How exactly does a blown head gasket clean pistons?

Discussion in 'Gen 2 Prius Technical Discussion' started by pasadena_commut, Jan 25, 2025 at 12:58 AM.

  1. pasadena_commut

    pasadena_commut Senior Member

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    It is a common observation that when coolant leaks into a cylinder after a while the carbon deposits on the top of the piston are removed. The common wisdom is that this is because of "steam cleaning". That is all well and good, but the details are lacking. What is the proof that steam is really the cleaning agent?

    The results of combustion in a normal cylinder is a mix of gases containing quite a lot of water in the form of steam. Yet that steam does not make pistons all shiny. Coolant is mostly a mix of water and ethylene glycol (C2H6O2). The water is already water, AKA burnt hydrogen, and most of it will stay that way, although it could conceivably take part in some other reactions. The ethylene glycol, on the other hand, is going to react to turn into something else. Look at the structure and it has one hydroxyl on each end of a two carbon chain. At high temperatures some of it will burn, perhaps some of it burns incompletely, or just breaks bonds, producing methanol or ethanol. Those might go on to burn completely later, but some should be there at least transiently. Which makes me wonder, can very hot alcohol molecules in a gas scour varnish off of metal? Gasoline, on the other hand, doesn't have any hydroxyls in it, to speak of. When it breaks apart during combustion it can produce chunks like methane, ethane, propane, and so forth (again transiently, as these will go on to burn), but none of these products will have a hydroxyl available to carry out other reactions with the carbon deposits.
     
    #1 pasadena_commut, Jan 25, 2025 at 12:58 AM
  2. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    I didn't do well enough in organic chem to give you a proper answer, but I'd suggest that you consider it in terms of relative mass.

    Even a single drop of coolant leaked into a given cylinder through a head gasket breach represents considerably more water (by mass) than you would get from natural fuel combustion- so there's there's lots more of that steam than there otherwise would be.
     
    #2 Leadfoot J. McCoalroller, Jan 25, 2025 at 2:41 AM
  3. Tombukt2

    Tombukt2 Senior Member

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    Back in the old days guys like Big Daddy Don garlits of drag racing theme their mechanics and engineers used to pour straight water down the carburetor to break up carbon deposits on top of the Pistons behind the valve so on and so forth I guess the steam and the water under pressure Getting into the cylinder bore under pressure and kind of pressure washes the tops of the pistons and so on . Should be really easy to Google and see what the folks like Reeves Callaway and the old diesel guy in the trucks and all that they can give you videos and big words like mayonnaise and all that so you can put it away in your mind for a long time but this is really old stuff It's just what happens You can get all the explanations and the big stuff no problem online get the real scientific skinny with all the big words No interest to me It just happens and that's the way it is You're not going to stop it You want to stop it and not have head gasket leaks that's the end game
     
    #3 Tombukt2, Jan 25, 2025 at 10:53 AM
  4. Mendel Leisk

    Mendel Leisk Witness Leader

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    If coolant leaking into combustion chambers reliably cleans the piston tops, I don't see knowing the "how" being that important. Besides impeding compression, it's likely stripping oil off the cylinder walls.
     
    #4 Mendel Leisk, Jan 25, 2025 at 11:44 AM
  5. ETC(SS)

    ETC(SS) The OTHER One Percenter.....

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    Drag racers replace their engines even more often than G3 Prius drivers - so I would not look to them for longevity advice.

    Water and heat makes steam which is a good way to clean the bore - but water does not compress very well.
    If too much of it gets into the bore, it can (will) strip away lubricants and cause pressure spikes that will pop the gasket - which is really supposed to be like a fuse to keep you from bending the rod.
     
    #5 ETC(SS), Jan 25, 2025 at 12:13 PM
  6. rjparker

    rjparker Tu Humilde Sirviente

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    It is a diagnostic tool. Obviously one can deny it's moisture doing it by speculating moisture is in combustion so there "is no proof".

    Usually those folks question if the earth is flat and if men have gone to the moon. But they are the first to believe in aliens visiting earth and government covering it up because the public could not handle it.

    But moisture as a result of combustion does not cause plugs to misfire for many seconds at startup or bend connecting rods. Perhaps it is easier to believe Toyota practices planned obsolescence through carefully engineered time delay flaws.

    Just a few years ago it was egrs causing the misfires primarily because it was dirty.
    prius busted Head Gasket.jpeg
     
    #6 rjparker, Jan 25, 2025 at 12:16 PM
    Last edited: Jan 25, 2025 at 1:15 PM
  7. Mendel Leisk

    Mendel Leisk Witness Leader

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    I have this unproven theory, that "dirty" EGR systems, more succinctly, EGR sytems clogged to the point nearly no exhaust gas is supplied, in particular to cylinder one, were causing the engine to run disproportionately lean and overheat, in particular at the cylinder one end, and head gaskets failed as a consequence.
     
    #7 Mendel Leisk, Jan 25, 2025 at 12:51 PM
  8. pasadena_commut

    pasadena_commut Senior Member

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    People's ideas of how things work, even if they sound reasonable, are often wrong. In this case what is being introduced into the cylinder is half water, half something else. Why isn't it reasonable to consider the idea that the other substance is actually the cleaner?

    If somebody had demonstrated the effect using plain water as coolant on a car that had just developed a head gasket leak that would have been proof. I was unable to find an example of that.

    There are about a million YouTube videos where people try to clean the gunk off pistons using various solvents. I have seen only one where they claimed to clean the carbon off using steam (piston out of car), with "very little brake cleaner". The piston in that case didn't look like it had baked on carbon and varnish though, more like soot:



    There are all sorts of high temperature high pressure steam cleaning machines available. They are great for blasting dirt, grease, and oil off of things. If steam was a reasonable way to remove varnish and other baked on carbon from pistons one might have expected to see them used for this. (I mean, other than washing the dirt and oil off the outside of the motor before disassembly.) Some shops use hot tanks for part of this cleaning, but that isn't steam, it is a big vat of very hot water full of detergents.
     
    #8 pasadena_commut, Jan 25, 2025 at 3:02 PM
  9. AVTrainz

    AVTrainz New Member

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    I was just watching a Gasket Masters video the other day and they were showing blown Gen 3 engines. Every leaking cylinder was cleaned of deposits, and the reason they gave was that rather than steam or coolant cleaning the cylinder and piston, it's actually unburned gasoline. When the coolant leaks into the cylinder, the engine can't compress it like the actual gas and can't ignite the gas properly. Thus what happens with the cylinder cycle is that gas, air, and coolant enter the cylinder, but as the coolant and most of the gas doesn't burn its sent right back out. In that process the unburned gasoline cleans the piston and cylinder. Of course, I don't know the everything exactly but that is what I heard and saw on the video.
     
    #9 AVTrainz, Jan 25, 2025 at 7:27 PM
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  10. pasadena_commut

    pasadena_commut Senior Member

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    That would make quite a lot of sense.

    Gasoline is an organic solvent, and all that carbon stuck to the metal is going to have mostly chemical bonds which are either carbon to carbon or carbon to hydrogen. Materials like that dissolve in an organic solvent more than they would in a polar liquid like water. (Ethylene glycol is closer to water than gasoline in this regard.) Also, in this case the gasoline will be a very hot organic solvent, which would increases its power as a solvent. (For most things that dissolve in it anyway. There are always exceptions in chemistry.)
     
    #10 pasadena_commut, Jan 25, 2025 at 9:11 PM
  11. mr_guy_mann

    mr_guy_mann Senior Member

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    A heavily used cooking grill or frying pan gets alot of burned crusty (carbon) buildup. If you heat that surface up (very hot) then pour water on it, the flash expansion into steam will "blast" the carbon loose.

    Use PHYSICS, the miracle cleaner!
    Before- baked on, caked on, stuck on grime. After- surfaces wipe clean (with no scrubbing)!

    The water that results from combustion is already in vapor form (because of the huge amount of heat released). There is no rapid phase change (expansion) so no "blast cleaning".

    Posted via the PriusChat mobile app.
     
    #11 mr_guy_mann, Jan 25, 2025 at 10:32 PM
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