[Lear35A]

I have no suspicion that I have a vacuum leak, but with some time to kill (stripped power seat nuts) I thought I'd check. I made a rig that I think is similar to one that others have made and posted about lately (picture below). When I pressurized the intake, I got exactly what I expected. That is all the air goes out the open intake valves. What am I missing here? I see no way to pressurize just the intake manifold and therefore be able to check the many vacuum hoses.

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[Cloud9...68]

Good question - I don't get it, either. I've been chasing down a "lean code" error, which is usually caused by a vacuum leak on the intake side, on my daughter's E46, and I used Tama's cigar-based home-made "smoke machine" method, which identified a cracked intake boot. In my searches for different methods to track down intake vacuum leaks, I've never seen anyone pressurize the intake like you did, and then watch for the leak rate on the gauge. I think you're right - unless you first remove the camshaft, the air will simply leak past the open intake valves. I think you need a lower-pressure source of air that you can allow you to visually detect the source of the leak (smoke, in other words).

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[flash]

perhaps i just continue to get lucky and have the magic spot in the rotation, but if the motor is cold, and has no oil pressure at the lifters, there should be no escape. if there is, it's at the exhaust valves, and a tennis ball in the tailpipe will take care of that.

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[MB968]

Anyone on the forum ever made a polar plot that shows the open/closed position of each valve versus crank rotation? Should be fairly easy to do and see where the best window might be. Of course deflated lifters would impact this, don't know if there is any easy way to add this feature into the mix.

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[flash]

there is one here somewhere. i remember moving it around. lol - i'd have to look for it now. i think it has "cam" in the title though

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[tamathumper]

There is one as part of the timing document I have, although I don't know that it's "polar".



Page 4 of this document...



http://www.evernote.com/shard/s20/sh/314...0743e4c3c7

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[Lear35A]

Does not compute. Anyone who has installed cams knows that even if the engine has been cold for six months and there is virtually no oil in the lifters the cams push the valves down plenty. (Even a special Porsche tool to do it. ) Add the valve overlap frequently mentioned here, and the pressurizing air goes right into the head and out the exhaust and down into the crankcase. Maybe, maybe there's a sweet spot that minimizes this, but nowhere that would allow a leak down test of the vacuum system.

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[banditsc]

I guess the intent here was lost, if the system is properly sealed it will hold air for a few minutes which gives you enough time to walk around and find any leaking hoses or connections. I've done this numerous times on my 928 and 968 and found little issues here and there that required a little tightening or a new vac hose connection, etc..

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[Lear35A]

No I know the intent. That's why I went to the trouble of making this thing. But I don't see how this could hold air for seconds much less minutes. If I put 5psi of air into the <acronym title='mass air flow'>MAF</acronym> and close my little ball valve the pressure falls to atmospheric immediately. The air isn't going out some tiny vacuum leak, it's pouring out the intake valves to the exhaust and crankcase.

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[banditsc]

Don't know what to tell you, maybe play with the crank position. I and other have been able to hold air in the intake system for minutes.

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[flash]

ditto, and on more than one engine.

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[Cloud9...68]

This is indeed a mystery. The only logical explanation seems to be the lifters. If they're not pressurized, they won't overcome the spring tension, and the valve will close. Could there be that much variation in the tendency for the lifters to remain pressurized from engine to engine?

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[MB968]

OK, I finally took the time to look at the charts that are ref'd above. It is unlikely that you'd be lucky to find a spot where all the intakes were closed due to overlap. But, you can choose a spot where one intake is open, and the exhaust on that same cylinder is closed. So, you will get leakdown past the rings, but you should be able to pressurize the system. Its just that one cylinder will also be pressurized. Somebody check this out to verify. But I think this is the reason you can find a window where pressurizing the intake will work.

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[tamathumper]

My experience is, consistently, that the pressure leaks down immediately from 5 psi to about 3 psi, then falls a bit more slowly from there to atmosphere, with a loud hiss emanating from under the intake manifold *somewhere*.

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[flash]

no idea where your loud hiss is, other than a leak somewhere. i don't remember a quick drop from 5 to 3 either, but i'd have to check it again to know for sure.

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[968Syncro]

It seems the tried and true method to find intake system vacuum leaks? With the engine idling, spray small shots of starting fluid at various hose connections - when the engine increases RPM you found the leak.

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[Lear35A]

I'm a little skeptical and right now feel I should have spent my time making a smoke generator rather than this pressurizing device. I'm in Germany this week, but when I get home I'll play around with the crank position and try to get it to seal.

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[tamathumper]

I've had no success with that method, but maybe I'm doing it wrong. I've tried sprays of starting fluid, carb cleaner, and even an unlit propane torch to find vacuum leaks with the car running. Unfortunately there is never a blip in the idle, even when I put the open propane torch right up to the air filter.

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[flash]

it takes a pretty decent amount of starting fluid, in just the right spot, to make the idle race. it works, but you have to get it right.

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[tamathumper]

Do you have to unplug the O2 sensor or something to disable the mix adjustment?

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