The R170 Blues
I notice some of you are having engine problems with ragged running of the engine getting CELs or just not running at all. I have a few suggestions to fix your problems and a code reader isn't necessary. In fact, it might just complicate matters for you.
The typical path to diagnois engine problems is to first check for fuel and then spark. I keep forgetting that these cars are fuel injected and not carbureted making the fuel check more difficult. For the moment, let's pretend that fuel isn't the problem.
The first thing I'd do is to replace the fuel filter which is some place in the back of the car. I believe it is connected by rubber fuel lines, so be careful removing it. Don't pinch or bend the lines. They may even need to be replaced.
Next clean the electrical connecton on the Crankshaft Position Sensor (CPS) with a an electrical contact cleaner and small brush. Here is a thread from Pelican Parts about the CPS:
Pelican Technical Article - Mercedes SLK 230 - Camshaft Positioning Sensor Replacement
The next thing I'd do is remove the cover on top of the engine exposing the coil packs, plug wiring, and access to the plugs. Again from Pelican Parts:
Pelican Technical Article - Mercedes SLK 230 - Spark Plug Wire and Coil Replacement
Here you can replace the plug wires and take out the plugs. Using a spark plug color chart, you can gain a lot of information about your engine. At this point, you might replace the plugs.
The last thing you can do is check the coil packs. You need a volt-ohm meter to do this. With the meter on the Ohm scale read its resistance. If it's zero, it is shot and needs to be replaced. The coil packs should read a certain resistance to determine if they are OK. You'll have to do some research to find the value. Also if you see any electrical connections, take them apart and clean them especially any leading to the ECU.
If you have a code reader, you might be able to pinpoint the problem. If you get a CPS error, you should still clean the electrical connection before replacing the CPS to test if the CPS is really bad.
Common codes are engine misfires:
P0300 - random misfire or any cylinder in no particular order
P030N - where N is the cylinder misfiring.
You can check the coil packs by switching their positions. For example, if you get a P0301, switch the front pack with the rear to see if you get a P0303. If you do the pack is bad.
Beyond these simple steps, you'll need some heavy duty diagnostic equipment to figure things out. At this point, you'll likely need an Indy or MB. BTW, check this out: