450SEL 6.9 ENGINE ROUGH IDLE

[revilla]

Hi Sebastian,

That's partially good news. Your System Pressure (SP) at 5.5 bars is right in the middle of spec range.  Great start.  That means your fuel pump and pressure regulator are doing its job just fine.

However, the rich/lean condition of the system is determined by the Control Pressure (CP).  Such pressure determines the height of the fuel metering piston in the middle of the fuel distributor.    This Pressure is critical for engine to run smooth and for quick cold/hot starts.  To measure CP you need a fuel gauge equipped with a valve that you can manually open/close.  The valve needs to be positioned between the gauge/dial and the WUR.  When the valve is closed you will read SP.  When the valve is open you will read CP.  The spec for cold engine CP depends on temperature when reading is taken.  At 20 deg C should be aprox between 1 and 1.5 bars but that depends on the WUR part number your car has.  Tables are available in the documentation.    At operating temp your CP spec should be between 3.4-3.8 bars.  If your CP is outside this range, you will get the rough idle (hot) you described.  The way to adjust the CP is through calibration of the WUR and/or reconditioning of o-rings and shims in the pressure regulator.

If your mixture is off, you will also get rough idle with random misfires.  This can be fine-tuned (after SP and CP are in spec) through the 3mm allen screw covered inside fuel distributor.  Many times rough idle is due to this.  However, be cautious not to fall in the trap to adjust the mixture screw to compensate for a CP out of spec. It will throw your whole system out of spec. I assume you have checked for air leaking into the engine and all is sealed.   There are many stories of WUR, FD, AAV, CSV, TT SW, rotor, cap, leads, etc adjustment/replacement leading to no improvements because the root cause was a simple vacuum air leaking into the engine.

Finally, you have Static Pressure, or Rest Pressure.  After engine shut off, pressure should be at 3.0 bars.  After 1/2 hour it should be no less than 2.5 bars.  If not, you will have hard hot starts and usually (but not always) this is due to a faulty accumulator, or leaking injectors (including the cold start valve), or o-ring in pressure regulator.

So next in your to do list is to read the value of your CP before jumping into any conclusion.  I myself spent weeks guessing what was the root cause of my idle/start issues before i decided to read/analyze/adjust each of the 3 pressures described above.  Most experienced people here will converge in the same approach. 

Happy to be corrected by the more experienced members.

Cheers

[bnunes39]

Hi, I managed to get time to work on this 6.9 , which I have put in many work hours, but still the same problem, engine rough idle, today I connected a vacuum gauge to the rear connection on the intake manifold which provides vacuum to the A/C unit and central locking.

I run the engine and at idle I get around less than 9 on the inches of mercury on the gauge, can anyone tell me what could be the reason, as I have checked most of the setting on this engine, good reading should be around 17.

Thanks,
Sebastian

[daantjie]

Have you checked your Aux Air Valve for function?

[bnunes39]

Hi, I have replaced the valve , but the same problem.

Thanks.

[daantjie]

Dig you replace with a new one or swapped an old one in? Best to test a used one in hot water to ensure it is actually fully closing.