Mike Frye

I usually try to put things in a new thread so that someone (maybe me) in the future can search for them and find what they're looking for. Today I have two more or less unrelated questions though, and hopefully it won't be a problem to combine them.

The first is about testing the intake system for leaks. Some of you may have seen my DIY intake tester. It seems to work ok for showing leaks except for one small problem: It's damn near impossible for me to isolate the sound of leaking air when it's near the throttle body/air guide. I've spent several hours on it in the past week and finally gave up and took it all apart again.

My first question is this: Should I be able to pressurize the intake to 5psi (.3 bar as per the manual) and have it maintain pressure or will it naturally leak out past the valves or something? I think that it should be able to maintain pressure for several minutes at least since the closed valves should stop any air from passing to the exhaust and the crank case is sealed (ideally) and pressurized also.




The second one has to do with the TB cover vents. I looked up a discussion from a while ago where the direction of air flow was discussed and debated:

TB cover vent discussion thread

I know that the guys with SC units just put a little vent filter on top of the cam cover vent but that doesn't force the air to circulate as it would if it were going up into the intake tube as it was originally designed. My question here is: How important is this circulation and have there been any ill effects to just putting that little filter on there? Part B of this would be: Has anyone who's done this ever had anything clog up the vent filter or is it (as I suspect) always clean and therefore probably not doing anything but covering the hole?

Fabio421

To answer your first question. No you will not get a seal. There will always be a leak somewhere whether it be through an open valve or even something as minute as the shaft bushing on the throttle plate rod. To test for leaks, pressurize the system with 10 psi or so and then spray ample amounts of soapy water onto the intake piping that you suspect is leaking.

As for your second question. The intake tubes draw air from the timing belt covers. This is supposed to keep the belt cooler. If you put filters there you will not have the same airflow. Most likely you won't have any.

aggravation

I'm curious about this also because I'm chasing down a possible leak. I can disconnect the supercharger and pressurize the piping leading toward the intake with 12 psi and it's tight all the way past the intercooler to the MAF but with an ultra sonic leak detector you can hear a large volume of air moving somewhere down deep. It's hard to tell exactly where it is escaping...externally under the intake or internally.
I'm assuming the throttle plate won't seal air tight but maybe it should be able to.
I would guess that once past the throttle plate there is no stopping it without plugging the exhaust because I think some of the exhaust valves will be open no matter where the engine is in its rotation.

All guess work on my part though, hopefully some of the smart guys will help with understanding the process...

ZEUS+

It is nearly impossible for an airtight throttle plate. Have seen many improperly adjusted(other manufacturers). If closed to much the plate will stick in the bore and cause a sticky throttle amoung other things.
There will be leakage past valves etc. With a smoke tester this is not an obstacle, just the price of the unit.
I am in the process of designing my sc system. I plan on routing the tb vent on the inlet side through filters so dust or debris will be trapped.

Mike Frye

I'm trying to picture the valve leak issue for the first part.

You've got the cylinder with the rings separating the combustion chamber from the crankcase and the crankcase is pressurized anyway, so the only issue is the valves, right?

So if the intake valves are open, the exhaust are closed and if the exhaust are open the intakes are closed and in most cylinders they're both closed, aren't they? How does the air leak from the intake past either the intake or exhaust valves to the open exhaust system?

Fabio and Zeus, that's the way I was picturing the tb vent. You need to have that, right? Is anyone having issues with the little filter on there? Maybe an auxiliary vent or fan system could be added to just cycle air through there for systems that have pressurized intake air...

Fabio421

Some valves will be open, others will be closed. What exactly are you looking for? Soapy water and a little pressuized air will show you your leaks.

As for the timing belt vents and if they are necessary. I don't have that answer. Some are running without them and they seem to be OK. Porsche felt that they were needed. You make the call.

hacker-pschorr

For starters the flappy bearings on both ends will leak some air, so do the bearings for the throttle plate.
Everytime I've pressurize a 928 intake it holds whatever pressure it's set to. These small leaks in the bearings are not an issue.

The throttle plate is also a non-issue. With my intake pressurized, opening / closing the throttle doesn't change anything.
Just to recap, I've pressurized mine up to 40psi.

The #1 problem you are going to have with a 928 not setup for boost is keeping the MAF seated under boost.

Mike Frye

That's what I mean, some will be open, and others will be closed, but in no cylinder are the intake and exhaust BOTH open, so how would the air leak through?

For the vent, I know it's my call, just wondering if anyone that has used a separate little filter has noticed it actually doing anything.

That's what I was talking about. So you didn't continually pump in air at 40 psi, you pumped it up and it maintained 40 psi until you let it out. So if mine is leaking VERY slightly, it's probably from the bearings. If I hear a hissing, there's a problem I have to fix.

Thanks for the confirmation Erik!

aggravation

Hacker are you saying you can pump it up and "valve it off", so to speak, and it should hold the pressure? Or that you can pump it up and it leaks out slow enough to have the target pressure built up in the intake while you test for leaks but removing the air hose it would leak down right away?

Bill Ball

The stock cams are non-overlap. So, at no time are an intake and exhaust valve opened at the same time. Therefore there is no path for air in the intake to escape through the exhaust. It either doesn't get into the cylinder or if it does, it is trapped there, unless a cylinder has a leakdown problem. Leaks past the throttle plate bushings or the flappy bearings should be quite small, unless there is a problem there. As Hacker says, it should hold pressure and any hissing noises should be tracked down.

Adam Geist

Mike,

I have a leakdown tester you may borrow. Please double check that the pcv-type valve at the oil filler/oil vent hose was installed in the correct direction. I know, I know....I'm being very **** again.
Adam

Mike Frye

Hey Adam,

I appreciate it. I have the whole thing apart and as I mentioned I'm going to come up with a way to test the whole throttle body/air guide as one unit before putting it in there so I can eliminate all of those connections as possible culprits when it's all together. Once all the intake runners are in it's hard to isolate anything underneath the 'cage'.

The PCV valve is in with the narrow end down in the tube and the wide end at the top the same way it came out. But thanks.

I appreciate your attention to detail, keep the suggestions coming!

Thanks everyone.

hacker-pschorr

I have an air compressor set to a certain psi. I've never valved it off to see if the pressure would hold.

So I fall under option B

Bill Ball

For continuity's sake, here is the follow-up thread by Mike with a successful ending!...

https://rennlist.com/forums/928-forum/446728-throttle-body-leak-is-there-a-fix-yes-there-is.html

Mongo

What?! in that picture Porsche doesn't have a special tire pump they can sell you for $500?! Amazing!