On the shelves of gas stations and auto parts stores you'll find dozens of bottles with big promises: clean your injectors, reduce fuel consumption, protect the engine, extend the life of your DPF filter. The question we hear in our workshop at least once a week is always the same: does this stuff actually work? The answer isn't simple because some additives do have proven benefits, while others are pure marketing. Here's what we know from hands-on experience.
Fuel additives and injector cleaners
Injection system cleaners (Liqui Moly, Wynn's and similar reputable brands) can genuinely keep injectors clean when used preventively, roughly every 10,000-20,000 kilometres. The principle is straightforward: the additive contains detergents that dissolve deposits on injector tips and in intake channels, maintaining proper fuel spray patterns.
The key word here is preventively. If an injector is already mechanically faulty, say the needle doesn't seal properly or the spring is cracked, no additive from a bottle will fix that. In such cases you need diagnostics and possibly injector repair or replacement. A bottled cleaner is no substitute for bench work, but it is good prevention to keep you from needing bench work too soon.
The only condition is that you use proven products rather than the cheapest option with an unknown label. A bad additive can do more harm than good, especially on modern common rail systems with tolerances of just a few microns. Cheap cleaners of unknown origin sometimes contain aggressive solvents that can damage rubber seals in the injection system.
Oil additives and why we don't recommend most of them
This category includes various "stop-leak" products for oil leaks, metal conditioners, friction modifiers, and additives that promise quieter engine operation and lower oil consumption. In practice, most of these products mask symptoms instead of addressing the root cause.
Stop-leak additives work by swelling rubber seals from the inside, temporarily reducing leakage. The problem is that this swelling is uncontrolled, so the seal becomes brittle and cracks after some time. Instead of replacing the oil seals when you should have, you now have both a leak and a damaged seal.
Metal conditioners and friction modifiers promise to reduce friction by depositing a protective layer on metal surfaces. Quality synthetic oil with the correct specification already contains all the additives your engine needs. Adding another layer of chemistry to that mix can disrupt the oil's formulation, and in the worst case clog lubrication channels. If you're interested in choosing the right oil for your engine, check out our guide to selecting engine oil.
A particular concern is additives that promise to reduce oil consumption on an engine that already burns oil. An engine consumes oil for a specific reason: worn piston rings, worn valve stem seals, or damaged gaskets. No additive can fix that, and masking the symptoms only delays diagnosis and repair.
Engine flush before an oil change
Engine flush is a concentrated solution poured into old oil before draining. The engine runs for 10-15 minutes at idle, then everything is drained together. The idea is to rinse out deposits before fresh oil goes in.
On engines that have been regularly maintained with quality oil, a flush is unnecessary because there are virtually no deposits to remove. On engines with low mileage and a clean service history, flush is money wasted.
The danger arises with high-mileage engines and neglected maintenance. Such engines have accumulated deposits that, unwanted as they may be, maintain a certain equilibrium. An aggressive flush can break off chunks of deposits that then travel through oil channels and clog the oil pump screen or lubrication passages. The result is a drop in oil pressure and serious engine damage. In our workshop we've seen engines where a flush filled the lubrication channels with debris, and that's a job we wouldn't wish on anyone.
If you think your engine needs a deep clean, don't pour in a flush yourself. Visit a workshop that can assess the internal condition of the engine, check oil pressure, and decide whether a flush is safe or risky. On some engines it's simply better to shorten the oil change interval to 5,000-7,000 km for a few cycles and let quality oil gradually clean out the deposits.
DPF additives and engine decarbonisation
With DPF filters (diesel particulate filters) the situation is specific. Factory DPF additives based on cerium, known commercially as Eolys, are built into the system from the factory on certain manufacturers (Peugeot, Citroen, some Volvo models). They lower the temperature at which soot burns, helping filter regeneration. These additives work as intended because they're part of the engineered system.
However, aftermarket DPF cleaners from a bottle that you pour into the fuel tank rarely solve an already clogged filter. If the DPF filter is seriously clogged, you need a forced regeneration with a diagnostic tool, or in more severe cases, physical cleaning or filter replacement. A bottle from the store won't help there. When a filter is 80% or more clogged, a fuel-tank cleaner simply doesn't have enough contact with the soot to make a difference.
As for HHO decarbonisation (machines that introduce hydrogen into the intake manifold to clean carbon deposits), our workshop experience is this: on older engines with heavy deposits it sometimes produces a noticeable result, but on modern engines with regular maintenance the difference is minimal. HHO decarbonisation is not a substitute for proper servicing, yet it's advertised as a universal solution for all engine problems, which simply isn't true.
When an additive actually helps and when it causes damage
There's a clear line between prevention and repair. Additives can help with prevention: keeping the system clean, slowing deposit buildup, improving fuel properties in winter conditions (winter diesel additives that prevent fuel gelling are an example that genuinely works).
Additives cannot fix a mechanical fault. They can't replace a worn oil seal, a clogged oil channel, a failed pump, or a mechanically defective injector. When used for that purpose, they usually mask the problem until it becomes more expensive to repair.
A few specific scenarios:
- Injector cleaner every 10,000-20,000 km as prevention - yes, it makes sense.
- Winter diesel additive in sub-zero temperatures - yes, it prevents gelling.
- Stop-leak for oil leaks - no, replace the seal.
- Flush on an engine with 200,000+ km and irregular servicing - no, you're risking engine damage.
- DPF cleaner from a bottle on an already clogged filter - no, you need diagnostics.
The best additive for your engine is the one you'll never need to use: quality fuel from a reputable station and regular oil changes at the prescribed interval. If you're unsure whether your car needs any additive, or you have symptoms you're trying to fix with a bottle, get in touch with our workshop. It's better to diagnose a problem than to mask it.