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Vacuum Leaks & Boost Pressure Leaks

[Saab 900, 9000, 92X, 93, 95, 99]

This section was primarily built for the Saab 900 79-94 but the actual article itself will apply to all Saabs in general. There is some good info on how to test for Vacuum leaks and Intake leaks that will apply across the board.

A Special thanks to Matt Hoffman for the contribution of this material. It is guys like him that make the DYI guys keep on going! Kudos Matt!

Intake system (vacuum) leaks are one of those things that don't get a lot of popular press. They are a little harder to deal with than somewhat more straightforward maintenance issues, like changing the spark plugs every 30,000 but they can have a real effect on your car's performance. The upside is that as you find and eliminate leaks, you will be rewarded with much smoother engine operation and better drivability.

What causes leaks? Almost all intake system leaks can be traced to rubber degradation. Over time, oxygen in the air (or carb cleaner) will decay the rubber part of the hoses, bushings and fittings on your engine, and make them brittle. In some cases heat and oil can erode the rubber, and vibration causes you airtight connections to leak. In the case of Saabs with turbochargers, there are two leaks that cause problems, since the engine may be operating at a positive pressure or negative pressure in the intake system.

Why do we care?
The only engine parameter you can control from the driver's seat is the amount of air going in the intake by pushing the throttle. Everything else is done automatically with a fuel control computer called an ECM. Vacuum leaks confuse the fuel system by leaning the mixture and by sending incorrect data to the fuel injection computer. Leaks under boost richen the mixture, and lead to other wear problems.

How do I repair the issues?
Some worn things are easier to fix than others, so it makes sense to get the most reward for your effort. The Bentley repair book recommends checking the fuel injector o-rings and replacing them if low performance and poor fuel economy occur. Personally, I don't think that's a great place to start. Removing the fuel injector rail is a lot of work and requires a lot of care. I recommend looking in a few easier to reach places first, and then move to the more difficult fixes. This will help in that you can see for yourself how much you want to pursue a leak-free system. A good way to test the injector o-rings is to pour water over them one at a time to see if the engine changes its beat. If it does then you need to replace the corresponding ones. This method of testing also works with the vacuum hoses as well. Do this test at idle.

Fixes:
General advice: some of the hoses and fittings can be so worn that they will fail when first handled. Some of the plastic tees on the intake manifold get very brittle with age, so you may want to buy some spares or have a way to deal with a failure if the car is your daily driver. Bear in mind that you don't have to do all the work at once. You can get a length of hose that will cover one or two connections and go from there. The typical hose to use is 5/32 in diameter which a bit smaller than came on the care originally but seems to eliminate issues with hoses flying off especially on turbo cars. Use a great grade of silicone hose and squirt the ends of the hoses with brake clean just prior to installing them onto any fittings. Once the brake clean disappears they will be difficult to pull off the fittings.

STEP 1 Look at the hoses and connections on the intake manifold, valve cover, and sensors. Hoses get brittle with age, and the thin wall hoses on Saabs can crack just past where they are plugged into a fitting. There is a chart on the fender that illustrates where to follow each hose. If you do replace hoses, make sure that you replace one at a time to avoid confusion.

Tip - if you are not ready to replace a hose with a new one, then just visually check the hoses, as well as listening for hisses while the engine is idling. Sometimes there will be a good seal on the brittle connection, that will be destroyed when you move it. )
STEP 2 Check the rubber bushings in the intake manifold, which hold the little plastic ports where the hoses plug in. If the plastic port inserts can be wiggled easily in their sockets, then you have a leak there that is a nice quick fix. See the pictures for comparison between an old and new bushing. If you have not personally replaced these on a car that is 8 years old or more, then I suggest just buying all three, and the valve cover tee bushing (they are cheap) and plan to replace them outright. It's a fine place to start on this project. Be careful not to pull on the small port when you remove them, since these get brittle and break easily. You might want to buy replacements of these with the bushings, just in case. Click photo below to enlarge! (this applies for the Saab 900 79-94

STEP 3 Note the intake tee on the valve cover and the bushing it rides in. This is another place where the bushing can wear and allow a large leak. The tee gets manifold vacuum and also direct air pressure from the air cleaner tubing. While you are there, note the valve cover gasket's condition. Since the tee is bringing manifold vacuum to the valve cover, an oil leak is also a sign of an air leak.
STEP 4 Check the hoses from the air filter (and all the turbo hoses, if equipped) to the intake manifold. Any hose that is much softer than the rest are probably soaked with oil, and should be replaced. Clamps and connectors should be properly tight.

Improvements:
None of these things will actually improve your car's performance, based on factory specs. But if you bought your Saab used, with old hoses and bushings in place, then you have never had an engine performing to factory specs, even with careful attention to other repairs. Going through the intake system is a great complement to a weekend tune-up and should give you a smoother idle, less vibration at highway speeds, and more low-end horsepower, which is easier on clutches and more fun to drive with.

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