Diagnosing Mercedes-Benz Misfire DTCs

What makes diagnosing misfire diagnostic trouble codes (DTCs) so difficult on many late model Mercedes-Benz vehicles is not only their sensitivity to misfire but the multiple causes of misfires. It can be caused by something as simple as a lack of maintenance on the ignition system, a fuel control problem or other fuel, air or exhaust restrictions. It can even be due to internal engine problems.

A fully functional Mercedes-Benz compatible scan tool is indispensable for misfire troubleshooting, as it will allow you to view the Powertrain Control Module (PCM) misfire counter data blocks. As always in this type of diagnosis, you first need to cover the basics. Use the scanner to read all DTCs, and then record and clear them. Inspect air filters, check for vacuum leaks, false air leaks, crankcase ventilation leaks or restricted hoses, and test fuel pressure both at idle and under load. Give the spark plug wires, if equipped, and spark plug or coil connectors a thorough inspection for any signs of arcing, burning or excess brittleness of the resistive materials. Any oil or fluid leaks on ignition-related components, such as valve covers or leaky sensors, need to be fixed.

For engines with two spark plugs per cylinder use an ion-current sensing system for misfire detection. This is far more effective at detecting misfires than engine systems that use the crankshaft sensor/speed analysis detection method. As with all the late-model Mercedes-Benz engine control systems, injector pulse is shut off by the PCM on the misfiring cylinders to protect the catalytic converter.

Let’s look at three different hotline calls we’ve received on Mercedes-Benz misfires and the three very different diagnostic resolutions.

Car 1: 2002 ML 320, a rough-running, zero-maintenance vehicle with about 110,000 miles. The completely worn-out spark plugs had just been replaced; the spark plug wires and air and fuel filters were new Mercedes-Benz parts. DTCs P0300, P0301, P0302 and P0303 would not reset until the SUV was driven aggressively. Identifix recommended that the technician test the fuel pressure first, both at idle and under load and also test the fuel volume. These tests came back with no errors. The next test was to check for excessive exhaust backpressure. Since the technician didn’t have the necessary test equipment, he loosened the exhaust pipes at the manifold. The misfire DTCs didn’t return after a test drive and the fix was to replace the flow-restricted catalytic converters. The clue to solving this problem was that all the misfire DTCs were on one bank of the V6 engine.

Car 2: 2001 SLK230, with DTCs P201B-1, P0301, P201B-2, P0303, P201C-64, P0304, P201D-2 and P0171. (Note: The Mercedes-Benz codes are listed first, with the generic equivalent DTC after). This little two-seater would start and idle fine but wouldn’t get out of its own way when throttle was applied. Because of the fuel trim DTC and the pattern failure of the mass airflow sensors (MAF) on these vehicles, the technician was not asked to conduct preliminary tests (fuel pressure, spark plugs, etc.) but to inspect the air filter and supercharger hose connections, and then use the scan tool data to check the MAF output. Idle readings were 2.2 grams/second (g/s) and a hard throttle snap yielded only 21g/s maximum.

The inspection found that the MAF was coated inside with engine oil and the PCV hoses on the front side of the engine were split and collapsed. Identifix recommended that the technician clean the MAF with a MAF cleaner, clean out the intake hoses, remove and clean out the intercooler, replace the hoses for the crankcase ventilation system, and remove the valve cover and clean out the passages for the breather inside. When this was completed, the car ran smooth and MAF output under load came up to 110 g/s. Though this was still a little less than desired, the car never set DTCs again.

Car 3: 2002 S600 originally had multiple misfire DTCs that would set intermittently on all 12 cylinders. Mercedes-Benz had many problems with these vehicles setting misfire DTCs when the vehicles were under warranty and issued a special troubleshooting diagnostic guide. The technician had already been through the guide, forward and backward. Using a star diagnosis machine to view the data screens for the ignition, all the input PIDs looked OK. After many repairs the car continued to set a code P0307 intermittently. The code would only set after a road test; running the car in the shop all day never produced a fault.

After using up all our good ideas, the only thing left to do was a complete pinout of the ME-SFI PCM, which had already been replaced along with almost everything else. This led to the discovery of a seemingly insignificant voltage drop at the #7 injector. Disconnecting everything and load testing the harness with an old-fashioned headlight bulb showed a dim bulb on the #7 injector connector. Stripping the wiring harness back a few inches from the connector located a poorly made crimp splice in the harness. Fixing and soldering over the crimp splice took care of the intermittent misfire on the #7 injector. The moral of this one … don’t give up!