Many owners of late-model Mercedes-Benz cars equipped with 3.0 and 3.5l V6 engines (M272 or M273), as well as some of the later V8s, have or will likely soon experience a near-universal Mercedes Benz intake manifold problem. Generally, we find problems in performance, which include poor idle, loss of power, and a Check Engine Light.
The Mercedes Benz intake manifold problem is caused by one of the inlet manifold tuning flaps breaking, causing unexpected and deficient performance depending on the orientation of the flap that has broken. To review the system, let’s describe what the Mercedes intake system is doing and why variable-intake-runner intake manifolds are becoming such popular design in modern cars.
The intake runner is the tube going into the cylinder head that the intake air travels down to get to the cylinder. As the intake valve closes, the air going into the engine hits the back of the valve and ‘bounces’ back. The pressure wave will ricochet back up into the intake manifold until it hits the back, where it will bounce back down the runner. The trick is to have the pressure wave arrive back at the cylinder right as the valve opens, achieving the densest possible air mixture passing into the combustion chamber (picture a high-pressure standing wave right behind the open intake valve).
To work in the RPM range you are targeting, you can tune the intake runners to be a very precise length. Unfortunately, scavenging only works in a narrow RPM range, so most engine designers build the intake runners to achieve this in a very useable RPM range. For a road car, this is around 3-4,000 rpm, where long intake runners promote a smooth even vortex.
How It Works
A variable-length intake runner manifold can switch between two sets of intake runners with a flap built into the manifold. This way, you can have one set of intake runners optimized for when the car is idling, and picking up low RPM speed. Then if you get on the throttle and request power for sportier or more demanding driving, the flap switches over to the second set of intake runners that is optimized for the higher rpm range. Many of these intakes also have specifically shaped flaps that cause a vortex, adding even more efficiency.
How It Fails
The most common cause of the Mercedes Benz intake manifold problem is one of the actuators for the variable system, causing the interior flap to become disconnected. Unfortunately, as the intake gets gunked up with oil and crud that comes out of the PCV system, the flaps have to work harder and harder…until the force becomes so great that the plastic linkage and actuators break and fail, turning on the dreaded Check Engine Light, and resulting in poor engine performance and economy.
Usually, the Mercedes intake manifold failure comes on as the vehicle gets on in miles, we typically see these at 80,000 to 100,000 miles. The resistance on the actuators and flaps running this system (they are maintaining an airtight seal, after all, so they are constantly moving along the walls of the manifold) becomes too much, causing the weakest part to break. In this case, it’s the cam lever in the center of the whole thing. Other parts can fail as well, including the Mercedes inlet manifold flaps, the actuator mounting arms, and the vacuum diaphragms.
The Proper Repair
You can find many “repair kits” out there with metal linkage that promise to correct the problem, but in our experience, the only true and permanent repair is to replace the intake manifold with the newer, and an updated design, while properly cleaning and servicing the intake ports in the cylinder head. Any “repairs” are usually very short-lasting and risk having failed parts fall into your engine via the intake ports, resulting in very expensive engine damage, and quite possibly ruining your engine. It’s just not worth the risk, do it right the first time, and it’s good to go for another 80,000 to 100,000 miles.
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The Atlantic Motorcar Center Service Team