Compression test

[Hammerhead]

(01-16-2018 )dhirocz Wrote:  The idea is to just have a better idea on the condition of what is going on. For all I know I have two cylinders that are weak and leak down quickly, or one that hardly has any compression. Or maybe they are all a little low. Maybe a leaking injector cup in the head glazed a cylinder. Might even detect a head gasket starting to go. Who knows?

Maybe I'm being anal. Anal works for me. The whole idea is to verify if this is actually a condition caused by wear or something else. The cylinders still looked pretty good when I last had it apart, which at this point doesn't mean much. I did take a look around with a boroscope. A compression test would give me a better basis in which to make an educated guess.

I want to know if one or two stand out. If they are all a bit low, which wouldn't surprise me, then so be it. Good point on the injectors though, it would take a couple hours to run the test...which would be a pain and pretty much commit me to an overhead afterwards. If someone was going to be in it doing injectors though, it wouldn't necessarily be a bad deal!

Anyway, the idea was to pull all 6 injectors and disable the fuel system, take another peek with the scope, then hook up the tester and give her a few cranks...on each cylinder. Record the data and look for the outliers.

The leakdown might be the hard part without the air pressure. Even with the valves closed and the cylinder aired up to say 30 psi, there's that missing dynamic of the piston actually moving as the cylinder is filling (as if the engine were running) compared to testing it stationary that would lead me to question it. Still could provide some useful data.

I also considered blocking off the air system as if I was doing a leak test and monitoring the crankcase for pressure past the rings into the crankcase. Problem is, it would also loose air past valve guides as well.

Anyway, I don't have it all figured out yet either. That's why I'm asking questions. Some of what I know of engines applies to these, and some doesn't. What I do know is it's almost 1am and I just saw my wife in a nightie so adios for now!

For whatever my opinion is worth, I think you're onto something here...
A leak down/cylinder differential pressure test would be a better way to go to start. They should also be done at as close to operating temp as possible, so all the same time to access of a compression test apply, but you could actually trace the leak with this test as opposed to "just a compression psi number" of a compression test.
You'd have a much better idea of where the pressure is escaping than how much psi each cyl creates statically.
I have used a leak down tester on gas engines for extended periods of time both seeking answers and demonstrating to someone that he was "barking up the wrong tree", so IMHO this is a much better idea..,that you'll read tomorrow.