Smoke Testing

Smoke testing is a way of finding leaks in pipes, which has been common in the motor industry for generations. A unit which creates smoke is attached to pipework and the smoke is forced through, revealing any leaks.

However, this is not of much use when attempting to find rain water leaks, as the car bodywork is full of holes. It would also be disastrous to introduce smoke (often created by burning mineral oil) to a car interior because of the smell.

However, we do use fogging machines to deliberately introduce chemicals to car interiors, usually to remove odour, in which we will fill the entire car interior with fog. Although it is not a method we commonly use to find rain water leaks, it can occasionally reveal them and is a tool, we have in our arsenal.

What it means

Smoke testing is a way of making invisible leaks visible. A technician uses a special machine that produces harmless, visible smoke and gently pressurises either the whole cabin or a particular section of the car. Wherever the car is not properly sealed – a gap in a membrane, a pinhole in a seam, a poorly sealed vent or a missing grommet – the smoke will creep out. By watching where the smoke appears, they can work out where water and air are likely to be getting in or out.

Why it matters

• Finds leaks you cannot see with water alone: Some leak paths are tiny or hidden behind trim, so you may never catch water in the act. Smoke can seep through hairline gaps and show up as a visible wisp where a normal hose test looks fine.
• Saves time and guesswork: Instead of repeatedly stripping and reassembling parts based on hunches, smoke testing gives direct evidence of where to focus repairs, reducing trial and error.
• Helps with wind noise and vent issues: The same technique that finds water paths can show where air is escaping, which is useful for diagnosing whistles, draughts and problems around rear vents and door seals.
• Non-destructive and repeatable: When done correctly with the right equipment, smoke testing does not harm the car and can be repeated after repairs to confirm that gaps have been closed.

Where you will see it

You will see smoke testing mentioned in leak diagnosis reports, specialist estimates and technical notes. Typical comments include smoke test cabin to trace leak, smoke testing shows leakage at door membrane, smoke escaping from rear vent housing or repeat smoke test after repairs to confirm seal. It is often recommended when leaks are intermittent, hard to reproduce with a hose, or have already defeated basic checks of rubbers, drains and membranes.

Context

Smoke testing is one of several tools used to track down stubborn leaks. It sits alongside controlled water testing, borescope inspection, lifting carpets and checking drainage pipes, rear vents, door membranes and scuttle areas. A specialist might, for example, seal vents temporarily, introduce smoke into the cabin and watch for it escaping around doors, glass, vents, pillars or under the dash. The key is interpreting the results properly: smoke usually takes the easiest path, which may be the same as water but not always, so findings have to be combined with knowledge of how the car’s drainage and membranes are supposed to work.

Common mistakes

• Assuming every place smoke appears is automatically a water leak, without considering how water actually flows and drains in that area.
• Using improvised smoke sources or too much pressure, which can contaminate interiors or force smoke through gaps that water would never realistically use.
• Doing a single quick smoke test with half the trims still in place, then declaring there is “no leak” because nothing obvious was seen.
• Fixing the most visible smoke escape point and skipping a follow-up smoke test, only to find there were additional leak paths that still need attention.

Written by Danny Argent. Last updated 08/12/2025 16:49