DC_//S4

Hi guys, I decided to take my newly tuned-up S4 (new plugs, oil change, brakes, etc.) for a nice drive this weekend and after a few spirited launches and flings through the back roads, the "check engine" light came on and when I pulled over and opened the hood- it sounds like the engine is sucking air like a vacuum and the belts sound like they're running louder. Not to mention the rough idle- it kind of shakes the whole car like something's off balance. At normal highway speeds, it's fine.

And the brakes work fine for a while then go completely manual (have to push really hard to get the car to stop). Then they come back again, randomly.

What the hell did I do???

deeprecon1

You have a answer in your question... vacuum leak !

start spraying carb cleaner around your hoses the idle should speed up .. you might also have to wiggle while spraying.

Vacuum sucks !

EuroFreak

Exactly, vacuum leak. Try looking listening to where the leak is coming from - often you can hear rushing air. I have run across this with several cars and in many cases its a simple fix.

ScottGo

Check the throttle body boot. I would venture to guess that most guys have ripped at least one in their travels.

ScottGo

thejulex

TBB is mu guess. The air is getting sucked in via torn TBB instead the normal route and it might be the source of the sucking sound.

Texas Twin T

Maybe it’s the big vacuum like that is on the back of the engine that plugs into the firewall?

DC_//S4

Yeah, the hair dryer/vacuum sound is coming from the back of the engine, near the firewall.

I had to drive it a little bit today and it felt a little better, but the brake pedal was hard as a rock. Then later in the day, it started up fine, idled fine and the brakes worked again! WTF???

lovely09

Here are some information in knowing if there's a vacuum leak on your car.

1.Too fast an idle speed. If an engine without computerized idle speed control is idling too fast and refuses to come down to a normal idle speed despite your best efforts to back off the carburetor idle speed screw or air bypass adjustment screw (fuel injection), air is getting past the throttle somewhere. Common leak paths include the carburetor and throttle body gaskets, carburetor insulator spacers, intake manifold gaskets, and of course, any of the engine's vacuum fittings, hoses and accessories. It is even possible that leaky O-rings around the fuel injectors are allowing air to leak past the seals. Another overlooked item can be a worn throttle shaft.
2.A rough idle or stalling. A performance cam with lots of valve overlap can give an engine a lopping idle, but so can a vacuum leak. A really serious leak can lean the air/fuel mixture out to such an extent that an engine won't idle at all. An EGR valve that is stuck open at idle can have the same effect as a vacuum leak. So too can the wrong PCV valve (one that flows too much air for the application), or a loose PCV hose. The rough idle in these cases is caused by "lean misfire." The fuel mixture is too lean to ignite reliably so it often misfires and fails to ignite at all. Lean misfire will show up as elevated hydrocarbon (HC) readings in the exhaust, enough, in fact, to cause a vehicle to fail an emissions test.
3.Hesitation or misfiring when accelerating. This may be due to a vacuum leak, but it can also be caused by a weak or inoperative accelerator pump in a carburetor, dirty injectors, or even ignition problems such as a cracked coil, worn spark plugs or incorrectly gapped plugs.
4.An idle mixture that defies adjustment. When setting the idle mixture adjustment screws on a carburetor, the idle speed should start to falter as the adjustment screws are turned in to lean out the mixture. If the screws seem to have little or no effect on idle, you have either got a carburetor problem or a vacuum leak.

DC_//S4

Thank you so much for your detailed response- again, I drove it for a couple of miles around town today and it was doing the rough idle and occasionally the idle speed would rise from a normal of 10K RPMs to maybe 15K. Nothing major, but not usual for my car.

After a couple of quick trips around town, the car idled fine and the brakes started to work normally again. I don't get it! It's like it fixes itself and then freaks out a drive or two later!

Can anyone address the reason why my brake pedal will be hard as a rock and then after a while will feel normal?

Finally, is this something I can fix myself? Is there a guide? I'm pretty handy with simple stuff like brakes and hoses.

Thanks again!

donromani

Check the big hose that comes off of the back of the intake manifold and runs to the firewall, roughly in the middle. This is your brake booster hose and it supplies pressurized air to the brake system to assist with braking.

Check this for leaks at the connections at the intake manifold and especially where the hose passes through the firewall.