Case Study - E38 - Smoke And Rocking Idle

Date of manufacture	March 1995
Mileage	88,000
Model	740i
Engine	M60

Symptoms

This is every driver of an M60-engined 7-Series nightmare! It is the exact symptoms for bore-wear due to failure of the Nikasil hardening process. Starting is also affected to some extent with a small amount of blue smoke initially. As the fault progresses the tick over can get considerably worse with a misfire so pronounced that the car rocks. Quite often there is a distinct trail of blue smoke as the car is accelerated hard. Initially the fault looks pretty terminal.

Repairs made

The initial checks were made to the ignition system, oil in the sparkplug wells can cause a serious misfire. However, replacement of the HT stubs and cleaning the oil from the wells made no difference. The plugs were slightly sooty but in good condition, all cylinders had been firing and the plugs were all the same colour.

Diagnostics showed no errors stored in memory (although the same fault with the M62 engine would show 'Banks A and B adaptations reached limit'). A replacement MAF was tried along with a Magic Reset, neither made any difference. The good news was that all the plugs were the same colour, where Nikasil failure causes bore-wear it usually affects the front two cylinders first. A worn valve-stem also affect single cylinders, generally making one plug a different colour to the rest. Being M60 engine (or M62) the most likely cause is the PCV valve!

The fix

There is one major difference between a PCV failure on a an M60 compared to an M62, the M60 smokes at start-up whereas the M62 (as a general rule) doesn't. The rest of the symptoms are much the same. This difference is due to a long pipe that runs from the PCV body to the throttle body within the M62's inlet manifold. The M60 manifold does not have this pipe:

M60 Inlet Manifold and PCV (Item 2)

M62 Inlet Manifold and PCV (Item 2)

The pipe that runs from the PCV to the throttle body means that on the M62 engine any oil passing the PCV is vaporised whereas on the M60 the oil usually pools at the bottom of the manifold. This is why M60's tend to smoke and the M62 doesn't when the PCV valve fails.

The PCV valve is used to keep a constant volume of air passing between the crankcase and the inlet manifold, this is needed to allow the cyclonic oil separator to function correctly. When the PCV fails it either gets gummed-up or splits, either failure allows oil to pass into the manifold. Where the diaphragm splits it also allows excess air into the manifold which is the cause of the rocking at idle.

Repair of this fault can be found here.