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It constantly bleeds off, but in my experience the speed of the bleed is important.
When I had a plenum leak the gauge would drop fairly consistently all the way to zero.
Once sealed up, the rate of drop would slow down dramatically as the pressure drops.
I rationalized it like this. When there was a leak the pressure had an easy escape all the way to zero. Without a “leak”, the normal places it can bypass (like piston rings) have a lesser effect as the pressure approaches zero.
Here’s a video I took to document a good check as a reference point. In hindsight I’m really disappointed I didn’t take a before video to show the difference.
@F350towing My detector is similar to yours but I use a small pressure regulator with a gauge that has a short hose on it. I put the regulator between the tester and the hose from the compressor. I use the regulator for other things like my smoke machine and plasma cutter so it's has quick connects on it. I never put a ball valve on it to see how long pressure holds, I just put about 20 lbs on and use soapy water at the connections. Next time I'm going to use my smoke generator and see how it works.
Also, it's easy to chase problems you don't have when checking for boost leaks. In particular, unless you also have a 100psi guage, don't compare the rate of the needle sweep on your guage to the video above. (The KC guage, for example, only goes to 30psi, and so will sweep fairly fast whether you have a leak, or not.) Lastly, if you put your thumb over the CCV port and the sweep slows, you know that much of the flow is not coming from your intake system. Don't leave the CCV port plugged, as you're then pressurizing the crankcase, but you already know that.
To do a pressure test I've always removed the hoses and eliminated the CCV section so no pressure was blowing in the crankcase, just connected directly to the turbo with the short hose.
In particular, unless you also have a 100psi guage, don't compare the rate of the needle sweep on your guage to the video above. (The KC guage, for example, only goes to 30psi, and so will sweep fairly fast whether you have a leak, or not.)
Yes, I used a 100psi gauge because it was in some random parts I inherited from my grandfather and I wanted to put it to use. A 30psi gauge would be better.
I still think the sweep rate is relevant though. A 30 psi gauge will drop much quicker than mine, but the rate of drop should still slow drastically as it’s getting to 10 psi and below in my opinion. Guess what I’m saying is compare the change in rate of drop within the same gauge between 30 and 10 psi. My sweep rate basically remained the same all the way down when I had the leak.
To do a pressure test I've always removed the hoses and eliminated the CCV section so no pressure was blowing in the crankcase, just connected directly to the turbo with the short hose.
Agreed, and I do the same, but air leaks around the rings and into the crankcase, which then vents out the doghouse
Originally Posted by RacinJasonWV
Yes, I used a 100psi gauge because it was in some random parts I inherited from my grandfather and I wanted to put it to use. A 30psi gauge would be better.
I still think the sweep rate is relevant though. A 30 psi gauge will drop much quicker than mine, but the rate of drop should still slow drastically as it’s getting to 10 psi and below in my opinion. Guess what I’m saying is compare the change in rate of drop within the same gauge between 30 and 10 psi. My sweep rate basically remained the same all the way down when I had the leak.
Based on that I’ve got some more looking to do, and going to have to build a better boost leak tester.
While something like the KC tester is very nice...if you go to your local hardware store, you should be able to get what you need for little money, that's what many did years ago. This is what i made...it's ugly but does the job. I have a gauged regulator i hook onto it; which also allows me to turn on/ off the air supply right there. I can also hook up a smoke machine to it, which makes life easier in finding leaks
Watching @RacinJasonWV 's pressure leak down was like watching grass grow. Clearly, he's not loosing any potential boost.
The weeds around my place grow faster than that. Heck, I could mow the weeds, come back inside, play Jason's video, and by the time the video ended, it's time to mow the weeds again. But who has time to wait that long?
Yet to be hired CNA's will approach my recliner to put me down for the night and ask "Are you still watching that video?"
"Why yes of course," I'll groggily reply. "I'm still waiting for Jason's pressure to drop!"
I hope it’s still that good. It certainly wasn’t before sealing the plenum but I had no idea of what a decent test looked like therefore I thought all was well since the truck could make good boost numbers. Seeing the difference in leak versus no leak is what prompted taking a video after the repair.
One thing I’ve learned is that the system needs confirmed any time it’s been opened. As much as I don’t want to drag the stuff out for the test, it is worth the few minutes of time.
I’ve got some more looking to do, and going to have to build a better boost leak tester.
If you can lay your hands on one, a 30 psi gauge is preferable, as earlier respondents above have concurred.
If you don't want to have to position your face or body in the trajectory zone of the pressure cap to read the gauge, then incorporate some short lengths of hose and fittings.
