Volkswagen's fourth-generation Jetta isn't any more prone to vacuum leaks than any other car -- and probably a lot less than most. But little failures can and do happen, and the eight different engines offered with this car have their fair share of vacuum lines and potential leak points.
Turbocharged engines have been a Jetta staple for many years now and the 2005 Jetta is no exception. Many Jettas left the Wolfsburg factory with VW's exceptional 1.8-liter four-cylinder turbo, which changes the air leak equation a little bit. With a normal vacuum leak, the engine will suck extra air in througha leaking hose -- or, more likely, hose fitting -- and the engine will run lean. Turbocharged engines have pressurized intake tracts, so leaks will result in air blowing out from the pressure generated under boost.
Oh, those Germans -- they do think of everything. The Jetta depends on engine vacuum to perform dozens of different functions, not least of which to operate the brake booster and to keep rogue fuel vapors from leaking out of the car. It is for these reasons that the Jetta uses a supplemental vacuum pump that speeds vacuum buildup at startup, provides vacuum when called for and helps the computer to detect leaks. The pump kicks on whenever vacuum is lower than it should be; if the pump runs when the computer says it shouldn't, the computer will trigger a vacuum-leak warning code.
All of the Jetta's different engines uses a unique vacuum tube arrangement, and VW was kind enough to assist you -- or their mechanics, anyway --in tracking them down. A sticker that details the location, routing and purpose of every vacuum line on the engine can generally be found on top of the radiator support of every Jetta. You'll refer to this diagram on a regularly when tracking down leaks; just go by the diagram and check line-by-line till you find the leak.
The computer may tell you exactly where to find the leak, depending upon its location and severity. But, if it doesn't then you can fall back on this old mechanic's trick: Pick a line on your routing diagram and trace it back to the engine. Have an assistant briefly start the car, and spray the line where it connects to the engine with a two-second burst of brake cleaner or ether starting fluid. If you hear the engine suddenly rise in rpm after spraying the connection, then you've found your leak. Keep working along the line, spraying every connection until you find the leak. When you get to the end of the line, pick another and start again.
