Sport45 wrote:
If you're suddenly seeing oil fouled plugs on all cylinders I'd suspect the PCV valve may have stuck open. Don't know if this has been mentioned or looked into already.
Sport,
Actually, this should have little effect other than an incorrect fast idle (+ possible lean die at stop signs). Because there is less airborn oil mist present at the low rpm/blowby conditions of idle rpm, these crankcase vapors tend to be cleaner air. On the other hand, the PCV valve is supposed to open up anyway at high throttle/low vacuum conditions when high rpms may create more misty airborn oil particles.
The prefered PCV test is to just remove the PCV valve and shake it. If the bobbit is metal, the weak bobbit spring is compressed by the mass of the oscillated bobbit and rattles when free (unstuck). If the bobbit is light-weight plastic, this test may not work. If one doesn't mind the horrid taste, a used PCV valve can also be tested by mouth. A quick suction will cause the bobbit to overcome the weak spring pressure and collapse into the same seated restriction mode at high vacuum idle. A lazy suction, just as open throttle provides far less vacuum, will allow free breathing around the unseated bobbit. Or, in lieu of a test, just replace the PCV; they are cheap.
The best designs of PCV systems largely avoid sucking oil particles by baffling them so the speedy oil particles cannot turn a corner as well as gaseous vapors, and the heavier droplets thereby crash into "walls" that cause the liquid oil to dribble back into the crankcase. This "best design" differentiation has become more important since the advent of Direct Injection (DI) in gas engines. Because some older marginally vented gas engine designs were merely modified for DI by changing only the cylinder heads, any excess intake oil mist still strikes the "tulip"(back of valve) as usual. But with fuel no longer in the intake mix (added directly later), the oil is not washed off and the hot valve soon tends to bake crusty, lumpy carbon deposits on the backside of the intake valve.
The profile of this debris, in turn, disrupts smooth intake flow and soon causes noticeable hp loss in modern, highly tuned engines. It is an expensive fix to clean this gunk off and can already be noticeable at every 10k miles. Add to this that many late DI gas engines are turbo-charged, which makes it worse. Turbo-charging tends to cause even more blowby because of very high combustion pressures (thus torque). The resulting excess blowby adds to the "wind" traveling through the crankcase, and drags more oil mist with it than naturally aspirated engines do. Note that not all turbo DI gas engine designs have this problem, but some are notorious for it.
FYI, diesels, also with both DI and turbo's, are less affected because of typical lower rpms, and the intake valves do not normally get as hot (lower cycle rate, more seat time, thus promoting engine durability). As OEM diesels are stressed for more hp, this advantage may change. Diesel hot-rodders sometimes trade durability for power. There appears to be no free lunch.
Wes
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