[flash]

no - the "race" would only last a second or two. that stuff burns really fast

warning! it is extremely flammable. do not get it anywhere near the exhaust manifold, as it can catch fire

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[tamathumper]

Mine doesn't "race", no matter what. A propane torch on wide-open at the intake, and it does nothing.

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[flash]

never done propane. no idea what that would do. i can't imagine much, based on the flow rate of the torch, and the ppm result. certainly a lot less than starter fluid right into the intake stream of a vacuum leak.



if i had to guess, i'd say your valves are all leaking. put a tennis ball in the tailpipe and try again with the pressure tester.

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[Cloud9...68]

Injecting the propane at the intake shouldn't do anything. The idea, as I understand it, is that if you have a leak in a vacuum line, the engine will run lean, and have a rough idle as a result (not sure why the sensors don't detect the lean condition, and enrich the mixture to compensate). Injecting propane at the point of the leak will temporarily cure the lean condition, and cause the idle to speed up. Never tried this technique, but that's my general understanding of it.

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[tamathumper]

Why would it matter where you introduce the highly flammable gas, so long as it gets into the cylinder?

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[MB968]

If it goes into the manifold past the <acronym title='mass air flow'>MAF</acronym>, then the computer sees it, whereas it if gets sucked in at a vac. leak point, the computer doesn't know it. However, I don't know how putting a flammable gas past the <acronym title='mass air flow'>MAF</acronym> would effect the idle?

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[tamathumper]

The way I see it, the same amount of gas (as in gaseous substance) is getting into the cylinder either way. If you introduce propane at *any* point in the vacuum system, including at the air filter, you're increasing the fuel mix and it should affect the idle, if that test is really valid. The <acronym title='mass air flow'>MAF</acronym> doesn't know if what's going past it is normal air or flammable propane, and introducing it there or at a vacuum leak still results in the exact same volume of air going into the cylinder.

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[flash]

the amount of air drawing it in at the MAF would be significantly more than at any vacuum leak. you would not be able to get much in there at a vacuum leak. it would likely not be enough to cause any changes. further, as it is a gas, and not a liquid, the relative effect would be less.

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[Lear35A]

Smoke machine sounding better and making more sense. And I don't even have a vacuum leak.

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[tamathumper]

Agreed.

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[flash]

the smoke will take the path of least resistance. it will not show you all of your leaks at once. you will have to do it a number of times, eliminating a leak and then testing again.



with a pressure tester, you can maintain the pressure, and find all of your leaks all at once.

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[Cloud9...68]

[quote name='flash' timestamp='1380213185' post='149922']

the smoke will take the path of least resistance. it will not show you all of your leaks at once. you will have to do it a number of times, eliminating a leak and then testing again.



with a pressure tester, you can maintain the pressure, and find all of your leaks all at once.

[/quote]

That is true. I recently did a smoke test (Tama's cigar method) on my daughter's E46, and quickly found a leak in the upper intake boot. I replaced it, let her drive the car several hundred miles, but the CEL didn't go off, so I had the codes read again, and the same one is still coming up. So, I'll have to repeat the smoke test to check for the next source. At least the test is very cheap and easy...

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[tamathumper]

The smoke comes out anywhere there's a leak, it doesn't only come out the "biggest hole". Anywhere the positive pressure is escaping, you'll see smoke.



I came across this on Pelican, which references the "disconnect the O2/ICV circuits or the idle won't change" idea I recalled, albeit for 944's but I have to assume some application for 968's as well.

Vacuum leaks are common and detract from high performance. The AFM (Air Flow Meter) measures the air being used by the engine. If there is a vacuum leak, the AFM output is in error and a lean mixture will result. This is especially important at idle and low rpms. A hesitation is often the result. If your engine uses an O2 sensor, the O2 sensor can hide symptoms.



In a non FI (Fuel Injected) car, like my VW Bug, I would spray around the intake with carburetor cleaner (STP) or a propane torch (don't light it!). Any vacuum leak will draw fuel into the intake and the idle will go up (rarely it would go down). This is how you know when you have found a vacuum leak. Test this with a shot of carburetor cleaner into the intake of the air filter. The engine RPMs will rise for a moment.



However, on an early 944, you have an O2 sensor that keeps the fuel mixture the same. This will defeat the test above. Disconnect the O2 sensor at the connector. On late 944s, you have an added idle stabilizer that keeps the idle at the same value. This system too must be turned off before fishing around for vacuum leaks with a can of carburetor cleaner. You have to short B & C on the test connector to disable the idle stabilizer. This too will defeat the above test since the idle would never change if the idle stabilizer is functional.



CAUTION: Using combustibles around a running engine. Keep a fire extinguisher handy at all times. Avoid spraying the spark plugs. Do not spray excessively.

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