R129 Issues Episode 2: MAF Sensors

The R129 SL was launched 33 years ago, moving this SL variant into the classic car territory. You wouldn’t think it when considering how technologically advanced the R129 SL was – the new creature comforts available at the start of the nineties are no different to those we are used to now. These electronic systems are often cited as the major issues with the R129 SL, but as the world’s leading specialist we continue to report a variety of issues not commonly known to owners and workshops around the world. This article focuses specifically on the MAF sensors used with late model six-cylinder (1996 onwards) R129s.

What does a MAF sensor do?

The MAF (mass air flow) sensor is an essential part of your fuel injection system and is responsible for calculating the total amount of air entering your engine. A MAF sensor that works correctly should accurately calculate the air flow, for the computer to inject the right amount of fuel into your engine. Over time, damage or contamination will occur to the MAF sensor and diminish its functionality.

How do you know if your MAF sensor isn’t working?

The downside to the R129’s technology is that there is no specific engine computer code to indicate a poorly operating or failed MAF sensor. It therefore takes an R129 expert to test the car in real world conditions and then investigate further by looking at the operating conditions of your R129’s engine.

What are the common symptoms of a dysfunctional MAF Sensor?

A MAF sensor can cause a variety of issues with performance; these include stalling, jerking, hesitant acceleration and a general reduction in power output. Too much or too little fuel in the chamber will cause untimed detonations and result in unpredictable acceleration. Callum who leads our Modern Classics workshop revealed that ‘’during a road test you can generally tell when there is a MAF sensor issue, simply because of the contrast to a R129 that is performing impeccably.’’

Two Scenarios:

Your engine is running rich:

This means that there is too much fuel and not enough air in each combustion cylinder, resulting in the following:

• Black smoke from your exhaust
• Reduced economy
• Rough idling

Your engine is running lean:

This means that there is not enough fuel and too much air in each combustion cylinder, resulting in the following:

• Engine hesitation and surging
• Hard engine seizures
• Hard starting & Misfires

The Solution to a Faulty MAF Sensor:

Your MAF sensor is located at the back of your engine bay close to the bulkhead. It is clearly visible and has a mesh looking top. We advise that you have this replaced or cleaned thoroughly. Before doing this, have an expert investigate the above symptoms to establish if your R129 does indeed have a MAF sensor fault. As previously reported, there is no specific engine fault code or warning light to indicate a failed MAF sensor.

SLSHOP is the world’s leading SL specialist, with a 50,000 sq ft. facility dedicated to classic and modern classic Mercedes; with a team specifically configured to service and restore this era of Mercedes-Benz. They possess all the diagnostic equipment and real-world experience necessary to identify and rectify any form of mechanical, structural and cosmetic issue. Find out how we can work with you to perfect your Modern Classic.

Alternatively, you can purchase the replacement MAF sensor from us and have this fitted by your local garage. Simply visit parts.theSLSHOP.com and search for your replacement R129 SL MAF Sensor.