How do you diagnose a turbo (exhaust side or boost pressure side) leak?
 

My son's 1999 is getting only 12 psi boost and isn't making that until 2500+ rpms. How would I look for turbo leaks.

The intake side are the most common and easiest to find. You can make a simple adapter to hook regulated shop air to the intake of the turbo to pressurize the system then listen for leaks and maybe have a squirt bottle of soapy water handy for verification. There was a boost leak tester being shared amongst members, but I have no idea where it is currently or how long the list is for people signed up to use it. I'm sure a search would turn up the thread simple enough. Otherwise the parts to make your own are not too expensive and it's a simple enough setup. I'm pretty sure there is a write up in the tech section for details.

Would that mean put air into the intake where the air filter feeds the intake air boot?

Wouldn't it just send that air pressure through the turbo, intercooler and associated fittings, then into the engine? Wouldn't that bleed off the pressure since it would pass through somewhere? I'd think intake and exhaust valves may be open partially on one or more cylinders. Maybe not... I'm not familiar with valve timing.

use a 4in fernco coupling with a reducer to a 1/4 in NTP bushing and hook up compressor to turbo inlet at 20 PSI

for exhaust hookup a leaf blower to it

have fun with some soapy water looking for leaks

Go to the plumbing department, grab a 3" PVC cap, drill a hole in the middle, screw your male air chuck into it, it will cut its own threads. Take 60 grit sandpaper and rough up the outside of the cap

Remove the air filter to ccv tube, remove the CCV, rotate the turbo to CCV hose, put the PVC cap in, tighten up clamps, set air compressor regulator to 10psi, spray soapy water everywhere at every boot or sensor from turbo to intercooler to plenums

Excellent and thanks! So the air won't just pass through the engine? Sorry if that's a dumb question. Never done it before and wonder if the valves would be open enough to allow air to flow through the engine and out the tail pipe.

Built my own leak tester with PVC as mentioned above. KC Turbos has some good boost and exhaust leak testing videos linked below. Both helped me diagnose boost and exhaust leaks on my truck. For boost leaks, I sprayed soapy water at every connection with pressure on tester. For the exhaust, it showed right away that my oem up-pipes were leaking when I mashed the throttle pedal while holding the brake.

Intake and exhaust valves would both have to be open at the same time on a cylinder to allow anything to just pass through.

To quote myself, we tried it and not much passes through the exhaust. I should have listened or had someone listen at the tail pipe, but what I do know is that the intake intercooler and turbo system take a LOT of air and hold pressure for maybe 20-30 seconds after the air is supplied and removed.

It won't hold air like a tire. Keep the air on and spray the soapy water everywhere. Tighten stuff down where it bubbles till it don't no more and reassemble the intake

Yeah, just leave the air supply connected, but drop the pressure at the compressor to 20 psi. Give it the full 150 and you'll make a leak somewhere, LOL.