I was wondering, what are the possible consequences of overfilling when changing the engine's lubricating oil?
Can the oil pressure rise?
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No not the oil pressure but is really overfilled the engine can burn excessive oil and contaminate your cat converter. Best to let out the excess do it when cool.cold to avoid the hot stuff up the arm challenge. Peter
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If you overfill to the extent that the crank is thrashing through it, then it can be turned into a 'foam' and you actually can get oil starvation.
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I'm sure Aprilia will agree that the crankshaft doesn't even need to touch the surface of the oil for the oil to be picked up. If the crankshaft just passes close to the oil surface the air movement it creates can pick up the oil, an effect known as 'windage'. Windage can cause overheating as well as foaming and oil-burning.
Two other possible problems with overfilling are oil getting by the seals and, on diesels, oil being drawn up the breather system and feeding the engine to cause 'run-away' (i.e. the engine speeds up totally out of control).
There is obviously a bit of leeway and I wouldn't expect just a very small level of overfilling to cause the above problems. Best to play safe though, if in any doubt.
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"Two other possible problems with overfilling are oil getting by the seals and, on diesels, oil being drawn up the breather system and feeding the engine to cause 'run-away' (i.e. the engine speeds up totally out of control)."
I have read else where of the possibilities of this happening with a diesel. They still need air though, so one way to stop the engine is clog the airbox inlet with a tee shirt, or let off a CO2 fire extinguisher up it.
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I have read else where of the possibilities of this happening with a diesel [engine run-away]. They still need air though, so one way to stop the engine is clog the airbox inlet with a tee shirt, or let off a CO2 fire extinguisher up it.
Good points, except that run-away doesn't usually occur until the vehicle is subjected to sharp motion, like turning onto a motorway slip-road, when the oil surges. Difficult to smother the air intake when at the wheel of a car accelerating flat-out up the bank of a motorway!
Ironically, a dirty air filter will contribute to the early onset of run-away - it creates a greater depression to pull the oil from the sump up the closed-circuit breather pipe and into the engine air inlet system. So stuffing a tee shirt in the air intake is perhaps not such a good idea anyway.
There are devices to detect the onset of engine run-away and prevent it happening, and breather circuit designs that will prevent the possibility of it happening anyway, but I don't know what the situation is regarding modern diesel cars. Is anyone here up-to-date on these things please?
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According to the warnings on VW diesel engines, over-filling with oil can damage the catalytic converter. This could lead to an eventual MOT fail.
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It'll do it on petrols as well.
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So, the way I understand it, the resulting problems are mainly located in the area right under the piston: the seals may not be able to stop the excess oil from getting above.
By the way, isn't some form of foaming supposed to take place to lubricate the crankshaft / piston rods / cylinder walls? Or they are oiled internally as well?
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No, you don't want foaming.
Some older engines had little 'scoops' that picked up oil from the sump and threw it up behind the pistons. Others (inc. some old MB engines) have/had little oil holes on the top of the big end to send a jet of oil up behind the piston. These features are still used on the odd modern engine.
These days the oil is mostly delivered from the pump via the drillings in the mains/crank/b-ends/con-rond/little ends etc.
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The oil going up inside the piston is also used to cool the piston.
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Yes, oil takes away about 20-25% of the heat, more on some engines such as rotary engines where about 1/3 of heat is removed by oil (hence they a substatial oil cooler).
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I constantly get told by BMW (after I ask) that overfilling by 200ml or so won't do any damage, but 500ml or more can damage the cat - so why do they insist on putting enough oil in the engine to cover the max mark on the dipstick twice over every time it goes in for a service???
BTW min to max is 1 litre in my M5 - so I guess max to 1" above max would be another 1 litre or so!
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If the oil is 1" above the "max" mark then that is serious overfilling and you should remove some of the oil. Either carefully drain some out (cold engine) or draw a bit out of the sump using a vacuum extractor (eg a Pela).
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The Engine Oil Bible - www.chris-longhurst.com/carbibles/index.html?menu....l - includes an explanation of the dangers of overfilling an engine with oil. Well worth a read and bookmarking for future reference.
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The Engine Oil Bible - www.chris-longhurst.com/carbibles/index.html?menu....l - includes an explanation of the dangers of overfilling an engine with oil. Well worth a read and bookmarking for future reference.
Apologies, but that link doesn't work as advertised. Open it, and then select The Engine Oil Bible at the top left hand side to open the correct page.
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That's a very useful site, DGW. His 'come to Britain' page is very good too - spot on in fact!
www.chris-longhurst.com/cometobritain.html
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There are some marvellous scare stories from the days of semi-diesels as used in old boats. They were designed to run on any old fuel. When worn, so much oil could be drawn up the bores that the engine started running on the lubricating oil, and uncontrollably ran faster and faster. In the end they flew apart, siezed up spectacularly, or ripped the bottom out of the boat.
At least with a car, or a petrol engine running on, it is possible to stall it.
With any car engine, especially if it has covered a few miles, anything that puts the seals under greater pressure is undesirable, as leaking oil can get on the clutch or timing belt.
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