I am in the process of repairing my 1997 Toyota 4-runner with a v-6 engine. I have only had the vehicle for a couple of days. I was doing a routine maintenance check on all the fluids. What I found was the antifreeze to be white in color with a brown oily substance on the reservoir. The rest of the fluids in the vehicle where all clean. Oil was very clean and no signs of white or discoloration, smelled normal. Transition fluid was clean, had a normal reddish color, did not smell burnt or bad. I performed a radiator fluid to clean all the white and slug out. I refilled with all purpose 50/50 antifreeze. After driving for about a half hour I recheck the antifreeze and the reservoir had a brown oily residue on the sides. I opened the cap and ran the engine without the cap. The antifreeze that spilled out of the radiator had this brown flaky oily substance in it. All other fluids where rechecked and still clean. I have reason to believe that it is the radiator leaking or the oil cooler leaking. I am trying to figure out if it is transmission fluid or engine oil in the antifreeze? that should tell me if it's the radiator or the cooler? or are they the same thing. Is the cooler located in the radiator?

  • 1
    Do you have an automatic transmission? If so, does this particular model have the transmission oil cooler built-in at the bottom of the radiator? To figure it out, check how may inlet/outlet you have. 2 larger one for the coolant and 2 smaller one at the bottom for transmission oil. Nov 4, 2013 at 11:54
  • I wonder if leak detection dye (UV dye) added to the engine oil might be a way of isloating the leak to either engine or trans. Not sure the concentration of the dye would be high enough by the time it makes its way into the coolant system though.
    – mac
    Nov 4, 2013 at 17:33
  • @mac they make UV dye that's safe for oil? that's awesome!
    – vlsd
    Nov 7, 2013 at 18:25
  • Any luck with that nasty sludge? I've got the same issue in my BMW E46 316i. It's in coolant, but not in oil pan. I have done the compression test and the results are just fine. How to figure out whether it's oil or transmission fluid?
    – user11999
    Aug 30, 2015 at 20:25

2 Answers 2


I think the likely suspect here is engine oil. If your head gasket is blown it is not unusual for oil to get into the coolant or the other way around. Coolant is being circulated through the engine very close to where oil is being circulated through the engine. Consider doing a compression test and/or a leak down test.


Flush the radiator. Some of the red coolants, especially if they used dexcool will look like that normally. If you've got oil, or transmission fluid going into the coolant you will likely have coolant going into the oil or transmission fluid, so check those fluids closely for contamination. Once you've done that, monitor the coolant closely and see if the condition comes back.

On a side not if you bought this vehicle recently from anyone but a private seller, or auction house take it back to them for analysis before doing anything else. They make take care of it for you.

  • I believe he said he already flushed the radiator and the oily stuff came back.
    – vlsd
    Nov 5, 2013 at 2:31

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .