Is My Lawnmower Toast?

September 23, 2007 9:56 PM

Is my lawnmower toast? Today I put oil in my lawnmower and made the (dumb) mistake of not draining the old oil out first. When I ran the lawnmower afterwards, large amounts of white smoke appeared and the engine started making a "knocking" sound. I stopped mowing, figuring it wasn't safe to continue on in this condition. Am I out a lawnmower or is this fixable? Can I just drain out the oil and start again or is it possible irreparable damage has occurred?
posted by The Gooch to home & garden (14 comments total)

If it was an electric mower - yes.

Seriously though - two stroke?
posted by puddpunk at 10:14 PM on September 23, 2007

Drain the extra oil and give it a smoke test. but you may have damaged the conrod or piston or bent the crank, there is a little tang on the connecting rod that splashes oil around in the crankcase, to much oil and your piston is slamming into a huge wave of oil this can do damage.
posted by hortense at 10:33 PM on September 23, 2007

Sounds like it was a four-stroke mower; you don't change the oil in a two-stroke...

If you stopped promptly I don't *think* it would have been ruined mechanically, but it may need to be torn down and cleaned out. At the very least you need to drain all the oil out and try again, probably after pulling the plugs and cleaning them, maybe cleaning the carb too.
posted by Kadin2048 at 10:33 PM on September 23, 2007

Did you overfill it?

Personally, I'd just drain it all out and start fresh, and see what happens.
posted by Kickstart70 at 10:33 PM on September 23, 2007

Drain out all the oil and put in new oil, except substitute Rislone™ for half a quart of oil. Run that for a few mows and then drain it out and put in new oil again. That should clean out most of the tar and particulate you generated when you burned a bunch of excess oil in the cylinders.
posted by ikkyu2 at 11:06 PM on September 23, 2007

I would just change the oil, put new gas in it. Use some carb cleaner and change the air filter. It is not a big deal if you tip your mower over and get oil into the piston. It may smoke a bit, but is should be fine. These engines are very resilient. Don't spend $150 to get it fixed if all is lost, go buy an electric and save on the environment.

If it has a place to put the oil, four stroke. If you have to mix the oil and gas, a two stroke.
posted by plethora1 at 12:25 AM on September 24, 2007 [1 favorite]

How much oil did you add, and how did you know to use that amount? The only way to fill a small engine, as I understand it, is to pour in a little bit and check with the dipstick, then add more and check again, until you're up in the comfort zone. Blindly adding oil, even if you did drain it previously, is likely to leave the wrong amount in the engine.

I don't think you damaged anything. Just drain it, fill it to the dipstick line, and try starting it again. What's the worst-case scenario?
posted by Myself at 3:33 AM on September 24, 2007

Meh, I bet it's muy fino.

As others have said, drain oil, probably drain gas. Maybe consider a new spark plug, since you likely fouled up the old one. Refill, rescrew, restart, and mow away.
posted by TomMelee at 4:49 AM on September 24, 2007

You're probably just getting some blow by because of the extra pressure. Drain out the excess oil and try it again.
posted by electroboy at 6:02 AM on September 24, 2007

The knocking and white smoke is a pretty good indicator that you somehow got oil in the fuel supply. I suspect that if you empty the gas tank and re-fill (maybe high octane would help a little) your lawn mower will forgive you.
posted by Huplescat at 8:46 AM on September 24, 2007

I overfilled a two-stroke motor earlier this summer. Ran the mower for several minutes before the smoke started rolling off it in waves. Shut it down right quick and let it sit for a week while I considered my options. Next weekend I drained off the excess oil and cleaned the spark plug and now it works just as well as it ever did.

If it's a four-stroke motor you probably blew a seal / trashed a valve / other expensive damage. As mentioned above, try draining the oil and gas and replace with appropriate levels of new. Not a huge investment if the alternative is buying new. Unless you're attached to the old mower, it will probably be cheaper to buy new than to repair. A sad state of affairs given a long-term view of the landfill.
posted by Fezboy! at 9:18 AM on September 24, 2007

"Drain out all the oil and put in new oil, except substitute Rislone™ for half a quart of oil. ..."

If it's a common walking type mower, with a single cylinder air-cooled 4 stroke, vertical shaft engine, don't do this, as the empty oil sump capacity is generally only 5/8 quart, for vertical shaft Tecumseh, Briggs & Stratton and similar small engines. Rislone™ is not intended for these type engines. Just drain the engine's oil sump, refill with clean oil, and pull the spark plug and clean it.

In a major overfill situation, the condition hortense describes is possible, but generally, before the engine bends or breaks parts, oil foam in the crankcase gets past the piston's oil scraper and compression rings, creating the white smoke condition, and gunking up the plug a bit. Power drops immediately, as the fuel/air mixture is way too rich, and the oil can't be completely burned, so the engine kind of self-limits the force it creates to tear itself apart on the crankcase overfill. If you shut it down right away, there's little chance you did permanent damage, and very little possibility you got enough oil up through the piston and intake valve to even get to the carburetor, much less to the gas tank.

Be vigilant in draining the sump this time; many vertical shaft Tecumseh engines have to be tipped a good bit towards the drain hole to get the last 1/3 of the oil to drain from the sump. You can't just drop the drain plug and wait with these engines, you have to tilt them, and if you pour 5/8 quart of oil on what you "think" is an "empty" sump, you'll just be overfilling them again. Myself's advice about carefully adding small amounts of oil at a time, and checking with the dipstick is good, but you have to be patient in doing this, as the oil you just added generally coats the walls of the filler tube in most small engines, which is where the dipstick is inserted, too, making it hard to get an accurate dipstick reading for 3 or 4 minutes after you've just added oil. So, it can take you 15 minutes to add 1/2 a quart of oil, checking 3 times, doing it this way. Maddening, if you're impatient, but safe, if you're patient.
posted by paulsc at 9:43 AM on September 24, 2007

UPDATE: When I got home tonight I drained out the oil as well as the gas tank and refilled with new and it appears to be working again.

Thanks to all for the great advice.
posted by The Gooch at 10:12 PM on September 24, 2007

I appreciate the useful comments in regard to your post. I've taken one step further on your road to lawn mower baking/crisping. In a rush to beat sundown after re-attaching a sharpened blade, I attempted to replace oil I had lost during the process by adding directly to the "oil filter," a box with a piece of oil-soaked foam with what I now realize is the air intake, a tulip-shaped aluminum funnel. Of course, it is impossible to pull the starting rope. Since the mower only cost around $50.00, I'm planning to pull the spark plug, attempt to slowly turn the engine a few cycles to drain oil from the cylinder, find the oil filling tube and dipstick and correct the low oil situation, then try to start the engine with the addition of some carburetur cleaner. If no luck, it's back to the small engine repair shop that sold it to us several years ago. If I haven't destroyed the engine, I will write a personal letter to Briggs & Stratton to thank them for such a durable and foolproof machine.
posted by beginner at 3:51 PM on October 14, 2007