SLK/R171: '05 SLK350 Misfire
10-09-2014, 11:28 PM
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'05 SLK350 Misfire
Greetings,
I'll make this as short as possible without leaving out too much.
At the shop we have an '05 SLK350 which had a broken intake manifold butterfly fulcrum and a rocker arm which had shifted out of position and was holding an intake valve open continuously. This was causing random misfires on all cylinders and horrible drive-ability. Anyway, corrected the issues and replaced the intake manifold gaskets which should have fixed everything... no dice.
The car still has misfires, but only on cylinders #1, #2 and #3. The catch is, it doesn't always misfire. The car will run smooth as stock for many miles, even days, but out of nowhere it will start misfiring. You can then shut the engine off, start the engine again, and the misfire is now gone. The car will now drive flawlessly until it decides to do it again. It will also sometimes decide to start misfiring from a cold start... but not always.
-We have swapped ignition coils around in an attempt to isolate bad coils, but it will only misfire on cylinders 1, 2 and 3.
-The cold-start misfiring eliminates bad oxygen sensors since they're open loop from a cold start.
-The only codes it's throwing is misfires.
-When it's running properly, the catalytic converters are at their expected temperature. (Temps are even left to right)
-When misfiring, both oxygen sensors (only right side, up and downstream) read as constantly active and show no variation in readings with throttle position or loading.
-When running properly, the same oxygen sensors still stay active and show no variability in their readings.
At first glance you would think that one (or both) o2 sensors on the right bank need to be replaced, but why would the vehicle run fine 80% of the time, then start misfiring after days of proper running? That, and there's no o2 codes?
Are there any commonly known issues with the SLK350's that would cause an entire bank to perform this way, other than o2 sensors?
Any assistance would be greatly appreciated.
-Brett
I'll make this as short as possible without leaving out too much.
At the shop we have an '05 SLK350 which had a broken intake manifold butterfly fulcrum and a rocker arm which had shifted out of position and was holding an intake valve open continuously. This was causing random misfires on all cylinders and horrible drive-ability. Anyway, corrected the issues and replaced the intake manifold gaskets which should have fixed everything... no dice.
The car still has misfires, but only on cylinders #1, #2 and #3. The catch is, it doesn't always misfire. The car will run smooth as stock for many miles, even days, but out of nowhere it will start misfiring. You can then shut the engine off, start the engine again, and the misfire is now gone. The car will now drive flawlessly until it decides to do it again. It will also sometimes decide to start misfiring from a cold start... but not always.
-We have swapped ignition coils around in an attempt to isolate bad coils, but it will only misfire on cylinders 1, 2 and 3.
-The cold-start misfiring eliminates bad oxygen sensors since they're open loop from a cold start.
-The only codes it's throwing is misfires.
-When it's running properly, the catalytic converters are at their expected temperature. (Temps are even left to right)
-When misfiring, both oxygen sensors (only right side, up and downstream) read as constantly active and show no variation in readings with throttle position or loading.
-When running properly, the same oxygen sensors still stay active and show no variability in their readings.
At first glance you would think that one (or both) o2 sensors on the right bank need to be replaced, but why would the vehicle run fine 80% of the time, then start misfiring after days of proper running? That, and there's no o2 codes?
Are there any commonly known issues with the SLK350's that would cause an entire bank to perform this way, other than o2 sensors?
Any assistance would be greatly appreciated.
-Brett
