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I recently overfilled a 4 stroke lawnmower engine with oil. It has been run for about 1.5 hours while overfilled. It smokes a lot right after being started but after 30 seconds or so the smoke clears. Now that I have figured out what I did I plan to drain the oil down to the "high" mark and keep running it. What sort of damage might I have done?

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I drained out oil and ran the engine today for about 45 minutes. It is no longer smoking even on startup and I can't detect anything wrong with it. Hopefully I just added a bit of extra wear and tear. –  Greg B May 23 '11 at 2:32
    
Thanks for all the input as it explains what happened to my ~5 year old 17.5 hp Craftsman riding lawnmower. My son was mowing the grass when it threw the crank through the case out of nowhere. He later asked if filling the oil too much would do this. Didn't think that it would as I have done it before on older Craftsman with just smoke as the result. Looks like this is a newer design engine and must have bent the crank and possibly lost lubrication enough to destroy the motor. Good thing I bought a 1 year old Troybilt Pony with the same engine for $300 from a friend because it didn't cut very –  user1979 Jul 7 '12 at 14:26
    
good. Can put the engine on the Craftsman, & use the Troybilt frame for parts, for a whole lot less money than buying or rebuilding the Craftsman motor. Sometimes you win. Again, thanks for everyone taking the time to share your knowledge and experience! –  user1979 Jul 7 '12 at 14:26
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4 Answers

up vote 10 down vote accepted

Over filling an engine with oil can cause it too "foam" the oil, which reduces it's lubrication properties. If you lower the level to proper amounts you may have added some wear, but it's unlikely that you caused serious damage.

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A few things can happen:

  1. The crankshaft could be bent
  2. The seals and gaskets could be destroyed
  3. Very high crankcase pressure can lead to oil coming out of the crankcase ventilation (which might destroy your pistons if it enters your cylinders through your intake system)

Eric Fossum's remark about turning the oil to foam is a very important one. It leads to reduced lubrication inside the engine which can lead to terminal engine failure.

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If the oil level is too high it can actually bend the crankshaft. If the crankshaft is striking the oil when the engine is running it can ruin the engine. It doesn't sound like you have done any permanent damage with what you have described.

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Long story to include a fairly short answer, but it's pretty amusing, so I'll give the whole story. :-)

My Dad once WAAAAY overfilled our cheap-o mower (Briggs & Stratton engine on it) due to filling the entire oil compartment when all you're supposed to do is wet the sponge in it. This occurred after we realized it had no oil in it, which we finally figured out after a Summer of having it quit working and be seized up every 20 minutes (from overheating apparently, you couldn't budge the cord until it cooled off, then it would run fine again).

The effect of the overfill was to blow oil, both in liquid and smoke form, through the muffler like crazy for several minutes. Pretty much fogged in our whole street before it cleared up.

As a final note, we (several years later) retired that mower by parking it out back and just leaving it there. The new fancy mower tossed a rod a month after the 3 year warranty was up, so we grabbed the ol' beat up mower from where it had been abandoned. Started on the 3rd pull (with the same gas, oil, and plug that were in it when we left it to sit in the elements)... Apparently Briggs & Stratton make a seriously tough engine...

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Brilliant story :) –  jensgram Sep 22 '11 at 6:50
    
Yeah, Briggs and Stratton have a very good repuation for mower engines. –  staticsan Sep 26 '11 at 4:38
    
Just a note: Those old Briggs & Stratton engines last even longer if you take good care of them. :) –  jp2code Jul 9 '12 at 13:43
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How can they last longer than forever??? ;-) –  Brian Knoblauch Jul 9 '12 at 14:32
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