09 Feb Check Engine Light Diagnosis – SL55 AMG
Driving to Naperville from Ohio may seem like a long way to travel for check engine light diagnosis. For many shops, this would indeed be an oddity but for Fluid MotorUnion it’s just another day. An SL55 AMG came in for a check engine light diagnosis because this customer was at his wit’s end. The vehicle had been all over Ohio for diagnosis from various shops. The car was slightly modified, so shop’s that dealt with OEM Mercedes service didn’t want to touch it, and the performance shops he tried just weren’t very experienced in Mercedes diagnosis. The fault codes that were presented were P0106 and P0505. These are faults for air intake manifold absolute pressure and idle control valve. All of the intake gaskets, vacuum hoses, valve cover gaskets, MAP sensor, and throttle had been replaced already by other shops. The engine was smoke tested, and the supercharger was removed multiple times. These faults would show up intermittently only when the car was warm, and coming to a stop.
Our check engine light diagnosis began with our technician replicated the issue with a test drive and verified the codes with a scanning tool. He then began by smoking the engine again. While the intake manifold was airtight, there was a leak coming from somewhere under the supercharger. We removed the supercharger and tested again, finding the valve cover gasket to be the only culprit. This brings up a great point, just because something has been replaced recently does not mean that it cannot be faulty. We’ll never rule out a possible cause just because someone else already tried it. This is why we always test it ourselves, because if we trusted that the valve cover to be tight, we would have gone another way in our fault finding, costing the customer valuable diagnostic time.
Now at first thought, it doesn’t really make a ton of sense for a valve cover gasket leak to cause intake manifold pressure faults and idle control faults. Until you remember the PCV system. For those of you who may not know, PCV stands for positive crankcase ventilation. It is essentially a one-way valve that vents gases produced by the crankcase back into the intake manifold to be safely burned. The PCV valve is the only way for crankcase pressure to vent, as it is an otherwise closed system. Since the PCV system is an emissions component it is very closely monitored by the vehicles onboard diagnostics and emissions control monitors. This means that the various pressure sensors have set values that they are looking for when the system is sealed, and when those values are not met it will set a fault to let you know.
The leak from the valve cover gasket compromised the seal of the crankcase system, causing a new way for that positive pressure to escape instead of being forced through the PCV system. This in turn caused the intake manifold pressure to change and be outside of those values the car was expecting to see. Resulting in our check engine light issue.
So while Ohio may seem like a long way away from Naperville, when it comes to getting the best Mercedes repair mechanics, Fluid MotorUnion is your tops choice. Not only do our technicians know your Mercedes, but also know and understand what can happen when things are modified too! Live in the Chicago land area? Find out why people from all around the country bring their vehicles to Fluid MotorUnion for check engine light diagnosis!