In the Relative Compression Test Quick Tips video we showed how to do a relative compression tests using a lab scope and a low amps probe.
In the 16.2 and newer software in Snap-on scan tools you can now do that test with a scanner on most Ford vehicles, so let's run through that. The tool is hooked up and the ignition is on so let's power up the tool.
I want Scanner, and it's going to automatically ID the vehicle for me. It's going to be under Engine System, it's going to be a Functional Test, and that's down here where it says Relative Compression Test.
Safety note: Apply the parking brake, make sure the vehicle is in neutral or park gear position, fully depress the accelerator pedal and apply the brake pedal during the test.
The test will continue when all the below conditions are met: My ignition switch is on, that's met; make sure the battery has sufficient charge, that's met; fully depress the accelerator pedal to the floor and hold, that's met too.
We'll click continue again and then we have to crank the engine for 10 seconds.
The test is complete and there are our results. Cylinder one, three, four, and two, we've got a 10th of a percent difference between cylinder one and cylinder too.
So let's pull spark plug out and see what would be a bad result.
The spark plug has been removed so let's run back through this test again and see what it's going to look like without that spark plug.
Press the accelerator pedal and there we go. I took out the cylinder to spark plug and it's 20% down. That's 20% out of the whole system.
That's definitely my problem right there. It's going to point you directly to the cylinder where the problem is.
It's a pretty quick, little automated test, and now you can do it with a scan tool. That makes life a little easier.