JensMB

Hello!

I have got a MB 380 slc from 1980 that has the following symptoms: The car starts fine when cold, and runs pretty well. It seems to be firing on all cylinders. Idle is around 1500 rpm in a couple of minutes after cold start.

The car runs fine when warm, while driving, but after it gets up to working temperature the idle rpm, goes under 600 rpm and It starts stumbling at idle. Even stalls.

When the car is hot it doesn’t start until it has cooled down – around 20 minutes later…

This is what I have done:

• Replaced fuel accumulator
• Replaced fuel pump
• Replaced fuel filter
• Replaced o-rings at fuel pressure regulator valve
• Checked the Cold start injector for leakage – which it doesn’t..

None of the above has changed anything…

I have measured the pressures:

• Cold controlpressure is 0,0 bar at 5 degrees C
• Warm control pressure is 2,5 bar after ca 5 minutes and it doesn’t go any higher
• System pressure is 5,1 bar
• Residue pressure 0,0 bar 5 seconds after shuting of the fuel pump.

Whats the next thing to check? What could be causing the absence of residue pressure and low pressures at the WUR?



Would be really grateful for any help or ideas…



Regards Jens

Vin_16

Hi JensMB, I hope this will help. Given the symptoms you've described and the actions you've already taken, it sounds like you've covered several potential culprits. However, there are a few additional components and checks to consider:

1. Warm-Up Regulator (WUR): The warm control pressure not increasing beyond 2.5 bar could indicate a problem with the WUR. It might not be adjusting the fuel mixture properly as the car warms up. A malfunctioning WUR could cause the issues you're experiencing with the idle and stalling when hot.
   
2. Idle Air Control Valve (IACV): The IACV controls the idle speed of the engine. If it's malfunctioning or sticking when the engine reaches operating temperature, it could cause low idle RPM or stalling. Cleaning or replacing the IACV might be necessary.
   
3. Vacuum Leaks: Check thoroughly for vacuum leaks. Leaking vacuum hoses or gaskets can lead to issues with engine performance, especially at idle and when the engine is warm.
   
4. Engine Coolant Temperature Sensor (ECT sensor): A faulty ECT sensor might not provide accurate temperature readings to the engine control unit, resulting in incorrect fuel mixture adjustments. This could affect idle performance when the engine is warm.
   
5. Oxygen Sensor (O2 Sensor): A failing O2 sensor might not provide accurate readings to the engine control unit, leading to improper fuel-air mixture adjustments, especially during warm-up.
   
6. Throttle Position Sensor (TPS): A malfunctioning TPS might not communicate the correct throttle position to the engine control unit, affecting idle speed and performance.
   

Given the complexity of older car systems, using a good scanner like the youcanic scanner could help identify any hidden fault codes or readings that might not be obvious without scanning. This could provide more specific information about sensor readings or potential malfunctions.

Considering these checks might help in identifying the root cause of the idle and stalling issues, especially when the engine is warm. If these checks don't resolve the problem, seeking the expertise of a professional mechanic with experience in vintage Mercedes-Benz models might be the best next step. They could perform more comprehensive diagnostics and pinpoint the issue accurately.