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The car is a '98 Buick Lesabre with the 3800 engine and about 170k miles. It appears as though some oil was in the radiator, but I haven't seen any signs of coolant in the oil. The water pump recently went bad, so I'm not sure how much oil was in the radiator because all the coolant leaked out; I just found some on the inside of the cap. When the water pump went the serpentine belt came off. I drove it to the mechanic and they said I needed a new pump, belt and a flush. They didn't mention any other problems. I drove home and haven't started it since. It didn't overheat at any point.

Do I have a blown head gasket? If not, why would oil be leaking into the coolant, but no coolant leaking into the oil?

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Hi I have oil in my coolant also but no coolant in oil my car never over heats –  Rachael Oct 31 '13 at 6:32
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3 Answers

It's likely that the oil cooler is leaking if the vehicle is equipped will have one. Look for metal lines running into the radiator tank. If you have an automatic transmission there will be two sets of lines on each tank of the radiator.

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Well it's a '98 Buick Lesabre. I'm not sure if it has that, but I think you may be right. I did see lines running into the radiator that looked like it was possibly leaking oil. Thank you very much for your help –  wickedpissah Mar 10 '13 at 19:00
    
What would a mechanic do to determine if the engine is no good? About how much would it cost me to have a mechanic check it out? –  wickedpissah Mar 12 '13 at 17:20
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The 3.8 Natural Aspirated Engine was a rather solid design and were rarely known to blow head gaskets. Their weak point seemed to be the plastic Intake Manifold. Here are the three possible causes to your issue.

  1. If you are 100% sure that it's oil in the coolant then remove the intake manifold and check the Intake Manifold Gaskets upper and lower for any signs of seepage. Be sure to inspect the Intake Manifold for any cracks or signs of it leaking. If this is the case, this is how oil would find it's way into the coolant and not vise-verse. Here is a picture of the intake manifold for reference. This is a very common issue with these motors. And honestly I'm surprised that with the mileage your vehicle has it hasn't been done yet. 98LeSabre_IntakeManifold

  2. Make sure it's not transmission fluid. If you have a look at the radiator and you'll notice the radiator has two metal pressure lines on the same side one near the top and on near the bottom of the assembly. These lines come from the transmission to the transmission cooler that is integrated into the radiator. If it was transmission fluid then the vehicle will have shifting issues because the coolant would also get into the transmission causing the clutches and torque converter to overheat. 98LeSabre_radiator

  3. If the oil and transmission levels are not dropping then what's going on? Double check the substance in question, if you realize that it is nothing more than sludgy build-up in the cooling system (you will notice it on the bottom of the radiator cap and in the coolant overflow tank) and the vehicle has a Dexcool brand coolant in it, this is caused by air being introduced to the coolant system. Possibly when they changed the water pump. Here is a photo of a the inside of a radiator that had Dexcool in it. You'll notice the substance when mixed with coolant could resemble oil to the untrained eye. dexcool_radiator_sludge

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It seems unlikely that you have a blown head gasket if you don't have milky oil. In fact it's fairly common to see some trace of oil on your radiator cap. A little bit of oil works its way in there, especially if you replace parts of the cooling system. You should be safe to just keep an eye on it for now.

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A little bit of oil works its way in there, especially if you replace parts of the cooling system. How does this happen? Changing cooling system parts shouldn't introduce oil into the cooling system. –  Larry Mar 11 '13 at 14:56
    
Think about that little bit of lubricant that comes on a new water pump or a new thermostat, etc. Oil is lighter than water and coolant, so any trace of it is going to end up at the top, which is right where the cap is. –  hillsons Mar 12 '13 at 0:44
    
There are all sorts of ways head gaskets can fail. For example in my wife's car (Subaru), the gasket seal is reliable except under the extreme pressure of combustion, so the leak only goes one way: combustion gasses into the coolant. I'm not an expert bus I suspect there are HG failure modes where oil gets into the coolant but not vice versa. –  R.. May 10 '13 at 18:39
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