How To Perform a Compression Test - [EASY] 

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Worksheet is here: drive.google.com/file/u/6/d/1ffDlHM26EwTHvJf45JBfphKzt5NGkD5r/view?usp=drivesdk

Yeh makes no sense

Washing those walls running all the fuel in cylinder 3 😂

That's what I'm saying, I don't think I've ever seen someone check the compression while engine is running.

Me neither, but he could’ve easily disconnected the fuel injector from #3… This a GDI engine? Those pump a lot of pressure (~1500-3000 psi) directly into the combustion chambers, I know it’s definitely something to consider when working with GDI engines.

It's not about whether it's causing damage. It's about the fact that if fuel is being sprayed into cylinders being tested it'll effect the reading. It'll likely do one of two things. It's going to wash away anything inside the cylinder and get between the piston rings and the walls. Or the gas will act as a barrier sealing any loose gaps between the piston rings and the wall. That why there's dry compression tests, which is what he just did. And a wet compression test. Had he not diagnosed the issue using his method. The next step would've been pouring a small amount of oil into the cylinder (wet compression test) to see if it raised the pressure. The reason for this is that if you have bad rings, piston, or cylinder walls the oil will seal any gaps. If the compression doesn't change at all but is still low. That confirms that it's valves.

Agreed. I’m wondering if he disconnected the power from the fuel injector and just forgot to mention.

Oops…… here it is drive.google.com/file/d/1ffDlHM26EwTHvJf45JBfphKzt5NGkD5r/view?usp=drivesdk

If you are doing a cranking compression all off, running compression only the cylinder you are testing.

The fuel pump is what pushes the fuel through the lines and to the injectors. The fuel pump is off cause he pulled the fuse. So no fuel is moving in the lines or going to any of the cylinders.

What do you mean by that?

At the beginning he wisely defeated the fuel delivery for the static test. Later he put the fuel pump fuse back in but then did not disable the injector for the cylinder he was testing. This could explain why he didn’t get the 50% number he was expecting.

@@MartinWibertI was thinking the same thing. He never went over if he did or didn't, and I suspect he didn't.

That’s dependent on the engine. It’s always best to test a known good vehicle and begin to build your data base. Most manufactures no longer give us that spec in service information.

You can but without a dynamic (running compression) you’re not testing the engines ability to breathe and exhale (volumetric efficiency). Wont be a complete test if you skip running.

I'm kind of confused, on the 2nd test you put the fuse back , I thought the fuse wasn't supposed supposed to be in because of fuel going into the piston washing the cylinder walls

You’re correct- to get comparative readings on a static compression test you have to remove all spark plugs. You also need to hold open the throttle blade or remove the throttle body to get maximum airflow in the engine for all cylinders

thats why you cut off the fuel

He passes by everyday 😂

Unfortunately a cranking compression test is called a static test. “Begin with a normal ("static") compression test on a warm engine to eliminate obvious problems such as badly worn rings, burnt or bent valves or holes in pistons.” - AllData

Thts a bit harsh

MERICA!