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I need all your knowledgeable help! I own a '94 TT Auto with 69Kmiles with the car only having an air intake filter and downpipe. Recently, I just started to have a rough idle where my car's idle drifts from a normal 650RPM (or so) to a seesaw of 590-700RPMs (or so). This just recently started happening so it's got me a little worried. I need all possible causes of why this might be happening and or possible quick fixes. Thanks...
 

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You will probably have to look very carefully at all your vacuum lines to make sure you are not sucking air. Most of the time when idle surges it is getting air seeping in somewhere. Some vacuum lines especially on Turbo models get real brittle from heat under the hood and a vacuum line will develop a crack or slit and the only way to find them sometimes is while engine is idling is physically run your hands down the vacuum hoses or pull them off individually and examine them carefully and replace any that feel stiff and non pliable. and also listening closely to air being sucked in. My MKIII did this after the mechanic rebuilt my engine and forgot to install a vacuum hose on my intake tubing just after my MAF meter which was my power steering idle compensation. Needless to say My engine idle surged because it was sucking more air into the intake after the air meter, which the meter didn't register so it threw the air/fuel mixture off and car was running lean making the idle surge plus when I took a turn my idle would fall off and engine would die because hose wasn't connected to the steering vacuum assist. I also wound up finding another vacuum line had slipped off one of the tubing.Once all vacuum leaks were sealed off the ECU adjusted the air/fuel mixture and the idle settled down.Good luck with finding the answer to your problem, and hope I helped you with a solution.
 
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