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Compression Testing

6th March 2006 - By Paradochs


So something’s wrong with your car. No? Then somethings wrong with your truck, suv, motorcycle, boat, etc. Why else would your be reading this?
I recently began having problems with my truck running very roughly. I figured a misfiring cylinder and started off by replacing sparkplugs and wires. When that didn’t fix it, I rebuilt the distributor. That didn’t do it either. I decided to try and find out if I had a different problem with the cylinders. I did a little research and decided that it wasn’t very hard to check the compression. The basic idea is that if the compression in a cylinder is considerably lower than the other cylinders, then you have a problem. The problem could be a valve, piston, piston ring, or probably a few other things that I don’t know about. Here’s how to do it.


Compression Tester Gauge. ($10-$20)
Deep Socket set & Ratchet


The procedure is simple and a little monotonous if you have 8 cylinders. It doesn’t matter which cylinder you start with. I suggest starting with the most difficult to reach, or the one you think is bad. Go around and do one at a time. Go in order so that you don’t forget which ones you haven’t done yet or do the same ones multiple times.
The first step is to run the engine to get it up to operating temperature. If you can’t do this, it’s ok. This will give you a more accurate reading.
Once the engine is hot, turn it off and remove BOTH ends of the coil wire. For those of you who don’t know, the coil wire goes from the coil (usually a cylindric shaped object) and goes to the top center of the distributor cap (the plastic piece with a bunch of thick black wires coming out). Please don’t touch the wire while the engine is running as this can cause electric shock, pain, embarassment, and is usually indicative of stupidity.
Now you’re ready to test the cylinders. Here are the basic steps for each cylinder.

  1. Remove the wire from the spark plug going into the cylinder using your hands or small pliers.
  2. Make sure nothing (dirt, leaves, etc) is in position to fall into the cylinder when you remove the spark plug. A can of compressed air or an air compressor is a great way to blow all the stuff out of the area.
  3. Remove the spark plug going into the cylinder using the ratchet and deep socket set.
  4. Screw the Compression Tester Gauge into the spark plug hole. Tighten by hand only.
  5. Crank the engine so it turns over roughly 6 times. This should give you an accurate measurement.
  6. Record the reading on the gauge. (Yes, write it down. I know, writing is terrible. At least it’s not physics homework.)
  7. Press the pressure release valve on the gauge so it drops back to 0.
  8. Remove the gauge from the spark plug hole.
  9. Replace the spark plug into the hole the same way it came out.
  10. Repeat.

If you find one that is considerably less than the others (I read approximately 12% or more) then you have a problem. Obviously the more the difference, the worse the problem is and the more it needs to be fixed. As for fixing the problem, you’re on your own. My compression was fine, all cylinders right at 150 psi.
I just found out what was wrong with my truck. It turns out that a bad spark plug was the original problem. When I replaced them, one of the new ones I put in was also bad. As if that wasn’t annoying enough, I accidently switched the 7 & 8 wires. I never claimed to be good at working on trucks :). Good luck with yours.

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