Its the BMW smell !!!!! (archive)

The BMW smell is the smell of crankcase vapors being released into the engine compartment, during decel, when crankcase pressure is high. The smell then comes into the car through any crack or gap or open window that it can. It is caused by having more engine blow-by than the positive crankcase ventillation system that the car was designed with can handle. On bmw's it is a simple system. There is a hose from the valve cover to the air filter housing, that theoreticly routes crancase (blow-by) gasses from the crankcase to the air cleaner, where they will join the clean air comming into the snorkle, and be sucked into the engine through the carb, or throttle body to be burned in the combustion process. The problem with the system bmw uses is that on a closed throttle, when blow-by gasses are in high volume, the throttle is closed, and there is little or no air going into the engine, thus no vacuum in the air filter housing to draw the blow-by into the engine. Where does the smelly, oily blow-by gas go then? Out of the air filter housing and into the engine compartment, then into the passenger compartment where you smell it. Bmw wasn't that stupid though, and on many cars, there is a little vacuum hose from the intake manifold to a fitting under the air filter housing where the big valve cover hose connects. This hose is a constant vacuum source, even with closed throttle, that is supposed to suck all of the blow-by gasses into the intake manifold where they are burned in the engine. This hose is in reality, a very small diameter, and cannot handle all of the blow-by gasses of an engine with some mileage, and wear, so you get the smell.
On older cars without the charcoal vapor cannister, the fuel tank is vented directly to the atmosphere below the trunk floor behind the rear bumper, or directly into the air filter housing. When it is vented to the air filter housing, gasoline vapors are often mixed with excess blow-by , and enter the air surrounding the engine during decel, especially on long hills.
One solution is to put the car in neutral and coast down hills, so as not to cause an excess buildup of crankcase vapors, and blow-by gasses.
The other is to re-ring, and put new valve guides and seals in your head.
On a new engine, blow-by is at a minimum, because tolerances are tight.
Another solution, would be to increase the diameter of the vacuum fitting and hose to the air filter housing, so that more air is sucked into the intake manifold at all times. Increasing this flow will tend to lean out the fuel mixture, so a richer idle and cruise mixture would be nedded.
Most other makes of cars, the crankcase vent system uses a hose diameter of 3/8" about 10mm. The size of the hose bmw uses is 3.5mm. so it is easy to see where the smell comes from.

Fuel smells in the trunk come from cracked fuel hoses, including the filler hose, and worn out o-rings on the fuel sending unit. If the purge hose is not connected to the cannister in the engine compartment, and the intake manifold, or in other cases the air filter housing, then there will be no way for the excess fuel vapors to be purged from the vapor holding tank in the trunk, and you will smell fuel. Also, the purge line for fuel vapors is a nylon tube that runs down the right side of the car INSIDE the passenger compartment. If that line gets compromised, then you will smell fuel vapors in the car most certainly.
Food for thought and smells for the nose.

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Re: Its the BMW smell !!!!! Merdad 03:18 11/16/2000 (0)

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