Your plug-in hybrid could be saving you money at the pump. But if you’re never using the petrol engine, you might be creating another problem.
There’s a particular satisfaction in driving past a petrol station when everyone else seems to be queuing for a refill. Your plug-in hybrid (PHEV) is charged, your daily commute is running on electricity, and that little petrol pump icon on the dashboard barely seems to move. Job done, right? Well, not quite.
There’s a catch that doesn’t get nearly as much attention as electric range, charging speeds or how many kilometres you can squeeze out of a kilowatt-hour. Petrol doesn’t stay fresh forever, and an engine that rarely runs still needs looking after.
If you’re a PHEV owner who hasn’t visited a petrol station in months, or whose engine seems to have become little more than an expensive passenger, it’s worth understanding what’s happening under the bonnet. The golden rule? Plug in, enjoy the electric kilometres, but don’t forget the fuel and engine that make your PHEV a hybrid in the first place.
The beauty of a PHEV – and its hidden complication
Let’s start with why plug-in hybrids make so much sense for many Kiwi drivers. Unlike a conventional hybrid, which generates most of its own electrical energy through the engine and regenerative braking, a PHEV can be plugged into an external power source to recharge its traction battery.
That means you can charge at home overnight, head to work on electric power, run errands around town and potentially complete most of your weekly driving without using much petrol at all. Depending on the vehicle, battery capacity, driving conditions and journey length, that can mean a substantial reduction in fuel consumption.
For someone with a short daily commute and access to home charging, the savings can be particularly appealing. You’re using electricity for the journeys it handles well, while keeping a combustion engine available for longer road trips, towing or those spontaneous weekends away.
And that’s precisely the point: a PHEV is designed to offer the benefits of electric driving without completely abandoning the convenience of liquid fuel. The problem begins when owners become so good at driving electrically that they almost never use the other half of the drivetrain.
A PHEV can spend weeks, or even months, using very little petrol. The fuel remains in the tank, the engine sits idle, and the owner happily watches the electricity savings accumulate. Unfortunately, the engine and fuel system don’t get to retire just because the battery is doing all the work.
Petrol has a shelf life. Yes, even in your car.
Petrol is a mixture of hydrocarbons designed to deliver energy efficiently when it burns. But it’s not chemically unchanged forever. Over time, exposure to oxygen, temperature fluctuations and moisture can cause petrol to deteriorate. Volatile components can evaporate, while oxidation can create gums and varnish-like deposits. In simple terms, the fuel gradually becomes less suitable for doing the job it was designed to do.
How quickly that happens depends on factors such as fuel formulation, temperature, storage conditions, tank design and how much fresh fuel is added. There’s no universal expiry date that applies to every PHEV. However, fuel left sitting for months deserves attention, particularly when the car is routinely driven in EV mode.
Modern petrol can contain ethanol, depending on the fuel grade and market. Ethanol can attract moisture, and under certain conditions water contamination and phase separation can become concerns. These effects aren’t inevitable in every tank, but they are another reason not to assume that petrol remains perfect indefinitely.
And no, petrol doesn’t suddenly turn into something resembling treacle on the day it gets old. Fuel degradation is a gradual process, and the severity varies. The important point is that petrol which has been sitting for a long time may not perform as reliably as fresh fuel.
What happens when petrol goes stale?
As fuel degrades, its volatility and combustion characteristics can change. Oxidation may also produce deposits that affect parts of the fuel system.
Depending on the condition of the fuel and the vehicle, potential symptoms include:
- Hard starting: The engine may struggle to start or take longer to fire.
- Rough running: Combustion may become less consistent, causing uneven running or hesitation.
- Reduced performance: The engine may not deliver its usual smooth response under acceleration.
- Fuel-system deposits: Gum and varnish can contribute to fouling of injectors and other fuel-system components.
- Fuel-system damage: Severe contamination or corrosion can affect pumps, injectors and other components, potentially leading to expensive repairs.
In a PHEV, the consequences can be especially frustrating because the petrol engine may be called upon unexpectedly. You might be happily cruising along in electric mode when the vehicle decides it needs the engine for additional power, a particular operating condition or a manufacturer-controlled maintenance routine. If the fuel has deteriorated, that petrol engine may not be quite as ready for action as you expect.
The solution isn’t to panic if your petrol gauge hasn’t moved for a few weeks. It’s to understand that fuel age matters and follow the manufacturer’s advice about fuel replacement and minimum fuel usage.
Your petrol engine needs exercise, too
Here’s the second half of the story. Even if the petrol in your tank remains usable, leaving the combustion engine unused for extended periods isn’t necessarily ideal. An internal combustion engine is a collection of moving parts, lubricated surfaces, seals, cooling passages, fuel-system components and emissions-control equipment. It is designed to operate, reach appropriate temperatures and receive maintenance throughout its service life.
When it rarely runs, some of those systems may not experience their normal operating conditions.
1. Engine oil needs the right operating temperature
Engine oil lubricates moving components and helps protect them from wear. It also carries away heat and helps manage contaminants. When an engine runs long enough to reach its intended operating temperature, moisture and some fuel contamination in the oil can be reduced through normal operation and ventilation.
Short engine runs are a different story. If the engine starts briefly, operates for a short period and shuts down before reaching the appropriate temperature, moisture and fuel can accumulate in the oil rather than being adequately removed. Over time, that can contribute to oil degradation and reduced lubrication performance.
Some PHEVs deliberately start their engines for short periods under particular conditions. That’s not automatically harmful; the vehicle’s control system may be following a specific operating strategy. However, owners shouldn’t assume that occasional brief starts are equivalent to regular, fully warmed engine operation.
Follow the vehicle’s service schedule and any engine or oil-maintenance messages. Don’t invent your own engine-running routine without checking the handbook.
2. Lubrication doesn’t mean an engine should run indefinitely without maintenance
Engine oil degrades with time as well as use. Filters, seals and other components also have maintenance requirements. A PHEV’s engine may accumulate relatively few kilometres while still being subject to calendar-based servicing requirements. That means a low odometer reading isn’t necessarily a reason to postpone maintenance indefinitely.
If your vehicle’s manufacturer specifies servicing by time or distance, follow whichever interval applies first. The fact that the engine has barely run doesn’t automatically mean the oil is fresh or that every service can be skipped.
3. The fuel system still needs to be in good condition
Fuel pumps, injectors and associated components are still part of the vehicle, even when the electric motor handles most of the driving. Prolonged periods with ageing fuel can increase the risk of deposits and other fuel-quality problems. In some circumstances, moisture contamination may also contribute to corrosion. Regular fuel turnover, where appropriate for the vehicle, helps reduce the likelihood of relying on very old petrol when the engine is eventually needed.
4. Exhaust and emissions equipment need to do their job
A PHEV’s combustion engine is connected to an exhaust system and emissions-control equipment, which may include a catalytic converter and oxygen sensors. These systems are designed to operate as part of the engine’s normal combustion and temperature-management strategy. Repeatedly starting an engine and shutting it down before it reaches the appropriate operating conditions can be less favourable than a normal operating cycle. The exact implications depend on the powertrain and its control systems.
This doesn’t mean every PHEV engine needs to be taken for a weekly motorway blast. It means the car’s programmed engine-management strategy and manufacturer guidance should be allowed to do their jobs.
5. Don’t mistake an engine that rarely runs for an engine that needs no attention
This is perhaps the biggest misconception of all. A battery-electric vehicle doesn’t have a petrol engine to maintain. A PHEV does. Even if you use almost no petrol, you still have an engine, engine oil, a fuel system, an exhaust system and associated components that may require inspection and servicing. Your PHEV hasn’t become a full EV just because you’ve mastered the art of charging it.
But surely the car will start the engine when it needs to?
Often, yes. And that’s one of the clever things about modern PHEVs. Depending on the model, the vehicle may start its combustion engine when the battery’s available charge falls, when additional performance is required, when operating conditions demand it, or when the system determines that engine operation is necessary.
Some vehicles also have fuel-refresh strategies or maintenance modes intended to manage extended periods of electric-only driving. But these systems differ between manufacturers, models and model years. Not every PHEV behaves in the same way, and automatic engine operation shouldn’t be treated as a universal guarantee that old fuel will always be refreshed before it causes trouble.
Your owner’s manual is the authority here. If your dashboard displays a fuel-refresh, engine-maintenance or oil-maintenance message, don’t simply ignore it because you still have plenty of battery range. And if the vehicle requires a certain quantity of fresh fuel or specifies a particular refuelling interval, follow that instruction rather than relying on advice from someone with a different make of PHEV.
So, how often should you use the petrol engine?
There isn’t one magic rule that applies to every plug-in hybrid. Fuel formulations, tank designs, engine-management systems and manufacturer recommendations vary. Some owners’ manuals specify minimum quantities of fresh fuel over a particular period. Others describe engine-running or fuel-refresh functions. A Toyota PHEV manual, for example, may specify a minimum amount of refuelling over a 12-month period. That is a model-specific instruction, not a universal rule for all PHEVs.
Here’s the practical approach.
First, read your owner’s manual. Look for information about fuel ageing, fuel-refresh modes, long periods of EV-only operation and engine maintenance.
Second, keep track of fuel age. If you routinely use electricity for nearly every trip, make a note of when you last added petrol. That way, you’re less likely to discover that the same tank has been sitting there for a year.
Third, use fresh fuel when the manufacturer recommends it. Don’t wait until the engine develops a problem before checking whether the petrol has been sitting too long.
Fourth, let the vehicle complete any prescribed engine-maintenance cycle. If your PHEV temporarily starts the engine or limits EV-only operation to refresh fuel or meet another maintenance requirement, don’t assume something is broken.
Finally, keep up with scheduled servicing. Follow the manufacturer’s time- and distance-based maintenance requirements, even if the combustion engine has seen very little use.
If the fuel is already very old, contaminated or associated with poor engine running, seek advice from an authorised dealer or qualified technician before attempting to solve the problem by simply adding more petrol.
Should you deliberately drive in hybrid mode?
Potentially, but there’s a difference between sensible use and burning petrol for the sake of it. If your PHEV offers a hybrid mode, it may be useful on longer journeys where the vehicle’s control system can make effective use of both power sources. Depending on the model, you may also be able to select a mode that preserves battery charge for later use.
However, forcing the engine to run simply because you believe every PHEV needs a weekly petrol workout isn’t necessarily the best approach. The engine may run inefficiently when cold, and unnecessarily consuming fuel can undermine the financial and environmental benefits of electric driving.
Instead, use the vehicle as intended. Charge it regularly. Enjoy electric driving around town. On longer trips, use an appropriate driving mode and let the hybrid system manage the balance of power where practical. If the manufacturer recommends a particular procedure to exercise the engine or refresh fuel, follow that procedure. Don’t assume a short drive around the block will achieve the same thing. The aim is to keep the engine and fuel system healthy without wasting petrol.
The sweet spot: electricity for everyday driving, petrol for flexibility
For most PHEV owners, the best ownership strategy is refreshingly straightforward. Use electricity wherever it makes sense, particularly for regular journeys that fit within the vehicle’s electric range. Charging at home can make everyday running costs attractive, while regenerative braking can recover some energy during deceleration. Then, when a longer trip comes along, enjoy the freedom of having a combustion engine available.
Just remember that the petrol engine is still part of the package. Fuel needs to remain in usable condition, the engine needs to be maintained, and the manufacturer’s instructions still apply. If you’re considering a PHEV, this is worth factoring into your decision, too. Plug-in hybrids tend to make the most sense when owners can charge regularly and also undertake journeys that make useful use of their combined powertrain.
If you can charge at home, drive relatively short distances during the week and occasionally head away for a long weekend, a PHEV could be a very practical option. If you never plug it in, you’re likely missing much of the potential efficiency benefit. If you never use the petrol engine, you’re creating a different set of maintenance considerations. Neither extreme is the ideal way to enjoy the technology.
Tarmac Takeaway – plug in, but don’t forget the pump
PHEVs offer an appealing middle ground between conventional petrol-powered vehicles and full battery-electric cars. They can reduce petrol consumption significantly for drivers who charge consistently and make the most of electric range. But that doesn’t mean petrol has become an optional afterthought.
Fuel can deteriorate while it sits in the tank, and the engine still needs appropriate operation and maintenance. The consequences of neglect can range from poor running to fuel-system problems and potentially expensive repairs. So, if you’re a PHEV owner who proudly boasts that you haven’t bought petrol for months, congratulations on making the most of electric driving.
Now check your owner’s manual. Your car might be perfectly happy with your driving habits, thanks to its built-in fuel and engine-management strategies. Or it might have specific requirements that you’ve overlooked.
Plug it in. Drive it electrically. Enjoy the savings. But keep the petrol fresh, give the engine the attention it needs, and don’t confuse a plug-in hybrid with a car that no longer needs an engine. After all, the whole point of a PHEV is having the best of both worlds. It would be a shame to neglect one of them.
