N46. S..t happens

A symptom of the problem: permanent vibration in idle and also driving, decreased power of the engine.

In the same time: without any problems, the engine is able to maintain Stoichiometric mixture, indications (voltage) of control probes – correct.

With Valvetronic turned on, Rough run data shows around +4.0 .. +5.0 units for cylinder No.4. By completely switched off cylinder No.4: +8.0 units. Conclusion: the mechanical efficiency of a cylinder is reduced at least by 50%.

Testing of the compression didn’t display any problem.

The engine uses Valvetronic, sometimes the error messages regarding misfire of cylinder No.3 are recorded. Replacement of spark plugs, ignition coils, an injector for the cylinder No.3 has not given any positive effect. Also replacing the injector of cylinder No.4 (changing places with cylinder No.3) – no positive results.


Taking off the valve cover, the following picture was revealed:

As you see, the Roller cam follower is broken, Ramp part of the Intermediate lever – damaged, HVA – “exploded”. It’s clear, that the intake valve of current cylinder No.4 was closed for all the time. So, the area of intake tract of cylinder No.4 was only one half of planned (comparing to other cylinders).


The more interesting is a fact, that:

  • DME is able to maintain a Stoichiometric mixture, it means, even the mechanical efficiency, reduced by 50%, is not an obstacle;
  • despite such huge difference of mechanical efficiency and, accordingly, Lambda between cylinders – in idle no misfires observed;
  • despite such defect, the engine was using Valvetronic.

I think, this is a very good sample, that Valvetronic IS NOT one to blame, if the engine shivers in idle, and is a good confirmation, that even in case of severe mechanical problems DME is able to “keep” max correct performance of the engine.


Additionally: in a mode without Valvetronic (idle, looking at Rough run data) this defect significantly decreased. The cause of such situation s following:

  • in case of use of Valvetronic (modes with a small valve opening, for example, idle – when Rough run data are displayed) the amount of inlet air is determined by valve opening. If one of the valves don’t open, 2 times less air than necessary flows in. The efficiency of the cylinder is reduced by 50%;
  • if valves are opened with a max opening (for example, turning off Valvetronic in idle), also opening of one valve is enough, for the cylinder to let in almost normal amount or air (in idle, it’s enough time for it). Of course, the efficiency of the damaged cylinder will be slightly decreased, but the impact of the defect will be significantly smaller than in the previous case.


Related entries:


Siemens vs Bosch. Part 1

Siemens vs Bosch. Part 2