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Rookie question here: If I want to test for spark, the most obvious thing seems to be to disconnect the gas line, remove the spark plug, plug it back in, ground the outside turn the ignition, and check for spark. And there's a video where the guy does just that, but it's on a 1 cylinder lawnmower engine.

It seems that doing the same thing on a 4 cylinder car engine might be a bad idea, since (1) You now have gas-air mix spraying out of the open cylinder (obviously you'd want to distance that from the plug you're testing) (2) One cylinder is open, with no pressure, the others are closed, which might create some kind of mechanical stress on the engine.

So, yes, I'm aware that I could just get another spark plug and swap it in there, but I'm just curious how bad an idea it is to start an engine with an open cylinder.

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It doesn't hurt things to test the spark or spark plug by doing so. You obviously don't want to run it that way for long. Most fuel injected vehicles have a fuse in the under hood fuse box which is for the fuel injectors. If you pull that fuse, the injectors won't fire, though everything else still works just fine, like the firing of the spark plugs (as long as that's not the issue ;-). This allows you to do the diagnostic without the worry of the engine running or fuel coming out of the single hole.

There's another thing you can do besides pulling the plug. There is a piece of equipment called a noid. It is a piece which goes inline with the spark plug wire. If the electricity is going to ground, it will strobe for you. This won't show you if the electricity is passing through a bad spark plug which is grounding, but it will give you an idea.

Spark Plug Noid Set

If you believe you have a bad spark plug, you can also move the spark plug to another cylinder to see if the problem follows the spark plug or stays in the same cylinder. This diagnostic can help track down the issue.

One of the things you don't want to do is to not have a way for the spark to travel to ground such as just pulling the spark plug wire and leaving it be while trying to run the engine. With some vehicles it can burn the coil out or cause other issues ... just a word of caution.

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