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99 v6 Mustang with Oil in the Coolant

I’ve got a '99 v6 Stang with 252k miles on it. I recently went on a small road trip, about 5 hours of driving total, and on the way back home I started to smell oil for about a minute, but then the smell went away. I didn’t think anything of it for a week or two, but I checked the coolant level yesterday, only to find that there is a decent amount of oil in the overflow tank. I’m not sure why this happened. There is no smoke coming out of the exhaust. I daily this car, I’ve maintained it the best I can, and I need to figure out what to do ASAP since it’s my only way of transportation.

If the vehicle has an automatic transmission, check the tranny fluid.

The tranny fluid cooler is inside the radiator. And if tranny fluid is leaking into the cooling system, it can look like oil.

Because that’s what it is.



I just checked the Tranny fluid and it was at normal levels, same with the engine oil. The radiator is fairly new, I installed a new one around Christmas time, so its only been in the car for about 9 months. Otherwise I’m not sure if its something to do with a head gasket or a crack in the block.


99 Mustang.

When was the last the time cooling system hoses where replaced?


I’m not sure, I’ve only had the car for just over a year. They seem to be in fair condition. Just a guess though. I had a leak in the old radiator and the new one solved that problem.

On a 99 Mustang where the history is unknown, you replace the hoses when replacing the radiator.

The hoses themselves might be deteriorating internally. And that might be what’s causing the oil-like appearance in the coolant.



Would that have also caused the smell of oil while driving?

Forget the smell.

Focus on the visual.



You got a cool ride there OP. Does your engine use an oil cooler? You know, where the oil is cooled by the coolant. If so, give that a look-see, as a fault w/that part that can mix the coolant with the oil. You want to make sure the problem isn’t that mentioned above, the transmission fluid cooler in the radiator that’s doing the leaking, b/c that will also leak coolant into the transmission and quickly ruin it, something you don’t want. Once the transmission cooler and oil cooler are ruled out, then you’re looking at an internal leak inside the engine, usually involves the headgasket and/or the intake manifold gasket. Or a cracked block. It’s sort of unusual you speculated it might be a cracked block, as that’s a pretty uncommon problem. Is there a reason you think it might be that?


I thought it could be that due to that fact that I asked my Automotive Tech teacher this morning and he said that could be a possibility.

Very unlikely unless the engine has overheated at some point, and even then still unlikely. Has this car been used for drag racing or other types of racing? Or has it always been just a daily driver as far as you know?

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It has always been just a daily driver as far as I know. Last person to daily it was my dad’s friend’s daughter. She was 16 when her father got it for her, and it has obvious signs of a wreck or two. As I said earlier, I’ve maintained it the best I can.

Some ideas . . .

Drive the car a few miles to get the engine oil up to proper operating temperature

Drain the oil into a large and clean tub. If it looks like moccha frappuccino, you’ve pretty much verified coolant has gotten into the crankcase

You could also use a block tester to check if combustion gases have gotten into the coolant. If so, some possibilities are failed head gasket, failed intake manifold gaskets, cracked block


With the engine COLD, do the following. Make sure the engine is COLD so you don’t get sprayed with hot coolant.

Open the radiator or reservoir cap. Start the engine. Fluid movement is normal but bubbles in the coolant are not. If you see bubbles, you have some type of breach such as a bad head gasket or cracked head. If nothing looks odd then on to more tests!


Maybe the first thing to do is put the car up on a lift and carefully check it over underneath for any leaks at all.


It’s definitely oil. Hoses are in good condition. I’ve drained the coolant from the car and have the hoses taken off, not sure what else I should try and do today.

Have you read my earlier post?

Some possibilities are failed head gasket(s) and failed lower intake gasket(s)

You shoudl also do a cooling system pressure test.

It could also be a cracked head or block. If a cracked block then you are going to need to decide how much you really like this car and if it is worth fixing.

Then there are the “fix in the bottle” type deals you can use but I do not like these as they can gum up your cooling system but good… Use only if you want to keep the car from the junkyard a little longer but not if you want to eventually fix the car right and keep driving it.

I have the same problem - oil showed up in coolant, car runs great, 192,000 miles, six, auto - I did try to limp home a year or so ago, blew the top rad hose, bought a good hose bandage, tried to limp home - hose blew right next to the bandage - turns out electric cooling fan had gone out. Made sure temp gauge never reached red zone. Lots of miles later, oil showed up in oil - had system flushed, reserve bottle cleaned, didn’t show up for a while, showed up again, oil soaked hoses, replaced hoses. Dealership has now tagged engine as bad, though it runs great. Draining what coolant I can out this weekend, replacing with low/tox coolant - have been told it may be intake manifold gasket -