clanmcclure

Good afternoon,
Long time listener, first time caller. I’m working on my 1988 S4 Auto. I’m in the process of doing an Intake Refresh and Cam Covers. I had everything back together, following Dwayne’s write up (thank you Dwayne!). I had verified the fuel system was leak-free by bypassing the fuel pump relay and also verified the TPS was functioning correctly at idle and WOT. Following Dwayne’s steps, I tried to pressurize the intake via the MAF and I could hear a decent air leak (or at least the sound of air moving) somewhere near the MAF and the rear/passenger side of the intake. I couldn’t get the system to hold any pressure. I’ve verified the MAF and the plumbing parts will hold pressure on their own, so the loss is downstream of the MAF itself. I tried soapy water, inspection mirrors and couldn’t find the leak. All new hoses and vacuum lines. After giving up and pulling the Intake and fuel rails off, I wondered how the intake could hold any pressure if the intake/exhaust valves are open on any of the cylinders? I don’t believe that setting the engine to TDC closes ALL of the valves at once, so I’m scratching my head to figure out how you would pressurize the intake system to check for leaks. Can anyone advise?

Thanks in advance,
Kevin

The Forgotten On

45 degrees before TDC on a 32V engine closes all the valves.

Crank it to there and then do a smoke test to find any leak. You can do it with a machine or a piece of hose and a cigar. Choice is yours

clanmcclure

Thank you Sir

Michael Benno

A leak free intake should pressurize regardless of the crank position. Ive done the test a bunch and never set the crank position.

here is an example of it holding pressure without checking position.

getting an airtight intake is a challenge and will require some work.

first off get a smoke tester and see if there are leaks. This will save you a lot of headache. After 3 intake jobs i would say this is mandatory

here are common areas of leaks

1. bearings for flappy and throttle, replace with double sealed
2. plastic y pipes that attach on the sides of the throttle boot. I additional hose clamps around the throttle boot for an airtight fit.
3. Intake side covets, these need a little extra sealant.
4. valve covers, oil filler cap, dip-sick. All these can leak air.
5. of course all the hoses and clamps under the intake

clanmcclure

Thanks for the advice. I did the intake and cam swap with Roger @ 928sRus so the intake was refurbished by Dr. Blast and 928 Doctors replaced all of the internals (flappy bearings, side cover gaskets ,etc). All new vacuum lines, elbows. Oil filler cap O-ring, dip stick and hoses. The leak sounds like it’s around the MAF on the passenger side, so also around the intake gasket near the firewall. I agree that the smoke tester will be the quickest answer so I just bought one on Amazon. Because I bought it, I’m going to guess that when I re-install the intake with new gaskets, that it will magically be sealed correctly and I won’t have needed the smoke machine! I would love to have that problem.

FredR

Kevin,

Our motors are "four stroke" and in simple terms that means that the valves have to be open for two of those strokes and closed for two of them- cam timing modifies that concept a little but in essence it still holds. The engine has a 90 degree crank and it has 8 cylinders - the mathematics say it has to have valves open no matter the position of the crank.

Pumping air through the inlet system will create a small pressure and the more air that is pumped the greater the pressure but with the throttle plate closed there is a lot of resistance to flow given the small hole in the venturi and the static aperture in the ISV- open the throttle valve and any pressure shown on the gauge at the point of entry will collapse immediately. That is basically why a smoke test is the practical way to go as it will show leaks with the slightest of pressure.

45 degrees before TDC is an important parameter when initially setting up the cams on interference engines as when the motor is in such position the valves cannot clatter the pistons in any cylinder no matter the position of the cams. Thus kit like Ken's brilliant 32VR cam timing kit has a setting notch to facilitate cam timing with the motor at 45 BTDC or with the crank at 180 degrees further rotated.