Re: finding a vacuum leak on an 88' Audi 90

Adam, I'm not sure we're on the same page here about vacuum leaks. Yes, there's vacuum that's used to assist the brakes (assuming that the 90 had the old/new vac. assist brake system), adjust the climate control, etc.
That vacuum may be generated by an electric pump (like the power lock system) or a cam driven pump (like on the old 4k's) or syphoned off the engine intake using a check valve. When the (non-turbo) engine is running, the engine produces a vacuum - that is why air flows into it. There's a measuring device (in my 5k's, it's the CIS airflow meter) that figures out how much fuel to put in based on the amount of air flowing in. Between the meter and the engine, there's a vacuum. If there are any leaks in the plumbing between meter and engine, then unmeasured air will enter the engine and the mixture will get screwed up. From what I understand, the Smoke Test pressurizes the intake and leaks will let the smoke out. Most leaks will be found with this test, although I've heard that some leaks let air in, but tend to seal under pressure. The Bentley test is a good indicator of leaks in vacuum lines - which may be connected to things like emission controls, timing advance, etc. in which case the test is good. A Mity Vac is good for these tests, too, and gives you a gauge to read the leakdown. The test will not uncover leaks in any of the intake hoses, though. The propane gas test will suck in propane into the system, causing an increase in rpms - when the engine is at operating temperature and idle is stable, you'll have a fairly high intake vacuum to test with. Obviously, this is not a test to be done in a closed space....but you wouldn't be idling a car in there anyway, right? Cheers! Steve Sears 1987 Audi 5kTQ 1980 Audi 5k 1962 and '64 Auto Union DKW Junior deLuxes (SPAM Blocker NOTE: Remove SHOES to reply)
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thanks again Steve, The vacuum system I was thinking about was the one involving the Main Electrical Control unit which is involved in the cruise control system. I thought I might start here since my cruise control is not working (I know there are multiple reasos for this, but the vacuum test seems straightforward enough.) After this I'll graduate to the enigne/meter test and bust out the propane. I'm fairly confident that this engine/meter vacuum is where the leak is because of the low rpm power loss and because I can literally hear what can't be anything but an air leak, even over the noise of the engine. One more novice question, does it make any sense that this sound sometimes subsides somewhat after the engine runs for awhile? The low-end power loss is still present though.
I did not realize how many various vacuum systems there are. Glad I have this Bentley.
thanks for all your help sherlock vacuum. Adam
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Adam, The vacuum cruise is not too bad to troubleshoot - usually there's a leak in the vacuum switches at the pedals - it's a good idea to give them a clean as your feet always bring in some new dust to clog them up. When I first got my '87 the cruise did not work - I followed most of Huw Powell's instructions at http://www.humanspeakers.com/audi/cruise.htm to no avail - as a last step I went to check the CC "brain" under the steering wheel - all I found was a loose connector - no "brain"...damn.....pretty well sums up how I felt as well - sad considering it's part #1 on Huw's list. At least it's not like the "prototype" electric CC I have in my 1980 5k - which has a tendency to floor the accelerator after being unresponsive for 100 km - Audi switched to vacuum mid-1980 model year. A Mechanic's stethoscope is a good tool to use to look for the source of the leak sound. You can move the pickup around to pinpoint where the noise is coming from. Yes, leaks can change as the car warms up - metal parts expand, rubber contracts (somewhat) and some leaks seal - common with exhaust manifold leaks in 5kt's - ticking that goes away when the engine is warmed up can be either lifters and an EM leak. You might want to bookmark Huw's site - he has lots of stuff there on non-turbos there, including a troubleshooting checklist: http://www.humanspeakers.com/audi/tuning-cise.htm There are other 'net resources, such as: www.audifans.com - including their excellent Knowledgebase at http://www.audi-quattro.org/cgi-bin/twiki/view/Audi/WebHome , www.sjmautotechnik.com, http://www.isham-research.com/quattro/ etc etc. Cheers! Steve Sears 1987 Audi 5kTQ - vacuum cruise works when car is warm (vac. pump issue) 1980 Audi 5k - electric cruise only works to startle passengers 1962 and '64 Auto Union DKW Junior deLuxes - manual cruise works so long as I don't sprain my ankle.... (SPAM Blocker NOTE: Remove SHOES to reply)

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These are great resource. Thanks for the links. Adam
On Fri, 2 Apr 2004 09:55:32 -0500, "Steve Sears"
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