Mercedes always had a penchant for using manifold vacuum as a control device for various ancillary systems. As elegant as that may be, it does result in an engine compartment with a multitude of vacuum lines and connections, on top of an already-complicated fuel injection and air-management system. If that myriad of components has never been renewed on a 20-year-old car, there will almost certainly be leaks, and they can be tricky to track down.
Vacuum Leak Symptoms
Major vacuum leaks will make the engine very rough or lumpy at idle as the optimum air-fuel (A / F) ratio is compromised by the intake of unmetered air. The car may still drive reasonably well, because the relative magnitude of the leak becomes much less significant as the throttle opens and the engine draws in the large amounts of air it needs for combustion under load. Idle speed may also be elevated, as the vacuum leak frustrates the efforts of the idle-control system to regulate the amount of air admitted to the intake ports.
The economy gauge in the instrument cluster should be pegged left at idle (except in gear with the A / C on). But this gauge is not accurate enough for proper diagnosis. We really want to connect a hand-held vacuum gauge to the intake manifold. The best place to do this is at the back of the manifold on the passenger side, where the gray line to the interior accessories takes its feed. A healthy engine at idle, according to most textbooks, should indicate 17-22 in / Hg of vacuum; In the case of a V-8 Mercedes 126, we should be seeing around 20. A lower number can indicate many things, including improper ignition timing and generally poor engine condition. But if the engine is other sound sound, vacuum leaks will be noticeable here.
Best Detection Method
One way to find vacuum leaks is with an automotive stethoscope. You may be able to actually hear the intake of false air. But the best way is to use your car's oxygen senor as a diagnostic aid. The oxygen sensor on the early V-8's is a single wire device, sending a voltage close to 1.0 when the A / F ratio is very rich and close to 0.0 when the mixture is very lean. By connecting the sensor's output wire through a voltmeter to battery ground, we can use it to test for vacuum leaks. To do this:
- Get the sensor HOT by driving the car.
- Test the sensor's lean response by creating a massive vacuum leak. Disconnect the gray line at the back of the manifold, where you hooked up your vacuum gauge.
- Test the sensor's rich response by spraying carb cleaner into the intake. (The air cleaner needs to be off for this work.)
- If the sensor is not responding correctly, replace it. It is vital for proper mixture control and fuel economy.
- If the sensor is working, test for vacuum leaks by spraying small amounts of carb cleaner around suspected leak areas, such as fuel injector seals and breath hoses. If you see a brief spike in the voltage, you've found a problem area.
- Do not go crazy with the carb cleaner: it is hostile to old rubber.
Unfortunately, not all of the possible culprits can be reached with this procedure. The rubber "donuts" between the upper- and lower-halves of the intake manifold are almost impossible to test. But usually, some combination of this procedure plus temporarily isolating vacuum-powered accessories will expose the leak.
Finally, a word on a vacuum leak location that is easily overlooked, but which can be very serious if allowed to get too far. The vacuum line from the back of the intake plenum to the brake booster is critical for brake performance; these cars are very difficult to stop without servo assistance. The line itself is easy to inspect for cracks, but it's hard to verify the integrity of the check valve in the line. The valve can look fine from a distance, but hairline cracks may be developing right at that point where the thick tubing mates with the valve. Try flexing this area with the engine running and see if your id changes, and give it the carb cleaner spritz test, too. New lines, including the check valve, are $ 40-50 and worth replacing for peace of mind if the original is still on there. Do not ask me how I know …!