Mind the Gap When Changing Out Spark Plugs

  • Web Lackey
  • 7/27/2020

Dear Car Talk:

I just changed the spark plugs and the coil bar on my 2014 Chevy Sonic. The engine starts, but won’t go over 3,000 rpms. Did I do something incorrectly? -- Max

Given that this happened immediately after you worked on the car yourself, I’m going to put my money on “yes,” Max.

Actually, it may not have been your fault, but I’m guessing that something’s wrong with the spark plugs. If the spark plugs can’t make a big enough spark, the engine might run fine at lower speeds, but may fail when you try to rev it up and need that bigger spark.

You’ve heard of spark plug gaps, right? The manufacturer determines how much space there should be between the spark plug’s two electrodes. That determines how far the spark will jump, and how big a spark you get.

Most spark plug gaps are somewhere in the range of 1 to 1 1/2 millimeters. When we install spark plugs, we always check the gaps. Why? Well, some come out of the box perfect. But some don’t. Who knows why? Maybe someone dropped a crate of them off the ship in Long Beach while they were unloading. Or maybe you dropped one on the garage floor.

The size of the gap is very important.

If the gap is too small, there’s not enough room for the spark to jump. So the spark won’t be big enough and hot enough to combust all the fuel and air coming into the cylinder when you rev up the engine.

Conversely, if the spark plug gaps are too big, the spark can get blown out at high speeds. The amount of air and turbulence in the cylinder increases as the speed of the engine increases. And if the gap is too wide, the spark will just get extinguished at high speeds.

You can buy a gapping tool at your local auto parts store for less than $10. And then you’ll have a working car and a new hobby.

But before you even do that, check to make sure that you bought the correct plugs for your car. Using the wrong plugs could also cause this problem. And if there’s any question about whether you’ve got the right plugs, just go to your Chevy dealer’s parts counter and ask them to sell you four new plugs. That’ll at least guarantee that you’re starting with the right parts.

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