It can’t happen with fuel injection. In a carbureted engine, fuel is drawn into the cylinders by the vacuum of the pistons gong down the cylinders on the intake strokes. That draws the fuel from the float bowl. As long as there’s fuel (until the float bowl is emptied), air, and enough residual heat in the cylinders to initiate combustion, that’ll keep happening without the need of any electronics.
In an electronically fuel injected engine, the fuel is sprayed through a solenoid-operated valve (the injector) that’s opened when the ECU tells it to. When the engine is turned off, the injector reverts to its fully closed position. Thus, there’s no fuel available to support dieseling. A descending piston cannot pull fuel through a closed injector.
At first it just sputtered, but as it got worse, it would run on longer and longer until it was fixed.
The mower didn’t just sputter when turned off. It ran faster and faster and blew flames out the exhaust! Anyway, it probably wasn’t running on 100% oil but it was definitely burning the oil as there was no smoke once it warmed up and the flames just kept pouring out.
A mower has a governor, not a operater-actuated throttle. At best, you’d control the preload on the governor, adjusting operating RPM…but all mowers come to a stop with an open throttle.
If your mower runs after shutdown, I see three possibilities:
- The magneto is no longer grounded, and is still supplying spark, despite being “turned off.” (This is actually common.)
- The mower is dieseling on gas from the carb.
- The mower is dieseling on motor oil.
It’s harder to imagine a sufficient supply of oil getting in…but if it DID get in, oil is naturally far easier to pre-ignite than gasoline. (High cetane/low octane.)
This engine was drinking oil like water! I think it was using as much or more oil as it was using gas. I was using my old drain oil from car oil changes to keep it topped off and would have to stop it about every 10 minutes and add a quart. It would smoke like mad when first started, then switch over to flames coming out the exhaust. I had been using expensive premium oil and saw no point in dumping that into something in this condition.
It seemed like the only thing keeping it from spinning any faster was the amount of oil it was burning. You could tell so much was getting into the cylinders that it would misfire and slow down.
Check out this video. I don’t think this was all gas but gas was likely mixed with the oil and helping the fire.
I don’t know why that reminded me of my Morris Minor. I was just a kid and didn’t know anything but it had trouble starting and would drink gas like that mower drank oil. I took it to the next town 12 miles away and had to fill the tank again to get home. The carb was one of those SU English jobs and watching a youtube on Morris maintenance last night, it looks like part of it was missing. It had a big rubber piece sticking out of the top of it to try and seal that hole in the top, where the guy filled it up with oil every 1000 miles. Who knew? Maybe with a new carb it would still be running and the guy that bought wouldn’t want to kill me.
I feel it is environmentally irresponsible and reckless to continue operating an engine that consumes a quart of oil every 10 minutes.