How to Do a Compression Reading on Your Engine

Publisher's Note: Welcome to our eighth edition of Tech Tips Tuesday on "How to Do a Compression Reading on Your Engine".  We'll be re-publishing an edition of AMSOIL Tech Tips for you every Tuesday.  Click on the Blog Category "Tuesday Tech Tips" to see everything published to-date.

John Gardner: Wouldn't it be cool if you could understand the mechanical integrity of your engine? Well, you can. Welcome to this AMSOIL Tech Tip on taking a compression reading.

How do you do it? Real simple. Just go down and get yourself an inexpensive compression gauge. Here's what you do: you warm up your engine. Once you warm up your engine, be careful, don't burn yourself. Blow out all the spark plug holes, and then remove all the spark plugs. Then you can go ahead and block the throttle wide open so you get full airflow into the engine. Once you do that, go ahead and insert the gauge into one of the cylinders, then disable the ignition. You want the car to crank but not start.

Then what you want to do is turn it over for four compression strokes, that's four-cycle strokes. So, 'no, no, no, no, no, no, no, no, no, no, no, no, no, no.' And once it does that, go ahead and watch your gauge. Now, the first time, you want it to get at least half the compression, and then check your manufacturer's specification for your compression readings. You need about 90 for the car to fire.

Well, what happens if it's low? Here you go: 10 to 15 percent difference between the cylinders. As long as you have your manufacturer specification, you're fine. All cylinders low? Well, you have burnt valves, you have a timing belt issue, or just a plain and simple tired engine. One or more cylinders usually indicate just a cylinder issue. Two adjacent cylinders? Well, that's probably a head gasket problem.

Then, I'd rather not get to this compression portion because I think preventative maintenance, we're all about that. We can solve a lot of these problems prior to having a compression issue. Is that right?

Len Groom: That's correct. You want to run a simple test first, right, before you have to get more invasive with a compression test. And that can come in the form of fuel additives. 

John Gardner: And fuel additives, let's address the cylinder itself. Do you have something down in there?

Len Groom: Well, true, you're going to bring it in the same way, generally through the fuel. But what will happen is our Performance Improver or our Upper Cylinder Lubricant is going to do the same job. It's going to help clean deposits away that could be performance-robbing or could be causing a ring stick that would be a loss of compression. And what we've actually seen is carbon buildup in the actual cross hatches in the cylinder.

John Gardner: Wow, that's interesting. How can we prevent it all together? I mean, maybe get it before it even gets into the cylinder, right?

Len Groom: You're going to add it to the fuel. That's the easiest way to get the additives in there. Put it in the fuel and go from there. Fairly simple.

John Gardner: Well, there you go. You don't even have to take a compression reading, but now you know how to.