Why a Fuel System Cleaner Might Leave Your Car Smelling Like Gas

A Mercedes owner recently reached out with a puzzling problem: a persistent gasoline smell coming from the vehicle. The only change made in the past two months was using Red Line 60103 SI-1 Complete Fuel System Cleaner, a popular additive available online. Now they’re asking: what went wrong?

Fuel system cleaners are designed to remove deposits from injectors, valves, and combustion chambers. In most cases, they work without issue. But when used on an older engine or one with existing buildup, the cleaner can dislodge larger chunks of carbon or varnish. Those particles can then clog a fuel injector or upset the fuel-air mixture, leading to incomplete combustion.

That unburned fuel has to go somewhere — often out the tailpipe or into the crankcase, which produces that raw gasoline odor.

Another possibility: the additive itself may have degraded a rubber seal or O-ring in the fuel system. Some older components weren’t designed for modern concentrated cleaners, and a chemical reaction can cause swelling or cracking. A small leak near the injectors or fuel rail would explain the smell without triggering a check-engine light.

If you’ve used a fuel system cleaner and notice a gas odor, start by checking for visible leaks around the fuel rail and injectors. Look for wet spots or a strong smell near the engine bay. If everything appears dry, the issue may be internal — such as a stuck injector or diluted engine oil from fuel contamination.

Either way, it’s a good idea to have a professional take a look. A qualified mechanic can run a fuel pressure test and inspect the system for damage. The cleaner itself isn’t necessarily the culprit, but it may have revealed an underlying issue that was waiting to surface.