Acid (was: Drain valves versus plugs versus suction)

Larry C Leung l.leung at
Wed Nov 7 20:32:25 EST 2001

Most of the deposit was water, the black from the carbon by products of
combustion (not all of it becomes CO2 unfortunately, nor water for that
matter). It is also quite likely, as I have alluded to, and James has
re-visited, that there is a weak element of acidity due to the
combination of water with CO2 (very weak carbonic acid, naturally occurs
in rainwater, even in the absence of mankind's activities, makes rain in
the neighborhood of 5.6 to 6.0 in pH) and rather reactive Sulfuric Acid,
though at very low molar levels (it's a result of Sulfur being IN the
fuel, it doesn't get burnt, rather it passes through and is oxidized in
the combustion process as a secondary reaction into SO2 which combines in
combustion water to make H2SO4). The problem with this acid isn't in the
exhaust (it's there, it's weak, and probably has no notable effect on
paint, though since there is much accumulation in the atmosphere, it
becomes an issue in the air, making rain become somewhere from 2.0 - 3.0
pH downwind of industry, some 10,000 times the acidity of natural rain),
but in the crankcase (put there by even limited blow-by and EGR), where
it accumulates naturally, as there is no flush system except changing the
oil. Not to mention, the creation of Nitric Acid due to the combustion at
high temperature (any high temp in air application) in the engine's CC's.
This can only be stopped by having the car breath pure oxygen, as the air
is about 78% nitrogen. Breathing pure oxygen would stop the "bends" too,
but it's not practical there either.

So, to make a long story short, as to the black exhaust products, no real
harm to the paint, but yes, to a VERY limited degree, it is slightly
acidic (i.e. it's a technical point, nothing more.)


On Tue, 6 Nov 2001 20:52:23 -0700 "James Marriott" <marriott at>
>> My car was recently in the shop, and one of the mechanics had
>it parked
>> right in front of one of the bays.  Inside the bay was a beat
>up 4k that was
>> elevated a bit, and the engine had just been started up.  There
>> tremendous amounts of black "stuff" dripping out of the exhaust
>and all over
>> my car.  In fact, about 25% of my hood was covered with this
>"black water"
>> by the time it caught anyone's attention.
>> I told the mechanic, somewhat angry, "thanks for covering my
>car with
>> sufuric acid", to which he said, "What are you talking about?
>I haven't
>> heard that one before, thats just some harmless water!"...
>> It was dilluted sulfuric acid, wasn't it?
>Indeed it was. Virtually all non-gas petroleum-based hydrocarbon
>fuels (coal, diesel, gasoline, etc) contain sulfur compounds,
>which lead to sulfur dioxide emissions, which lead to acid rain.
>It also led to, for example, BMW replacing a bunch of 4.0
>all-aluminum V8s under (a very expensive) warranty (program) due
>to our higher-than- Deutschland sulfur content dissolving the
>cylinder walls.
>cu, James Marriott
>'87 4kq (alias "late-B2 90q") with rare NG engine, 181k
>'89 200q (MC1, ProconTen/no bag, 1.8 blah blah), 139k
>Boise, ID, USA

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