You will need to reduce this formula for bubble mix used by pre-schoolers. One cup of Dawn Dish Soap to 1 Gallon of Water.

Put it in a spray bottle and spray it on the cold Exhaust Pipe and start the Engine and look for bubbles indicating leaks. That will work unless the holes are large and also works to find leaks in intakemanifolds when the Turbo creates boost pressure.

Putting that rag in the Exhause Pipe would likey help bubble formation.

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84 300D, 82 Volvo 244Gl Diesel