black substance in coolant overflow.

my parents accord recently had a black substance (looks like oil) in
their coolant over flow in their 99 2.3L accord ex. just wondering if
there was a history of bad head gaskets on these cars, or if maybe
there is a transmission oil cooler in the radiator that could have
caused the mix.
Any help would be greatly appreciated.

Car is an Automatic with 160k on the odometer.

 

Please describe this "black substance" in more detail.

Is it flaky-looking? Oily liquid puddles? Tarry? Thick? Thin? Floating?
Sunk? What?

 

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That's not the 'overflow', but the 'reservoir'. If the level is below
the MIN line, whatever you see in there doesn't count. Fill it to MAX
and then check it next time the engine is cool and report back if
there's black stuff in it.

'Curly'

 

the best way i can describe it is as black tar, sunk to the bottom.
when i pull out the hose in the overflow tank, it looks like black
cake batter

 

Did your parents have their cooling system serviced at some
point(coolant change or hoses or thermostat replaced)? If so, it's
possible that whoever did it threw in a can of sealant "just in case".
This stuff sometimes doesn't get fully distributed and some of it
collects in the resevoir. Looks like what you described.

Ken

 

Car was recently purchased from a small lot dealer. We have no
knowledge of the coolant system being service. black substance is
below the minimum fill line on the resivior and there doesnt look to
be any oil or black substance floating in the radiator. any more
ideas?

 

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If you can see the stuff below the MIN line, the reservoir is EMPTY. The
reservoir is never supposed to be empty, so just add about 1.5 litres of
coolant to the system to bring it up to the MAX mark and then do it
again in a couple days after the system has had a chance to purge some air.

'Curly'

 

coolant is above the the minimum line, black slime is below it