Car Additives and Flushes Buying Guide

A car additive changes or supplements the fuel, oil or coolant already in the vehicle, while a flush is normally used to clean a system before the old fluid is drained.

Different treatments perform very different jobs. Some contain detergents that remove deposits. Others improve corrosion protection, stabilise stored fuel, alter fuel combustion or help seals remain flexible.

An additive may help when contamination, deposits or deteriorating fluid are causing a problem. It cannot repair a cracked component, damaged injector, failed sensor, split hose, worn bearing or physically blocked filter.

A fuel-system cleaner removes or reduces deposits in parts of the fuel system, potentially including injectors, intake valves, carburettors and combustion areas.

Deposits can disturb fuel delivery and combustion. This may contribute to hesitation, an uneven idle, reduced throttle response or gradually worsening fuel economy.

Most fuel-system cleaners are poured into the tank before refuelling. Filling the tank then helps mix the treatment through the fuel. The treated fuel carries the cleaner through the system during normal driving.

A general fuel-system cleaner is most useful when:

• the vehicle is running reasonably well but feels less responsive than before;
• it has spent a long time making short journeys;
• it has been stored or used infrequently;
• there is no clear evidence that one particular component has failed;
• an all-round preventative clean is preferred to a narrowly targeted treatment.

For an all-round petrol treatment, consider Liqui Moly Fuel System Treatment.

Diesel owners wanting a multi-dose maintenance treatment can consider Millers Oils Diesel Power ECOMAX .

Browse the complete range of fuel-system cleaners .

An injector cleaner concentrates on deposits that can interfere with an injector's fuel flow and spray pattern.

Injectors must deliver accurately measured fuel in a controlled spray. Contamination around the injector tip can affect that spray, contributing to poor combustion, hesitation, rough idle, smoke or reduced performance.

Browse all petrol and diesel injector cleaners .

A diesel additive may clean injectors, improve fuel lubricity, protect against corrosion, support cleaner combustion or raise the fuel's cetane rating.

Not every product does all of these things. Some are simple maintenance cleaners, while others are concentrated performance treatments or products intended for commercial vehicles.

Regular diesel treatments are most relevant for:

• high-mileage cars and vans;
• vehicles making frequent short journeys;
• seasonal vehicles or machinery stored with fuel in the tank;
• older fuel systems that may benefit from additional lubricity;
• drivers who prefer preventative treatment rather than occasional intensive cleaning.

Millers Oils Diesel Power ECOMAX Multishot is intended for repeated dosing and combines injector cleaning, lubricity and cetane-improving functions - one of our most popular additives.

For a single tank, consider Millers Oils Diesel Power ECOMAX One Shot Boost .

Browse all diesel additives and cetane boosters .

A fuel-borne DPF cleaner helps soot burn at a lower temperature, making it easier for a diesel particulate filter to complete a regeneration.

A DPF traps soot from diesel exhaust. The vehicle must periodically raise the exhaust temperature and burn that soot away. Repeated short journeys can prevent this process from completing.

A DPF treatment may be useful:

• preventatively on a diesel that regularly makes short journeys;
• before a sustained drive under suitable regeneration conditions;
• where soot loading is at an early or moderate stage;
• after the cause of repeated failed regenerations has been corrected.

A suitable fuel-tank treatment is Millers Oils DPF Cleaner and Regenerator .

Browse all DPF cleaners and regenerators .

An EGR or intake cleaner breaks down carbon and oily deposits in the air-intake and exhaust-gas recirculation system.

Exhaust soot can combine with oil vapour in the intake and create a thick deposit. Over time, this can interfere with EGR movement and restrict airflow.

Possible symptoms include:

• hesitation or poor low-speed response;
• sluggish acceleration;
• black diesel smoke;
• an EGR-related fault code;
• uneven running caused by restricted airflow.

Products such as NAPA EGR Cleaner are applied to the intake rather than simply poured into the fuel tank.

Browse all EGR and intake cleaners .

A heavily restricted manifold or physically seized EGR valve may need to be removed and cleaned manually.

Turbo and exhaust cleaners target deposits in components such as turbocharger vanes, catalytic converters, particulate filters and associated exhaust passages.

Products in this group are not interchangeable. Depending on the formulation, a cleaner may be:

• added to petrol or diesel fuel;
• sprayed through the air intake;
• applied directly to a DPF or GPF;
• intended principally for workshop use.

Check whether the treatment is specifically intended for petrol, diesel, DPF, GPF, turbocharger or catalytic-converter use.

Browse the range of turbo and exhaust-system cleaners .

A general petrol treatment may combine light injector cleaning, corrosion protection, moisture control and fuel stabilisation in one maintenance product.

It is a practical option for cars that are used infrequently, make repeated short journeys or would benefit from preventative fuel-system maintenance.

A multi-dose option is Millers Oils Petrol Power ECOMAX .

Browse all petrol fuel additives and octane boosters .

Classic-car additives can provide valve-seat protection, protect against the effects of ethanol-containing petrol and stabilise fuel during storage.

An engine flush is added to the old oil shortly before an oil change to loosen deposits and suspend contamination so it can leave with the drained oil.

The engine is normally run at idle for the period stated by the manufacturer. The old oil is then drained while warm and the oil filter is replaced.

Suitable examples include:

• Liqui Moly Engine Flush Plus
• Millers Oils Engine Flush

Browse all oil flushes and engine-oil additives .

A leave-in oil additive remains in the sump with the engine oil and is intended to alter properties such as detergency, friction, seal conditioning or film strength.

Products in this group may be designed to:

• reduce friction or wear;
• condition ageing seals;
• reduce light oil consumption;
• quieten hydraulic tappets;
• clean varnish gradually;
• provide additional protection in competition use.

Modern engine oils are balanced formulations. Adding another chemical package can change viscosity, friction characteristics and additive concentration.

This is particularly important in engines with wet timing belts, DPFs, GPFs, sensitive variable-valve-timing systems or strict manufacturer oil specifications.

Use an oil additive for a defined purpose. Do not use it to make an unsuitable engine oil appear suitable.

A cooling-system cleaner loosens rust, scale, old coolant deposits and oily contamination before the cooling system is drained and refilled.

A cooling-system cleaner is useful when:

• the old coolant is rusty, cloudy or contaminated;
• the coolant type is being changed;
• a radiator, heater matrix or water pump is being replaced;
• the vehicle has suffered from poor coolant maintenance;
• oil contamination entered the system and its mechanical cause has been repaired.

A suitable example is Liqui Moly Radiator Cleaner .

After cleaning, drain and rinse the system as instructed. Refill with coolant meeting the vehicle manufacturer's required specification. Coolant colour alone does not prove compatibility.

Browse all cooling-system cleaners, enhancers and sealants .

Start with a petrol injector cleaner or complete petrol fuel-system cleaner if the symptoms are mild and deposit-related.

Misfires, warning lights or persistent rough running can also be caused by spark plugs, ignition coils, air leaks, sensors or mechanical faults. Do not continue adding cleaner if the first correctly dosed treatment makes no difference.

Use a diesel injector or fuel-system cleaner where deposits are suspected, but investigate smoke, knocking or severe poor running promptly.

Diesel symptoms can also come from injector wear, boost leaks, turbocharger problems, EGR faults, fuel-pressure faults or excessive DPF loading.

A preventative DPF treatment may help support regeneration, but the vehicle must still be driven under conditions that allow the exhaust to become sufficiently hot.

No additive can compensate indefinitely for a diesel that never completes a regeneration.

Check the handbook first. A DPF cleaner may support an early regeneration warning, but severe warnings or restricted performance require diagnosis.

An engine flush is the appropriate pre-oil-change treatment where deposit cleaning is required.

Add it to the old oil, run the engine exactly as directed, then drain the oil and fit a new filter. Do not confuse a short-duration flush with an additive intended to remain in fresh oil.

A cooling-system sealant may help with a very minor seepage, but locating the leak should come first.

A leaking hose, water pump, expansion tank or head gasket needs a proper repair.

Use a fuel stabiliser or ethanol-protection treatment suitable for the vehicle, added to fresh fuel before the final drive.

Use lead replacement as well only if the engine requires valve-seat protection and has not been converted for unleaded petrol.

No ordinary additive can create a large power increase in a healthy standard engine.

A cleaner may restore performance lost to deposits. An octane or cetane booster may help an engine designed or tuned to take advantage of the altered fuel, but the benefit depends on the engine and calibration.

1. Identify the system. Confirm whether the product belongs in the fuel tank, engine oil, cooling system or air intake.
2. Check the fuel type. Do not use a petrol treatment in diesel or a diesel treatment in petrol unless the manufacturer explicitly approves both.
3. Measure the dose. More additive does not mean more cleaning. Overdosing can alter the mixture beyond the product's intended concentration.
4. Use one treatment at a time. Avoid combining strong cleaners unless compatibility is expressly confirmed.
5. Follow the full procedure. A DPF treatment still needs suitable driving conditions, while a flush must be drained after the specified period.
6. Stop if the problem is serious. Overheating, oil-pressure warnings, severe misfires, heavy smoke or major fluid loss require diagnosis rather than another bottle.