Hyper Alert: Engine Damage (ring/cyl wear killing ignition to running carb'd engine)

[gjgerhard]

That's true. I was surprised when I caculated that we're talking about several cc's of gasoline. All the more reason to use a fuel cut-off every shut-down, not just the forced kind. Is that what modern hybrids do?

Correction: it would be several cc's if the driver left the throttle at cruise position. Of course, no one would do that, you lift your foot when you turn the key off. Well, that's what I was doing, and that's why there is damage---never switch off an engine unless it's at idle.

I don't have the numbers in front of me, and I'm too lazy to do them again, but a 40mpg car going 60mph is using close to 1 1/2 cc's per second. So, considering my throttle plate was closed within a few milliseconds of power-down, the gas that was washing down my cylinders was probably not much more than one cc or so per episode. Divide that by four cylinders, and each was probably getting the equivalent of a couple of spritzes from a perfume sprayer.

[Bicycle Bob]

Gasoline in the oil is not much of a problem. It evaporates quickly at operating temperature. Arctic fliers sometimes added it deliberately to thin the oil for extreme cold starts.

[RobertSmalls]

Gas in the oil is also not a source of pollution, as it's sent into the intake by the PCV system, to be burned.

However, it's a terrible lubricant, and if you only use the car to go to a mile to chruch on Sundays and the store on Monday, gasoline and H2O in the oil can become a problem because the oil in the pan doesn't stay hot enough to allow gas and water to evaporate.

[Christ]

Originally Posted by RobertSmalls

Gas in the oil is also not a source of pollution, as it's sent into the intake by the PCV system, to be burned.

However, it's a terrible lubricant, and if you only use the car to go to a mile to chruch on Sundays and the store on Monday, gasoline and H2O in the oil can become a problem because the oil in the pan doesn't stay hot enough to allow gas and water to evaporate.

It's not a direct pollutant, you're right about that, but it's not all going into the oil, either. When you shut the engine off, if there is gas going in, it's also going out the exhaust ports until the engine stops, which (I assume) is what Bob was referring to when he said it was a pollutant.

[RobertSmalls]

I agree completely. Gasoline going out the exhaust valves would be a big pollutant, unless it gets ignited on the cat.

[Christ]

Don't think those older ones have a cat?

[rmay635703]

Originally Posted by gjgerhard

I just learned something the hard way and I'm making an effort to tell people about it.

However, I discovered that switching off the engine while in traffic can damage it, which is what happened to me. Twice.

Here's why: there are two ways to stop an engine that's not a diesel: you can switch off the ignition; you can shut off the fuel. If I ever go back to doing this, it will use the second method. What I was doing, and what caused the damage, is I would be driving along at elevated rpms, I would depress the clutch and switch off the ignition, and wait for the engine to stop turning.

While the engine is spinning down to stop, it continues to ingest a fuel/air mixture in the cylinder that is not getting burnt. Gasoline is horrible as a lubricant, and is actually quite effective for washing lubricants away. So what I was doing was washing away the thin layer of oil in my engine's cylinders, and eventually the rings started to complain.
Greg in Seattle

Sorry to hear about your motors, good reason to let them rev down a tad before key off.

Good thing my subaru 360 has a electric fuel cutoff when I shut off the key (a necessity on a gravity fed system) Also my 2 cycle continues to ingest oil as it has torque induction OOH.

[bestclimb]

Letting the engine run down while fuel is in the cylender and washing the oil off the rings then restarting is like doing a bunch of cold starts. Pretty much why fuel starvation is the only way to go for cut off. Keep the spark popping to burn out and use the energy of the fuel after cut off, (still some fuel in the intake on a fuel injected engine) Then you wont have the problem of fuel washing the oil.

The operators manual for my Cessna gives a table for oil dilution for cold starts. Nowadays most people use preheat though.

[George Tyler]

Are you sure that the fuel is what caused the problem? Iagreed in the beginning, but thinking about it, the engine has been running, the surface temp of the cylinders and head and the temperature of the pistons is way above 100 deg C, any fuel is going to evaorate on contact and go out the exhaust? Also, the small amount of fuel in the high vacuum in the inlet will instantly evaporate?

[SentraSE-R]

That gasoline vapor is still going to contact the cylinder walls and wash off the oil coating that was left from the crankcase side of the oil ring on its last pass.