Too much oil?


#1

(2010-08-21) This week on Car Talk, Vern from Ohio stands accused of killing his wife’s sister’s husband’s minivan by over filling it with oil. Is he guilty? More importantly, does it even matter, since technically the guy isn’t related to him. In fact, he’s nothing to Vern!



Tom & Ray rightly chided Vern for adding oil without checking the dipstick first. They let Vern off the hook because they heard evidence suggesting a pre-ruined engine, before Vern’s wife ever drove it. The rationale was that the oil light went on and the clattering resumed super-quick after the overfilling; and the owner (brother-in-law) presumably was having the same experience, because he was filling it too. OK, they make a bit of a logical leap and deduce it was wrecked before, and absolved Vern. A good time was had by all, and everybody’s happy except the brother-in-law. Plus maybe their relationship is wrecked too.



But what I really wanted to know is whether it’s even possible to kill an engine by adding too much oil. I’m just a lowly engineer, but I figure the engine would choke and splutter on excess oil for a while, slapping it around with the pistons, running rough, and blowing huge clouds of oil smoke. (Was there any blue smoke, Vern?) Then all that internal sturm und drang would spill and burn the oil until it was only slightly overfilled. Meantime, I think nothing gets damaged, if it’s a good engine.



If this is correct, then it’s an ironclad reason to let Vern off the hook. Anybody set me straight?


#2

When oil is overfilled it ends up being turned into whipped cream, all air little cream. The pump can’t pump whipped cream so the engine never gets the oil it needs. (Think this, each revolution moves X oz of oil, when the oil is whipped up, that same volume is pumped but most of it is air, not oil. Far less oil gets pumped. It does not take a engine long to get serious damage.


#3

The bigger problem than the pistons on the cylinders is the main and rod bearings. These ride on a pressurized barrier of oil, pumped into the crevaces through channels and holes in the bearing surfsces. When the oil is a froth, as Joseph described, the fluid barrier disappears and the surfaces come into contact and get damaged and can seize.

A crankshaft is subjected to considerable lateral forces, and contrary to popular belief it bends as it operates. Without a good fluid barrier to help keep the aforementioned surfaces apart, well, they’re kaput. Kaput is, as you know, a formal engineering term.