Oil in coolant overflow tank

Oil in coolant overflow tank

Postby ahiggin [OP] » December 2nd 2011, 5:37pm

My '98 just rolled over to 200,000 miles. But now I'm seeing oil in the coolant overflow tank. Possible causes?
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Re: Oil in coolant overflow tank

Postby Phantom » December 2nd 2011, 5:50pm

Depends on if that oil is engine oil or transmission oil

If trans fluid the trans cooler tube in the radiator tank is leaking , you would need to replace radiator and flush trans fluid out of trans cause coolant will be in the transmission an will damage it if not removed

If engine oil you may have blown head gasket , cracked head , cracked engine block or other way that the oil can enter the cooling system

remove radiator cap and check coolant and check oil dipstick for a milkshake looking oil
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Re: Oil in coolant overflow tank

Postby ahiggin [OP] » December 2nd 2011, 6:34pm

More data: I discovered the oil in the coolant tank on Sunday. At that time, I removed, emptied and cleaned the tank and filled it with fresh coolant. I then drove to Dallas and back, approx. 500 miles round trip. Now there is no more oil in the tank. Still a tiny bit of oil under the radiator cap, but no more than when I checked it Sunday. I checked the oil dipstick, and the oil is clear and has no unusual odor. I checked the trans dipstick and the fluid is clear with no unusual odor. Results: inconclusive.

I suppose the next steps are to pull the oil pan and trans pan, but I don't think that will help diagnose it either. Clearly oil is coming from one of the two sources. My bet is on the radiator.

Any other diagnostic suggestions? And thanks for your help!
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Re: Oil in coolant overflow tank

Postby Phantom » December 2nd 2011, 6:45pm

When you discovered the oil in the coolant tank , how did you determine it to be oil , what did iy look like ? what color was the coolant in tank? did it look foamy or have bubbles in it?

If there is a pinhole leak in the trans cooler tube it could be a slow leak of fluid into the radiator
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Re: Oil in coolant overflow tank

Postby ahiggin [OP] » December 2nd 2011, 7:06pm

It looked like a black oil slick floating on top of the coolant. I don't remember it looking foamy or bubbly. When I rubbed it between my fingers, it was much slicker than antifreeze.
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Re: Oil in coolant overflow tank

Postby Phantom » December 2nd 2011, 7:37pm

Certainly something to watch closely , if you have the engine oil cooler , it may have a pinhole slow leak just like the trans cooler tube . ....ever used a radiator stop leak product like Bars leak ?
it has some type of black slippery liquid in it.

sometimes I have seen black residue inside of coolant overflow bottles that is just old crudded coolant from age as many people do not clean the bottle when they change coolant , you can get a cooling system pressure test to make sure it is holding good pressure
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Re: Oil in coolant overflow tank

Postby ahiggin [OP] » December 2nd 2011, 8:16pm

Ah, good thought about the engine oil cooler. Yes, it has one - not called out as a separate RPO code, so must be part of the Z82 HD trailering package.

I used a Bars leak product years ago, but the system has been flushed several times since then.

As far as pressure-testing the cooling system, wouldn't that risk forcing coolant into either the engine or trans if there's a small leak? I could pull the oil lines and coolant hoses off the radiator, and cap the radiator hose connections to pressure-test the radiator in isolation.

I think I'll just flush the cooling system and keep an eye on it for now. It's overdue anyway.

Phantom, you've been very helpful. Thanks!!
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Re: Oil in coolant overflow tank

Postby ahiggin [OP] » December 12th 2011, 7:48pm

For those who might stumble onto this thread with oil in their coolant tank, the problem turned out to be a bad radiator. There was a crack in the engine oil cooling coil in the radiator tank causing a small leak. New radiator: $170, new cap: $7, Dexcool: $25, Coolant flush additive: $4, engine oil: $15, oil filter: $5, total parts and materials: $226, Labor: $253. Repair total: $479.
Last edited by ahiggin on December 12th 2011, 10:39pm, edited 1 time in total.
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Re: Oil in coolant overflow tank

Postby Jasen » December 12th 2011, 7:53pm

Fortunately it was small and in the rad and you discovered it in short order.
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Re: Oil in coolant overflow tank

Postby Phantom » December 12th 2011, 7:55pm

Good to hear you got it fixed :dance:


Sad that the labor charge was more than all the parts combined :(
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Re: Oil in coolant overflow tank

Postby ahiggin [OP] » December 12th 2011, 10:46pm

Phantom, thanks again for your help in hitting the problem dead on. I asked the shop to pull the trans and engine oil lines off the radiator before pressure testing the radiator, which they did. Yeah, I'm not used to labor charges at all, as I usually do all the work myself. But with a bathroom remodel and Christmas stuff going on, I didn't have time. Also doesn't hurt to have one of these repair statements occassionally to remind my wife that even though I'm retired, I'm still worth a little as a general fixit man. :D
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Re: Oil in coolant overflow tank

Postby RECox286 » December 13th 2011, 2:28am

Thank You !

We appreciate your follow-ups just as much as y'all appreciate the information that we provide.

Glad you got your truck back on the right side of the road.

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