Retard here.
Can someone explain the theory behind a wet compression test?
oil on the walls seal the rings.
What do you mean? Why does it work? If you have blown rings or some kind of compression leak, you'll see low compression with a gauge. The reading will be higher if you squirt some oil in, because it will seal the passages that were leaking before, temporarily.
The oil will help if the problem is with the rings, but not if it's the valves, so it can help you narrow down what the issue is. You can normally just hear the problem without doing that though, if it's a big leak.
>>14090241
I still think leakdown tests are better
Little or no air escapes past the rings and piston.
Provides a more accurate reading due to less leakage.
>>14090255
But don't bad rings allow oil to get into the cylinder?
>>14090266
but isn't the leakage the info you seek ? by doing a compression test
>>14090278
not if the engines not running.
>>14090291
You're still cranking it though during the compression test.
>>14090265
Neither is "better," different tests determine different problems
>>14090278
They do!
IT'S A PARADOX!!!
>>14090279
See
>>14090260
If you do a compression test dry and it shows a bad cylinder, then do it wet and it holds, you will know the problem was with the rings. If it still reads as bad you will know the issue is with the valves, head gasket, or a crack in the cylinder. It's just eliminating a variable.
>>14090307
good to know
>>14090298
>not hand cranking it to get an accurate reading.
>>14090299
wat
>>14090299
Regardless people still seem to disregard leakdown tests. Both shadetree /o/ harbor freight shills and us real techs who use snapon
>>14090337
https://en.wikipedia.org/wiki/Leak-down_tester
>>14090340
Nah, leakdown is superior, but can be tricky to accomplish properly with higher duration or high overlap cams, especially if valves are still open at BDC. If they are closed at bdc then a leakdown works perfect and is easy.
If not, you gotta know the closing events of the valves and then mechanically hold the car there by holding the balancer with a socket while pumping air in there, and trying to fight 100psi (or whatever you are using as regulated pressure)
At the end of the day, a wet compression test will only eliminate rings, but will not isolate with any accuracy the exact problem.
