You can rig up a leakdown test rig. Basically connect shop air to the compression tester if you want to read actual leakdown numbers. That usually isn't necessisary. You just need an air fitting the same thread as a spark plug. Like what you would use to replace valve oil seals. You can make one by breaking and removing all the porcelain from a spark plug and welding or brazing an air fitting to it.

With both valves fully closed on cylinder 8 pressurize the cylinder to 100-130 psi. If air is still being used to pressurize listen for the leak in 1 - the intake, 2 - the exhaust and 3 - the oil cap. If you can hear it in the intake then it's a bad intake valve. If you can hear it in the exhaust it's the exhaust valve (might want to listen at the O2 sensor bung with the sensor removed). If you can hear it leaking at the oil filler it's rings.

Ted

 

this is a 2008 5.3L .. you had the fuel pressure controller fail causing fuel pressure problems . I doubt it is a ring issue .. the valves may need replacing . if you find in your testing the valves are melted and not seat and leak on #8 I would remove both heads and replace all the valves.

the engine will not run properly with new valves on only one cylinder as the others are probably messed up ..the compression should be all about the same ..on all cylinders.

 

It's a 2007 4.8l engine.

The fuel pump module being changed did not change really anything other than me finding that my u0109 was from a loose fuel pressure sensor wire. Although having a new fuel pump module is peace of mind.

I still don't know if my p0308 makes my fuel pressure go up and misfire occurs but my main issue now is this low compression number on the 8th cylinder.

Leakdown test will be next but I'm already thinking the worst.

Anybody got an idea on how much a used 4.8l engine 2007 and up will go for?

 

if you drive this the converters will be damaged..with miss fires .
you posts with fuel pressure #'s did indicated poor pressure that will cause the valves to burn up..