In this article I want to walk you through what we actually do during that first inspection, why each item matters for how the LPG system will run later, and how the inspection saves you headaches after the install. This is the routine we follow at our workshop in Banja Luka, on every car, every time, even when the shop is full.
Why a pre-installation inspection is not optional
LPG is not a small accessory you bolt onto an engine. It is a second fuel system that grafts onto the existing engine and shares everything with it. If something on the engine is already weak, LPG will not fix it. In most cases LPG will make it more obvious. Petrol injectors that spray unevenly will spray unevenly when the LPG ECU calibrates around them, because LPG mapping uses the petrol injection signal as its baseline. A cylinder with weak compression will run hotter and more stressed on LPG. A lambda sensor that is on its last legs will not survive a winter on gas.
A pre-installation inspection does three things at once. First, it protects you from the situation where you pay for an installation, drive away, the car runs worse than before, and then there are arguments about who is responsible. Second, it protects us, because we are not going to take the blame for problems that were already there before we touched the car. Third, it tells us whether this particular car is a real candidate for LPG, or whether we need to have a different conversation with the owner. Every serious LPG installer does this. It is not a secret routine or some made-up procedure, it is how the job has to be done if you want it done right.
Engine condition check, the foundation of everything
The first part of the inspection is the engine itself. LPG burns at a slightly higher temperature than petrol, and any weakness on the engine becomes more visible under that load. That is why a compression test is not optional, it is a baseline measurement we run before any LPG goes near the car. A compression test cylinder by cylinder tells us whether each cylinder holds pressure, and if one is noticeably lower than the others, we know it before the install rather than after.
What else we cover in this part of the inspection:
- Oil consumption check - we ask the owner how often they top up oil between services, look at the dipstick level and how clean the oil is. A car that burns oil on petrol will burn more oil on LPG.
- Spark plugs out for a look - plugs tell us everything. Color, deposits, oil contamination, corrosion, all visible. A black or oily plug is a signal that something is off, before LPG even enters the picture.
- Idle behavior and engine listening - how it holds RPM, whether it stumbles, whether there are unusual sounds from valves or belts. Experience here means more than any diagnostic machine.
- OBD scan of all stored codes - not only active codes, also history. An old lambda code, a misfire from months ago, a lean condition flag from last spring, all of these are clues that help us understand the car.
- Cooling system - coolant level, radiator condition, thermostat operation, fan function. An LPG car runs slightly hotter, so a marginal cooling system that barely makes it through summer on petrol will not survive a summer on LPG.
We do all of this before we even start talking about which reducer or which gas injectors this car will get. If the engine is healthy, we move on. If it is not, we fix it first.
The petrol fuel system has to be healthy
This is something drivers often miss, but for LPG to run well, the petrol side of the car has to be in good shape. The LPG ECU does not work blind. It watches what the petrol ECU is telling the petrol injectors, and then opens its own gas injectors in the same rhythm. If the petrol data is strange, the LPG behavior will be strange too.
So in this part of the check we measure fuel pressure on the petrol system and watch whether it stays stable. We look for leaks at the pressure regulator, check that the pump cuts out properly when it should, and check that the petrol injectors are showing a clean spray pattern. Old injectors that drip a drop after closing, or clogged ones that do not deliver full volume, cause issues on petrol and on LPG, because the gas mapping inherits all of those errors.
A fuel filter that has not been changed in years is another typical finding. A clogged filter loads the pump and creates unstable pressure, and that is the kind of thing we recommend doing alongside the LPG install while the car is already in the shop. Same with the air filter, because it affects both fuels equally. If it is black with dust, we change it before we map the LPG, so the map is built on clean readings.
Ignition, spark, and what LPG does not forgive
LPG has a narrower window in which it ignites compared to petrol. That means any weakness in the ignition system, which on petrol may go unnoticed, will create misfires, power loss, or stalling on gas. That is why we go over the ignition system carefully.
Spark plugs we check not only by appearance, but we measure the gap. We look at which type of plug is currently in the engine, because for LPG operation it often makes sense to use a slightly different heat range, depending on the manufacturer and the engine. We measure resistance on the coils, because a weak coil that just barely fires on petrol will not fire at all on gas. On older cars with a distributor we also check the leads, clean the contacts, and inspect the rotor and cap.
The goal of this part of the check is that when you drive away with LPG installed, your spark is strong and clean, because that is the difference between a car that runs perfectly on gas and one that "runs sort of okay". We do not compromise on this part. If the plugs are at the end of their life, we say they get changed with the install. If a coil is suspect, we measure it and show the owner the numbers.
Exhaust, lambda, and the catalytic converter
LPG changes the composition of the exhaust gases, so the lambda sensor and the catalytic converter take on a slightly different job from what they had on petrol. If the lambda sensor is already tired and slow to react, it will cause problems on LPG. If the catalytic converter is clogged or damaged, the LPG system will be running into strange back pressures and the mapping will be unreliable.
In this part of the check we visually inspect the cat from underneath, look for mechanical damage or impact marks. On the diagnostic tool we read live lambda data, watch how it oscillates, how fast it switches between rich and lean, and whether it gets stuck in one position. A slow lambda is a lambda on its last legs, and if we replace it before the LPG install, the map starts clean from day one.
Electrical system and space for the new equipment
The LPG system arrives with its own small ECU, its own relay, its own fuses, and its own wiring loom. All of that has to fit somewhere, and all of it needs clean voltage and clean ground. So the inspection includes the battery, alternator, ground points, and the general state of the electrical wiring.
A battery that just barely starts the car cold will not give stable voltage to a new LPG ECU. An alternator outputting below spec, the same story. In this part of the inspection we measure voltage at idle, voltage under load (lights, fan, A/C), and check that the alternator holds the right values.
We also look at physical space. Where will the LPG ECU live so it stays out of moisture and heat, where the relays will go, where the main loom will run from the ECU to the reducer and the gas injectors. Every car is different, and this part of the inspection helps us make a real plan rather than improvising with our hands inside the engine bay later. Diagnostic work with the right tools is unavoidable here, and we do it with the same equipment we use for regular fault diagnosis, more on that part on the vehicle diagnostics in Banja Luka page.
The tank, the location, and the mechanical side of the inspection
The second large part of the inspection is where the gas tank will go. The most popular solution is a toroidal tank in the spare wheel well, but that is not always possible, and not always the best solution for every car. In this part of the check we look at:
- The condition of the spare wheel well and whether it is solid enough for the tank bracket
- Alternative locations if a toroidal tank does not fit (for example a cylindrical tank in the trunk)
- Mounting points where the bracket can be fixed safely, not on metal that is already rusting through
- The space and channel for the tank vent, because that is a safety item with no compromises
- Where the external filler will sit and whether the metal around that point is solid
- The route the gas line will take from the tank to the reducer along the underbody, and whether the underside is rust-free along that path
If we find that the underbody is heavily rusted exactly where the line needs to run, or that the bracket point is rotten, that is a conversation we have with the owner before the install. We do not install LPG on a half-rusted body, because that is not safe for the driver or for the workshop.
What we typically find and how we handle it
After years of doing this at Auto Gas Gaga, we know in advance what we will likely find on most of the cars that come in for a pre-install check. Typical findings and how we handle them:
- Tired petrol injectors - we recommend ultrasonic cleaning or replacement, and this is a good moment to do it because everything is already open
- Worn spark plugs or weak coils - replaced as part of the installation itself
- Low compression on one cylinder - this is an honest conversation with the owner. Installation may still be possible, but expectations have to be realistic and we say so directly
- Active check engine light - we do not install LPG on a car with an active engine fault. We resolve the cause first, then move to LPG
- Rust around the tank mounting area - we look for an alternative location or recommend body repair before the install
- Service debt - oil that has been in the engine too long, coolant that is five years old, a filter nobody has touched, all of this we recommend doing alongside the install in the same week
The goal is that the car coming in for installation and the car leaving after installation are not just "the same car plus LPG", but "a car in better shape than it was, plus LPG". That is how trust gets built and how customers come back and send their neighbors.
How long it takes and what it means for the owner
The pre-installation inspection usually takes one to two hours, depending on the vehicle and how many things need to be checked. We do it as part of the consultation, and if the customer decides to go ahead with the installation, the inspection is already part of the process and there is no separate charge for it. If the inspection shows that the car is not a good candidate for some reason, or that serious repairs are needed first, we tell the owner exactly why, explain it clearly, and there is no charge for the inspection itself either. Honest, no surprise invoices.
After the inspection we sit down with the owner, walk through the findings, and split them into three groups. What must be fixed before LPG, what is smart to handle alongside the install while the car is in the shop, and what does not need to be touched. Then we give a proposal: which LPG system suits this engine best, where the tank will go, and what the mapping plan looks like. The owner takes the proposal home, thinks about it, comes back when ready. No pressure, no "today or never" sales talk. If you want to read about whether LPG makes sense for you in the first place, take a look at our article on whether LPG is worth it in 2026, and if you are already considering what a service visit looks like once the LPG is in, the article on what gets checked at an LPG service covers that other side of the work.
A lot of our VW, Audi, Skoda and Seat customers come to us specifically for this approach, because they know we will inspect the engine the way it needs to be inspected on those engines, and we have more on that work on our Volkswagen mechanic Banja Luka page. The installation itself is then done according to plan, no improvisation, and the customer knows up front what they are getting, with more on that whole process on the LPG installation Banja Luka page.
If you are thinking about an LPG conversion, drop by Auto Gas Gaga in Banja Luka for a proper inspection. We look the car over the way it should be looked over, explain what we find, and give you a clear picture before any part is ordered. No sales pressure, just thirty years of experience and an honest conversation about the shape your car is in and what LPG can really do for it.