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Isolating Vacuum Leaks

on the 1992 300E (W124/M103)

by Robert Ryan ( r_p_ryan on the forum )


The goal is to identify the best way to isolate vacuum leaks. Experiments were conducted using a variety of techniques. The most effective technique turned out to also be the easiest and required the simplest tools. This entails observing the oxygen sensor while carburetor cleaner is sprayed in very small quantities at suspected leaks.

Skill Level
Diagnosing vacuum leaks can be done by both beginner and advanced shade tree mechanics. There are some techniques here that should be left to those with understanding of MB fuel injection. However the bulk of techniques can be performed by individuals with the following skillsets:

Advanced Techniques

Special Thanks

Useful Links

When I first did this I sprayed carb cleaner on all six injector seals and one showed as leaking and the other five were good. One week later I repeated the procedure and discovered they were all leaking except the one I repaired. This indicates that the carb cleaner actually caused vacuum leaks at the injector seals. While I haven't tried a less aggressive fluid such as WD-40, I highly recommend trying something other than carb cleaner. I found that the carb cleaner dissolved cured silicone, and suspect that it ruined the seals.

Spraying carb cleaner on a hot engine produces highly flammable and toxic vapors.

I experimented with a variety of highly flammable or dangerous gasses including propane, oxygen, and argon. Not only are these dangerous but also they produce zero benefit. The point here is that if you happen to have welding equipment there is no reason to use these gasses to identify vacuum leaks, as they just don't work.

The following narrative describes how I worked on my car. This narrative must not be considered the best practices way to repair a car. The work I have performed on my car is purely as a hobby and I am by no measure a professional mechanic. My real job has nothing to do with the automobile industry and I am self-taught. The tools and techniques I use are clearly amateurish. Therefore, if you are planning on working on your car you must obtain the proper training, tools and components to do the job correctly. Many of the techniques I employ could be considered hazardous and would probably get a professional mechanic fired from a reputable facility. I TAKE NO RESPONSIBILITY FOR ANY INJURIES OR DAMAGE INCURRED BY USING OR FOLLOWING THIS INFORMATION.



The stethoscope proved tedious and difficult
Listening and observing idle speed while applying flammables, accelerants and retardants was scary, tedious and not reliable
Observation of the oxygen sensor is a simple and reliable way to identify vacuum leaks

How to Use the O2 Sensor to Identify Vacuum Leaks

The engine will be idling for a considerable amount of time. I recommend checking the engine temperature frequently during this process. I also recommend occasionally revving the engine to 2000 RPMs.

Step One: Stopping the computers from setting the idle

The computerized fuel injection and ignition systems work as a team to produce a smooth idle. I decided it was best to have a warm engine that was disconnected from the computerized idle controls.

Upon Duke's tip, I should have just popped out the OVP relay. This prevents the computer from powering up. The benefit here is now clear – only one thing to unplug. (There may be a side benefit of not needing to reset the Check Engine light, don't know yet). The OVP relay lives behind the battery, has two push-in fuses on the top, and has an aluminum body. It is to the left of the computer (left from the driver's perspective). Do this and skip to step two.

(For the curious, here's how I did it)
I removed the air cleaner and then disconnected the O2 sensor, Idle Control Valve, Electro Hydraulic Actuator, and Throttle Switch (see pictures below). When the engine is started with these disconnections the Check Engine light will illuminate and will need to be reset. It is reset with a technique described by Steve Brotherton in the Links section above.

Disconnecting the idle control valve

The oxygen sensor wire lives under the carpet on the passenger side. The sensor plugs into the green wire. In this photo I am connecting the O2 sensor wire to a multimeter to read the voltage (black wire to yellow). The O2 sensor produces between 0.9 and 0.1 volts depending on the saturation of oxygen in the exhaust. You'll need a multimeter that has a resolution of 1/10 of a volt or finer.

Step Two: Isolating vacuum controlled components from the intake manifold
After the controls that modify the idle speed were disconnected I removed the vacuum hoses from the intake manifold and taped up the nipples (see pictures below). Several components are vacuum controlled and can contribute to vacuum leaks. These include the brake booster, transmission, ignition system, and chassis controls (heating, A/C, etc). Later on these systems will be sequentially reconnected to the manifold and the O2 sensor will tell us if they have any leaks.

This hose provides vacuum pressure to the EZL ignition computer

Here I have removed the hose from the nipple

And here the nipple is taped shut with duct tape

Removing and taping the connection for the heater, A/C vacuum

Removing and taping the vacuum for the transmission

Removing and taping the connection for the brake booster

Step Three: Connecting the multimeter to the oxygen sensor
Check out the picture in Step One. The (+) lead from the multimeter is connected to the plug that went into the green wire. I use a yellow test wire with clips and a black wire with a push-on connector to attach the multimeter to the sensor. The (-) lead of the multmeter is grounded to the chassis. After the O2 sensor wire is connected to the multimeter start up the car (the car should have been warmed up previously). The Check Engine light will be on. This is okay for now.

Reading from the O2 sensor. 0.9 volts is approximately at the richest end of the spectrum. If your car reads this high then you might need to have the advanced skills described at the beginning.

Here the O2 sensor is at the other end of the spectrum. By unplugging brake booster connection you can get a feel for how the O2 sensor responds. Also by shooting a very small amount of carb cleaner into the throttle body. This made the O2 sensor reading go from 0.1 straight up to 0.9 volts.

Step Four: Finding leaks on the manifold
At this point the engine should be warm. If there is a big leak the O2 sensor reading will be low. If the reading is near 0.9 volts then it is going to be difficult to find manifold leaks. This is because the highest reading is .9 volts and the carb cleaner increases the reading. Here is where the advanced users have an advantage: per Dan Landiss's write-up alter the enrichment so the reading is around 0.1. If this is too complicated don't despair, you can also just poke a small hole in the tape that covers the brake booster connection to allow some air in and lean-out the mixture. Just keep the hole as small as possible so the O2 reading is around 0.4 volts. Now shoot a small amount of carb cleaner at the throttle body. Does the O2 reading go up? If not then something is wrong and these techniques are not going to work for you - you might have a bad O2 sensor.

For the advanced users! If you modify this setting then you will need to reset it with a multi meter that reads duty cycles (and you still may fail the smog test)

Take the carb cleaner and shoot small amounts at the ICV hoses. If you shoot too much the fumes with enter the throttle body and alter the O2 sensor reading. This is where the sensor readings jumped up to 0.9 volts on my car.

(After I removed the ICV I found this cracked manifold hose)

Try some carb cleaner on the injector seals. I found #5 was leaking.

Once any leaks have been identified they need to be remedied prior to moving on to step five. I used some wire ties and some Hylomar sealant to temporarily plug the leaks I discovered in the manifold.

Step Five: Finding leaks in vacuum controlled components
The vacuum from the intake manifold powers lots of things, and any of these things can develop leaks. In this step I get the O2 reading up and then slowly reconnect the vacuum controlled components. If the 02 reading stays reduced after reconnecting a component then I know there is a leak in that component. The O2 sensor should be reading high, above 0.6 volts. The advanced users now need to enrich the mixture a la Landiss (get it around 0.85). If you made a small hole in the brake booster tape then plug it with more tape. If your O2 reading is still low then you might have difficulties with this step. With the engine still running, quickly remove the tape from the nipple and shove the vacuum hose back on. The sensor reading will dip down but should recover to exactly where it was before. I found no leaks in any of my vacuum controlled components.

The engine may quit when reconnecting the brake booster, so try to get the connection made as quickly as possible.

Putting it all back together.

1. Shut off the engine
2. Reattach the O2 sensor wire to the green wire
3. Reattach the air filter.
4. Reset the Check Engine light as per Steve Brotherton (link at top). The connector lives in front of the battery and has a lid on it. California cars have an LED and a push button for convenience. Later W124's have 16 contacts. There is a way to cheat, but you'll need to reset your seat position and radio stations. This might be destructive – I don't know so be warned and PLEASE let me know if YOU KNOW it to cause problems!
  1. Shut off the car
  2. Completely remove the battery and put it somewhere safe
  3. Turn the key to On
  4. Ground the (+) cable to the (-) cable for a second or two
  5. Turn the key to the Off position
  6. Reinstall the battery
5. Start the car.


- Robert Ryan

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