(The air cleaner needs to be off for this work.) Test the sensor's rich response by spraying carb cleaner into the intake. Disconnect the grey line at the back of the manifold, where you hooked up your vacuum gauge. Test the sensor's lean response by creating a massive vacuum leak. By connecting the sensor's output wire through a voltmeter to battery ground, we can use it to test for vacuum leaks. 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. But the best way is to use your car's oxygen senor as a diagnostic aid. You may be able to actually hear the intake of false air. One way to find vacuum leaks is with an automotive stethoscope. But if the engine is otherwise sound, vacuum leaks will be noticeable here. A lower number can indicate many things, including improper ignition timing and generally poor engine condition. 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. The best place to do this is at the back of the manifold on the passenger side, where the grey line to the interior accessories takes its feed. We really want to connect a hand-held vacuum gauge to the intake manifold. But this gauge is not accurate enough for proper diagnosis. The economy gauge in the instrument cluster should be pegged left at idle (unless in gear with the A/C on).

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 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. Major vacuum leaks will make the engine very rough or lumpy at idle as the optimal air-fuel (A/F) ratio is compromised by the intake of unmetered air.

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. 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. Mercedes always had a penchant for using manifold vacuum as a control device for various ancillary systems.

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