Good evening to all W202 Fans,

I am slowly about to go insane. I have this Problems with my car for over nearly a year.
The car is a 1995 W202 C180. Initially, the problem was that after the engine warmed up and idled for a while, the idle became unstable. It would fluctuate every few seconds between roughly 600 and 900 RPM, and inside the cabin, it felt like the engine had hiccups. Shortly afterward, the odometer, temperature gauge, and clock would cut out.

Since then, I’ve replaced a long list of parts:

• Engine wiring harness
  
• Ignition coil wiring set
  
• Refurbished throttle body
  
• Intake manifold temperature sensor
  
• Voltage regulator (alternator)
  
• Overvoltage protection relay
  
• Oxygen sensor
  
• Spark plugs (replaced last year)
  
• Coolant temperature sensor
  

Possible remaining causes I’m considering:

• Coolant thermostat
  
• Ignition coils
  
• Degraded catalytic converter
  

But things have taken a new turn. Recently, I replaced the ignition coils and spark plug connectors again, cleaned the throttle body. After reconnecting the battery, I performed a throttle body relearn.

Here’s where it gets strange:
When I first started the cold engine, the idle shot up immediately to ~ 2000 RPM. Then its slowly goes down to 1000 RPM. Then it starts rising nonstop. No Gear in, I press on nothing and it starts rising to 3000 rpm as if I was pushing on the gas slowly.

What’s also odd: when the engine is idling and I repeatedly press the brake pedal, the idle keeps increasing as if I were pressing the gas pedal. It’s like the brake is somehow affecting the throttle signal.

Before replacing the ignition components, the idle was just fluctuating. Now it climbs non-stop. Could this be related to a vacuum leak or even a misbehaving throttle body (which I got rewired, checked and should be fine)? Could a faulty brake booster or vacuum line also be involved?

At this point, I’m completely out of ideas and honestly at my wit’s end. If anyone here—especially any Mercedes whisperers—has any insight, I’d really appreciate your thoughts!

Thanks in advance.