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Old 12-11-2003, 01:06 AM
JimSmith JimSmith is offline
Join Date: Jul 2001
Location: Old Lyme, Connecticut
Posts: 3,596

There is another possibility to consider besides the valve guides and seals. I am not sure of the configuration of your vacuum pump, but, if you have a line running from the pump to the intake manifold, you may have a ruptured or leaking vacuum line. If the car smokes a lot at idle, as well as during a cold start episode, I would check the vacuum pump out. At idle the engine actually draws a vacuum as there is a throttle plate downstream of the air filter. This will lower the pressure and suck on the vacuum pump discharge connection at idle. If the diaphragm is leaking, it will suck engine oil into the intake manifold. But, if the problem only exists when you start up, it is likely a valve guide/seal issue.

For the same reasons noted above, at idle the intake valve guide clearance with the valve sees lower than ambient air pressure where it intersects the intake air flow passage above the valve seat. So, when you idle you draw oil into this area. When you shut down, the oil that was "on its way" all runs down on top of the valve. When you start up you get a little extra oil into the cylinder.

At higher speeds that throttle plate is moved out of the throttling position by the throttle system linkage (goes around to the intake side of the engine). So, the only pressure drop in the induction system comes from the air filter and the flow passage losses themselves. Not as much vacuum results, so the smoking stops or subsides. In the case of the vacuum pump diaphragm, once the throttle plate is cleared by the linkage, the vacuum goes away and the pump will not pump oil, so the smoking (can be quite spectacular at idle) goes away as well.

If you want better definition of the problem, have the compression checked. Do a dry, and, if the numbers are not good, wet (note there are members who say this could be dangerous, however, the local dealership does them wet and dry) compression test. If you want more definition the scope of the problem, try a leak down test. This should make it clear if there is a valve path or piston to cylinder wall path leakage problem and help decide the action you want to take before you invest the time to remove the head.

Good luck, Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)
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