I swapped out my radiator today, and when I disconnected the two thin hoses at the bottom, brown fluid that looked like oil came out. It kept slowly leaking out while I took out the old radiator and put the new one in until I connected the hoses to the new radiator. What exactly was that fluid? Was it transmission oil? Does transmission oil go through the radiator somehow? Or was it just filthy coolant that wasn't changed in possibly 170k miles?

 

Nissan and a few other manufacturers made a trans. Oil cooler inside radiator.
There is quite a bit of a reservoir in there you might be tad bit low on Tran fluid.

 

gear oil has its own chamber in the rad on some vehicles so it gets cooled off, completely normal for some ATF to come out when the lines are removed.

if it was brown i would pull your tranny dipstick and see what colour the fluid is, might be time to change the trans fluid

 

The fluid is transmission oil like stated above. You might want to change it.

 

Do not flush it

 

brown hunh?? wow, do a drain and fill and get the front 6-7 quarts emptied out and use nissan matic-d and do again in like 2/3 weeks so you have all new fluid in there. be careful to top off to proper levels and put in only what you took out.

 

No a good idea to drain it all considering it is already so far gone. Drain about 3-4qrts, thats what will come out the the first time, then see how it likes the new fluid. The Re4fo4a is very tempermental with fluid changes

 

trans isnt necessarily gone, trans fluid will naturally go brown over years from trans heat. burnt black is when you worry

 

if it's dark brown then the viscosity is gone which lubricates the gears, seals. solenoids, and changing it might be risky. might break free some metal shavings and cause more harm than good. that's why i'd i'd do it a little at a time and drive like granny smith to be on the safe side.

 

I thought all autos (in the last 30 yrs or so) have the tranny fluid routed through the radiator?

 

many do but some dont, depends on the manufacturer