It took me wayyyyyy to long to build one of these! This was dead simple and helped locate a few leaks within moments.
Prerequisites:
Air compressor with a reasonable amount of capacity. Doesn't need to be much if you rely on the soap bubbles more than the sounds of air leaking.
Materials:
-4" to 3" rubber coupler with hose clamps
-3" PVC pipe cap
-Male air fitting
-Pressure gauge
-Spray bottle with soapy water
I picked up the top 3 items from Home Depot and a low pressure gauge from Amazon. HD had pressure gauges but they're all too high which limits your needle movement.
How to build:
-Drill & tap 1/4" NPT holes in pipe cap
-Install gauge and air fitting. Use teflon tape, pipe dope, or other pipe sealer.
-Put cap into coupler
-Attach coupler to K-jet
How to use:
Disconnect and cap PCV and charcoal canister lines. They are guaranteed leaks that will only distract from actual leaks.
Set regulator for 5ish PSI, attach and start pressurizing.
Spray soapy water at all vac lines, manifold bolts, fittings, etc. Watch for bubbles
Fix leaks. Enjoy the lack of afterfires and stable idle.
Caveats:
Technically you are finding boost leaks, not vacuum leaks. Many rubber vacuum lines are fine under vacuum but garbage under boost. In my experience only silicone hose provides consistent leak-free operation under both conditions.
You will never see your intake hold pressure, it will always drop. I've only read a rule of thumb for "1psi drop per second is fine". No idea what to expect on our motors. I have to replace my "pipe of agony" because it's leaking like a sieve. Once that's done hopefully I can post my own numbers.
If you cannot hold any pressure and no leaks are found, it's possible (although unlikely) that a cylinder has valves in overlap (air going straight out exhaust). That or you have ring failure on whatever cylinder has an open valve.
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