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?
A few things can happen:
- The crankshaft could be bent
- The seals and gaskets could be destroyed
- 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.
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...
I found a craftsman push mower with an eager 1 or tecumseh engine(i think they're made by the same company)down an alley one day. the next day, i found a john deere push mower on the side of the road with a 3.5 horse briggs on it which appeared to be brand new and in mint condition, but when i brought it home, I noticed the right rear wheel was literally tearing off of the deck. this was obviously a P.O.S from walmart made with beer can thin sheet metal. So I kept the motor, handle, and blade. The craftsman mower had a broken cable, so I jerry rigged it by removing the brake (which stops the motor) and connecting a toggle switch, even though it was a little more dangerous. the handle was also a little bent, but the deck was heavy duty and it still served its purpose. after using it for a couple of weeks, the tecrapseh threw a rod(I probably should've changed the oil). I swapped it with the briggs from the beer can thin john deere deck, and luckily the handle fit right on to the craftsman deck too, and the cable was exactly at the right length, so I had a new engine and a straight handle with the safety feature in tact. The engine had been sitting crooked on my garage floor for a few weeks, so some oil had leaked into the piston and I had to pull the string without the spark plug a few times and oil sprayed everywhere.
I poured out the old oil and poured in some new stuff before even starting it. I had trouble seeing the full line on the dipstick, so I poured in a whole quart and thought too much oil was better than not enough. It started up on the first pull, and of course I was expecting some smoke from oil leaking into the cylinder head, but the smoke was so thick and was showing no signs of stopping. and of course the d-bag driving by in a prius was giving me a smug look. I was able to see the level on the dipstick after it ran for a few minutes and some old left over oil mixed in, it was quite over filled. I poured some out, runs like a champ with no smoke.
I don't think over filling will do too much permanent damage to small 1 cyl push mower engines, but it's probably much worse for larger riding mower engines. Either way, if you over fill, be ready for a hell of a smoke screen.