I'm new and need help with a boost leak test

[97gsx_zeclipse]

Anyone in San Diego help out with my first boost leak test . I got a 97 GSX. I looked at the vfaq and it seems simple but I just want someone to help me for the first time so I know I get it done right. Thats all. Hit me up

 

[97gsx_zeclipse]

Well i guess no one can help. I'll just do it and get it over with. Hopefully I do it correctly.

 

[Talesin]

Yeah, there are a few guys down in the SD area (along with Total Eclipse, IIRC) but a boost leak test isn't really all that hard. Just pull your intake, fit the adapter over the turbo inlet, and hook it up to a pressure source. If all the air comes out the exhaust, either pop a ratchet/extension into the little spot behind the driver's side wheel to let you turn the crank by hand, or just remove the boost leak adapter, get in the car and 'tap' the starter until the crank lands somewhere with no valve overlap. 20psi is good for a first test, and don't forget a bit of soapy water to help you narrow down where a leak is really coming from.. can be difficult to find them, sometimes.

 

[GimmeMoeBoost]

Correct me if I'm wrong, but I think for a first time you should block the throttle plate closed so you don't have to worry about the valves being open, and air leaking out. But of course doing it this way would only be checking leaks up to your throttle body. Doing it the way Talesin said, would be checking leaks past the throttle body through the intake manifold. Such as a bad throttle body gasket, manifold gasket, etc. I heard you should also plug the vacum line going to the inlet of the boost controller, if aftermarket. (I still need to perform this test myself, but just my 2 cents.)

 

[Talesin]

Er.. air's still going to get around the TB plate even at its 'fully closed' position. FIAV, as well as the fact that an engine should never have zero incoming air, so there's no reason for the TB to completely seal closed.. after all, a piston is essentially an air pump, more or less. And air pumps die when they try to pull against a vacuum TOO hard. Also, with no pressure in the manifold, the stock CBV ('BOV') will open and let all the pressure out. It really doesn't have much of a spring, and kind of relies on that pressure to stay closed. Since the intake is off for the boost leak test, it'll all just come out the stock CBV.

In addition, with the valve non-overlap, you'll also find out if you have any TB/manifold leaks, which also translate into vacuum leaks. It's really not that hard to do.. just grab a ratchet of the right size, turn your wheel all the way to the left, pop out the little round plug in line with the crankshaft, and slip the socket (and maybe a short extension) into the square hole. Then turn it slowly, I believe only clockwise. Keep the positive pressure line on the intake, and when it seals up, you're good to leak test.
Holding 20psi for about 30 seconds is a 'good' test result. That is, pressurize the intake tract to 20psi, and turn off your pressure source. Your aftermarket boost gauge or pressure gauge on the boost leak tester shouldn't be down to zero yet after thirty seconds, though some slow leakdown is generally acceptable, especially if the engine isn't hot (air past the rings).
If you get any loud hissings, track them down. A bottle of soapy water is helpful for the ones you generally know where they are, but can't quite find them. Soak the area and look for bubbles and drying-out.. bubbles are a lot easier to spot, of course.

 

[GimmeMoeBoost]

So if I'm right, if my 1g bov will only hold (say 18psi), it would leak, making it a weak point if I was to try and boost 20psi?

 

[Talesin]

Well, it WILL leak up there without being crushed. The primary reason it stays closed though it pressure equalization. The CBV line from the manifold runs to a diaphragm above the plunger in the CBV. This exerts downward force. The pressure in the IC piping is exerting upward force against the plunger. Since they're the same pressure, the 'spring' above the plunger serves to hold it closed against equal pressure.
When the TB plate closes, the manifold quickly comes under vacuum, robbing the upper diaphragm of pressure and actually putting it under vacuum. The pressure in the IC piping is more than enough to overcome the spring and push the plunger up, and allow the excess pressure to bypass back to the pre-turbo intake pipe.

Eclipses do not come stock with BOVs, they come with CBVs.. compressor bypass valves. This means that in addition to opening up to allow overpressure past, they're also open under vacuum... light driving and idle, to allow the turbo to be less of a restriction in the intake path while unspooled.

 

[97gsx_zeclipse]

Wow you explained that really good. I understood all of that. I have some time to do a boost leak test tomorrow. So I just tighten the screw under the plug next to the crankshaft clockwise, then pressurize the intake and listen for leaks? After I'm done I turn back the screw to where it was? Thats all right?

 

[Talesin]

Er, no screw. There's a plug in the plastic splashguard in the driver's side wheelwell.. it usually falls out after the car gets to a certain age. In any case, the end of the crankshaft pulley is directly through that hole, and has a square 'socket' receptacle in the middle. You can fit a ratchet extension into that, and then turn the crank slowly by hand with the ratchet, only going clockwise, until the valves have no overlap.

Don't turn the crankshaft back the other way when you're done.. wisdom of the ages says that can lead to skipped timing or some weirdness with the timing belt tensioner. Just remove the ratchet and extension, replace the plug in the wheelwell wall, replace your intake and start the car. You're essentially turning the engine over by hand with the extension and ratchet, until you find a spot where none of the intake and exhaust valves on the same cylinder are open at the same time. #1 TDC is bad, as either 2 or 3 has its valves overlapped at that time.