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Old 01-24-2010, 07:04 PM   #1 (permalink)
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Exclamation Hyper Alert: Engine Damage (ring/cyl wear killing ignition to running carb'd engine)

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

A little background: I bought my Honda Civic FE new in 1983 (the "FE" stands for "Fuel Efficient"). I have achieved over 50mpg in normal commuting. However, one of my techniques is what's called "forced stop," if I understand it correctly. I routinely switch off the engine when going down hill.

Just about everyone maintains that doing this is dangerous, but I take the arrogant position that it is dangerous for people who don't know what they are doing. The most obvious issue here is the steering lock. Cars have steering locks to prevent theft. I disabled mine so that it would not be a threat. Steering can get harder if the car has power steering. My little Civic does not. Yes, the brake booster looses effectiveness, but I have the ability to push the pedal as hard as it needs. This technique is for manual shift only. I always switch the ignition back on as soon as the engine stops and leave the transmission in the appropriate gear with the clutch depressed. Note that riding the clutch with the engine running will wear out your throw-out bearing, but has no effect at all when the crank has stopped.

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.

The symptoms of this damage are a little unusual. What I first noticed was a small cloud of oil smoke when I leave a stoplight (having idled a bit). Once up to speed, there is no apparent smoke. Oil consumption is not radically up. There is no smoke when starting cold.

My son has a 1983 Civic, too. I rebuilt both of these engines recently; mine is bored .020 over, his got new pistons. I was driving his car while mine was in the shop getting painted, using my usual techniques. It developed the symptoms I described, except that his got so bad that it was fouling plugs.

I got my car back and it was only a couple of weeks before it started smoking, too. That was enough to convince me that my switching off the engine at medium revs was the culprit. Note that switching off an engine normally occurs while the thing is idling, so the potential to create problems is much reduced.

It is likely that making sure the engine is idling before switching it off would prevent the kind of damage I have encountered, but I'm just going to leave it idling from now on. If you use this technique and have a way to shut off the fuel, use that. I hope this will save somebody out there from ruining his rings, for what it's worth.

Greg in Seattle

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Old 01-24-2010, 07:29 PM   #2 (permalink)
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Very sorry to hear your expensive story, Greg. I always let my engines drop to idling rpms before shutting them off during FASes. If I don't, they start right back up again.
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Old 01-24-2010, 07:42 PM   #3 (permalink)
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Thanks for the alert! That's a good point. It may be too late for my Previa but now I know. Luckily, I mostly drive a Prius now except for deliveries.

In a fuel injected engine, don't the injectors stop the second you turn off the ignition? There may still be some un-burnt fuel coming in at the last second though but surely not for the whole spin down.

Can this be observed with the MPGuino or scanguage?
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Old 01-24-2010, 09:04 PM   #4 (permalink)
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So... Did you save enough money from the amount of gasoline saved to justify the expense of the engine damage?

If not, it's called 'false economy'.
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Old 01-24-2010, 09:16 PM   #5 (permalink)
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Quote:
Originally Posted by orange4boy View Post
Thanks for the alert! That's a good point. It may be too late for my Previa but now I know. Luckily, I mostly drive a Prius now except for deliveries.

In a fuel injected engine, don't the injectors stop the second you turn off the ignition? There may still be some un-burnt fuel coming in at the last second though but surely not for the whole spin down.

Can this be observed with the MPGuino or scanguage?
Yes, they do. As soon as you cut power to the ECM, the injectors stop working, which means that there is no fuel being injected at all into the engine.

His Civic FE was/is carb'd, and unless he can find a way to close the venturi, it will continue to suck in fuel as it is spinning down. He shouldn't be shutting the key off until the engine is at idle, because all that extra unburnt fuel gets into the cylinders, flows past the rings, washing the oil film down, and mixes with the oil. Eventually, it can also wash the crank/main bearings and seize/warp the lower cradle. (The part of the block that the crank bolts into.)
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Old 01-24-2010, 09:43 PM   #6 (permalink)
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Of course not. I never would have indulged in this mileage maximizer had I realized it was harming my engine. I posted this as heads-up to others to not repeat the mistakes I made.
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Old 01-24-2010, 09:49 PM   #7 (permalink)
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Quote:
Originally Posted by Thymeclock View Post
So... Did you save enough money from the amount of gasoline saved to justify the expense of the engine damage?

If not, it's called 'false economy'.
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Old 01-24-2010, 09:50 PM   #8 (permalink)
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werd, thanks for letting people know!
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Old 01-24-2010, 09:51 PM   #9 (permalink)
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If you install a fuel injector kill switch, there is nothing to worry about. An engine kill switch will save you gas while prolonging the life of the engine.
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Old 01-24-2010, 09:52 PM   #10 (permalink)
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Yes, that's right, it's a fairly conventional carburetor. I thought about setting it so that the butterflies close completely, but there are at least two possible downsides to that approach that I can think of: one, shutting the carb completely will result in vacuum levels never intended...that would probably work the valve seals harder, among other things, and two, I believe that at least some carbs have a "dribble" port below the butterfly...the increased vacuum would suck that one harder, maybe even making the problem worse.

As you point out, letting it get down to idle is one way to avoid this problem. The other is shutting off the fuel. That is how diesels are stopped normally, and I remember that my Piper (Lycoming) had a fuel shut-off as well (Marvel updraft carb).

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