vapor from oil dipstick

blueskyMD

blueskyMD

Pre-takeoff checklist
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Bigfoot297
So after I came back from a short flight I decided to add a Quart of oil . When I took dipstick out there was vapor/smoke coming out of it. Question is it water vapor or smoking oil ?. And if it is water vapor then doesn't if make sense to open dipstick after every flight and let the vapor out since water is the biggest contaminant in the oil ?
 
It is water vapor. There is nothing wrong with removing the dipstick to allow moisture to exit after a flight.
 
You do have an oil breather tube that vents to the atmosphere. Trouble is, leave the oil cap off and then forget to put it on could be MAJOR. My advice it's not in the POH, don't do it.
 
I always remove the dipstick when putting the plane away after a flight. I cover the dipstick hole with a rag to let the moisture out and keep out dust. It is surprising how much vapor comes out. If I miss a big rag on the engine on preflight then I should not be flying anyway.
 
I too pull the dipstick out and prop the oil door up with it (for visibility) after a flight.
By the time I am done securing the aircraft, lost of vapor has had a chance to escape (hopefully helping the engine in the long term).
Before leaving the airplane, I do a final walk-around to check that all vents are closed etc and that's where I replace the dipstick and close the door. (hard to miss or forget in that case)
 
What made you "decide" to just add a quart of oil?
 
Hopefully you did not let too much of the magic fog out. That magic fog is what makes the airplane fly. Not enough of it and you will be slow to lift off next time.
 
Sounds like a procedure with little-to-no benefit, but potentially messy consequences. Just leave the dipstick in, and water vapor that doesn't make it out of the breather tube won't be enough to harm anything.
 
SoonerAviator said:
water vapor that doesn't make it out of the breather tube won't be enough to harm anything.
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That's one theory.

However, other owners hold to the view that less water is better, in the engine of a parked airplane.

Corrosion is such a huge issue for most planes, due to inactivity, that we should seize easy opportunities to get water out. The few minutes that steam comes out of the filler tube, after shutting down the engine, presently exactly such an opportunity. There's no reason to think 100% of that water escaping from the open fill tube will escape anyway, over a long period of time, from cold oil.
 
NoHeat said:
That's one theory.

However, other owners hold to the view that less water is better, in the engine of a parked airplane.

Corrosion is such a huge issue for most planes, due to inactivity, that we should seize easy opportunities to get water out. The few minutes that steam comes out of the filler tube, after shutting down the engine, presently exactly such an opportunity. There's no reason to think 100% of that water escaping from the open fill tube will escape anyway, over a long period of time, from cold oil.
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I understand that, but I don't think there's any empirical evidence that doing so will meaningfully reduce the amount of corrosion inside the engine. Moisture from the general atmosphere while setting will likely impact corrosion much more than the little bit contained in water vapor that didn't escape through the breather tube after being run.

It's sort of akin to placing the aircraft on jack stands after each flight to eliminate weight on the tires while parked. Sure you can do it, but are you really extending the life of anything?
 
SoonerAviator said:
I understand that, but I don't think there's any empirical evidence that doing so will meaningfully reduce the amount of corrosion inside the engine. Moisture from the general atmosphere while setting will likely impact corrosion much more than the little bit contained in water vapor that didn't escape through the breather tube after being run.
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Umm that prolly depends entirely on the climate. As for "little bit" maybe look at the products of combustion.
 
I've checked my oil on fuel stops on my long cross country's before and when the plane has just come back from a flight and have never come upon the vapor. I suppose this is due to me 1) being a student pilot and having relatively little experience 2) I rent so the plane has little time to accumulate water.

I would agree though that it's pretty pointless to let the dip-stick "air out". For the very little it may help the danger of forgetting about it is too great, plus I know of no evidence that suggests it helps in the slightest.
 
Clark1961 said:
Umm that prolly depends entirely on the climate. As for "little bit" maybe look at the products of combustion.
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Not sure what you're implying by "products of combustion." Those products generally exit through the exhaust, and what makes it into the crankcase can exit to atmosphere via the crankcase breather. I'm simply saying that pulling the dipstick isn't necessary because the vapor already has an escape route.


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SoonerAviator said:
Not sure what you're implying by "products of combustion." Those products generally exit through the exhaust, and what makes it into the crankcase can exit to atmosphere via the crankcase breather. I'm simply saying that pulling the dipstick isn't necessary because the vapor already has an escape route.


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Click to expand...
Maybe the escape route you imagine isn't really there.

There is a tremendous amount of water vapor in the products of combustion. Paerhaps none of it makes it into the oil. Perhaps not. Well okay, yes it makes it into the oil. Are you really suggesting that anything that may help dry the oil is bad?
 
Why would not an additional "escape path" be a good thing?

On a Cirrus, a milky white sludge can often be found on the oil filler/dipstick cap. Maybe this would help?

I say "maybe" because I doubt there's any empirical data to prove the point one way or the other.

In my Sky Arrow, I "burp" it with the cap off after every flight to get a good oil level reading. Had not thought about there being some small additional benefit. Or not.
 
If it was burning oil you'd definetly notice the smell
 
Since heated air rises it is going to vent from the highest location fastest. The vent tube originates higher on the engine case but then curves below the engine, but the dipstick tube is taller giving the air a path to flow straight up from the sump. I believe both vents help to expell the maximum amount of moisture laden air from the engine when both are used.
 
SoonerAviator said:
I understand that, but I don't think there's any empirical evidence that doing so will meaningfully reduce the amount of corrosion inside the engine. Moisture from the general atmosphere while setting will likely impact corrosion much more than the little bit contained in water vapor that didn't escape through the breather tube after being run.
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The moisture entering the case past the rings from combustion would be, I'd think, about a thousand times greater than atmospheric vapor entry. There was a discussion on POA some time ago about the atmospheric moisture in fuel tanks. Even in saturated air there is so little water in a tank-sized volume, and the expansion/contraction rates in the temperatures involved is so small, that it would take a very long time to get any significant water in a tank that way. Most water comes from leaky caps and in the fuel itself, as dissolved or entrained water, not so much as free water. The volume of a crankcase is faar smaller than a fuel tank and atmospheric water is going to play a very small role.

Someone told us, too, that burning a gallon of gasoline produces about 1.6 gallons of water. Or some crazy amount like that. ANY leakage past the rings is going to send plenty of water into the case, and it's far worse when the engine is cold and the clearances are larger. Add that to the fact that a cold engine isn't going to vaporize that water readily and blow it out the breather, and you realize that ground-running the engine without flying is obviously foolish thing to do. Better to let it sit.
 
Vaflier said:
Since heated air rises it is going to vent from the highest location fastest. The vent tube originates higher on the engine case but then curves below the engine, but the dipstick tube is taller giving the air a path to flow straight up from the sump. I believe both vents help to expell the maximum amount of moisture laden air from the engine when both are used.
Click to expand...

There are many engines where the dipstick tube extends into the case and below the level of the oil. The Lycomings using the long filler/dipstick tube that angles down and into the top of the case flange where it bolts to the sump is like that. Water vapor cannot get out that way if the oil is up near full. Many of the small Continentals, those with the kidney oil tank, sometimes have the oil filler/dipstick neck welded to the side of the tank well down the side, and they, too, are flooded with oil when full, and vapor can't get out that way either.

Use a small flashlight and look down there and see where the oil is at. Might surprise you.
 
Dan Thomas said:
The moisture entering the case past the rings from combustion would be, I'd think, about a thousand times greater than atmospheric vapor entry. There was a discussion on POA some time ago about the atmospheric moisture in fuel tanks. Even in saturated air there is so little water in a tank-sized volume, and the expansion/contraction rates in the temperatures involved is so small, that it would take a very long time to get any significant water in a tank that way. Most water comes from leaky caps and in the fuel itself, as dissolved or entrained water, not so much as free water. The volume of a crankcase is faar smaller than a fuel tank and atmospheric water is going to play a very small role.

Someone told us, too, that burning a gallon of gasoline produces about 1.6 gallons of water. Or some crazy amount like that. ANY leakage past the rings is going to send plenty of water into the case, and it's far worse when the engine is cold and the clearances are larger. Add that to the fact that a cold engine isn't going to vaporize that water readily and blow it out the breather, and you realize that ground-running the engine without flying is obviously foolish thing to do. Better to let it sit.
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I'm not disagreeing about the potential for water vapor to enter the crankcase, I simply said most of those byproducts exit through exhaust. Some residual amount will slip past the rings, and assuming an engine that was brought up to temperature during a flight, should be mostly evaporated from the crankcase via the breather tube. There will certainly be no way to eradicate all moisture from inside, and I haven't ever seen/read any basis for removing the dipstick to try and accomplish that goal.

I agree that the ability of the engine to pull out moisture when cold is greatly reduced, hence why it should be avoided if possible.


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SoonerAviator said:
Some residual amount will slip past the rings, and assuming an engine that was brought up to temperature during a flight, should be mostly evaporated from the crankcase via the breather tube.
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Not even close.

All of the "stuff" in the crankcase is from blow-by - mostly burned combustion products that contain way more water than the air can hold at normal ambient temperatures. Bringing the engine up to temperature will eliminate liquid water that would otherwise have accumulated in the crankcase, but it can't all evaporate out the breather tube because you have a constant supply of water vapor past the rings. Now, since automobiles went to positive crankcase ventilation in the '60s, you aren't going to find much data on water concentration in the crankcase, but the products of combustion are about 13% water. And, room temperature air can only hold about a couple percent water. Also, I can tell you that the blow-by volume in my Rotax 912 is enough that it is used to push the oil from the crankcase back to the oil tank (the crankcase is sealed, there is no scavenging pump) - so there is a significant volume even with cylinders that test at 78 to 79/80.

Net result - when you shut down your engine, it has a bunch of water in it and it's going to condense. Eventually, it will evaporate and escape - but remember - your crankcase is isolated from the atmosphere by about 4 feet of small diameter tubing - it's gonna take a while.
 
Capt. Geoffrey Thorpe said:
Not even close.

All of the "stuff" in the crankcase is from blow-by - mostly burned combustion products that contain way more water than the air can hold at normal ambient temperatures. Bringing the engine up to temperature will eliminate liquid water that would otherwise have accumulated in the crankcase, but it can't all evaporate out the breather tube because you have a constant supply of water vapor past the rings. Now, since automobiles went to positive crankcase ventilation in the '60s, you aren't going to find much data on water concentration in the crankcase, but the products of combustion are about 13% water. And, room temperature air can only hold about a couple percent water. Also, I can tell you that the blow-by volume in my Rotax 912 is enough that it is used to push the oil from the crankcase back to the oil tank (the crankcase is sealed, there is no scavenging pump) - so there is a significant volume even with cylinders that test at 78 to 79/80.

Net result - when you shut down your engine, it has a bunch of water in it and it's going to condense. Eventually, it will evaporate and escape - but remember - your crankcase is isolated from the atmosphere by about 4 feet of small diameter tubing - it's gonna take a while.
Click to expand...

Again, I get that, but the little bit of vapor coming out of the dipstick tube after shutdown contains how much of that crankcase moisture? 2%? 10%? I haven't seen it recommended in the operating manual that one should open the dipstick after shutdown to remove moisture . . . wouldn't the manufacturers be concerned about so much moisture sitting in the crankcase?
 
If the humidity level goes back up before you put the dipstick back in, you accomplished nothing. in fact, I'd theorize that exposing the warm engine to air as it cools will result in more or the same amount of moisture in the engine.

I don't think you're accomplishing anything at all.
 
SoonerAviator said:
the little bit of vapor coming out of the dipstick tube after shutdown contains how much of that crankcase moisture?
Click to expand...

Nobody knows.

Yes, that's the question. But nobody knows quantitatively the answer. I've never heard of a study.

I'm not sure how you could easily measure the moisture that's condensed onto surfaces, which is what you'd want to know. Maybe drain the oil, then wrap the engine in a hot insulated blanket and measure the water vapor coming off. Then repeat for engines that were vented after flight through the dipstick and engines that weren't. That wouldn't be easy.

What would be more valuable would be a longitudinal study, comparing over a long time several identical engines operated under identical conditions, half that were vented out the dipstick after every flight and half that weren't. That's what it would take to answer this question, and it's unlikely anybody will do it. A big flight school might be a candidate because they have a fleet of identical planes operated the same way, but the results wouldn't be meaningful to us because their planes are operated frequently. Ours have corrosion problems because we let them sit idle for weeks or months between intermittent flights.

The one thing we can probably agree on, and that the manufacturers like Lycoming are clear on, is that frequent flying is best, for avoiding corrosion.
 
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Salty said:
If the humidity level goes back up before you put the dipstick back in, you accomplished nothing. in fact, I'd theorize that exposing the warm engine to air as it cools will result in more or the same amount of moisture in the engine.
Click to expand...

Here's something on the amount of water in saturated air: http://www.engineeringtoolbox.com/maximum-moisture-content-air-d_1403.html

Do the math and see how much water could possibly enter that engine even if it was able to draw in an entire crankcase volume of air as it cooled, which it can't. There isn't much volume there, and much of the volume that is there is occupied by the crank and rods and oil and so on. It's really tight in there.

At 50°F, the air can hold as much as .000037 ounces of water per cubic foot. Not even a droplet. The water comes from combustion. Atmospheric water is almost small enough to ignore, and guys that have opened up engines that have sat for years since the last flight, without being ground run, find no corrosion in them.
 
To add fuel and confusion....My IO-550 has a nice rusty looking appearance up around the oil filler tube....My previous IO-235 had a plastic oil filler tube, no rust there. I suspect the amount of moisture depends on how hot the engine gets and ambient moisture.
 
coloradobluesky said:
You do have an oil breather tube that vents to the atmosphere. Trouble is, leave the oil cap off and then forget to put it on could be MAJOR. My advice it's not in the POH, don't do it.
Click to expand...

That.

I just let my vent tube do its job.
 
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