The real name for the pop testers is a Diesel Nozzle Tester. You can look on eBay to see what they look like.

The O-ring that the other poster was speaking of can cause Oil to mix with the Fuel.

The Injector Cut Out test that was spoken of is while the Engine is running you loosen one of the Injector Line Nuts (this cut so off the fuel to the Injector because the fuel squirts out, a little messy )and see what effect it has on the engine. The tighten it up and go to the next Injector.

If a Cylinder is making a knocking sound cutting off the Fuel by loosening the Injector Line Nut will stop the sound. And that identifies the problem cylinder.

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84 300D, 82 Volvo 244Gl Diesel