What cause oil in the coolant

There is oil already in the engine oil pan and lubrication system and apart from some condensation there should be no water. The only place water can come from in any quantity is from the cooling system and although cracks and leaky water heated inlet manifolds and gaskets can be the cause the most common source of coolant ingress is the cylinder head gasket.

A failing head gasket doesn't very often mean there will be oil in the coolant.

Check your oil for coolant. Looks muddy or diluted. Check for air in the cooling system.
How To Bleed Cooling System EricTheCarGuy

Sounds like the head-gasket blew and allowed oil into the coolant and coolant into the oil. Unfortunately, it's time for a tear-down and rebuild of the engine.

Over heating can be caused by many things. The first question I would have is when did the problem begin? If you or someone recently added coolant, perhaps the wrong type was added. The second question would be when was the last time the coolant system was flushed? Preston sells a product with easy to follow instructions to flush out debris in the system. The third is are there any hose leaks? Low levels of fluid would certainly case over heating. The fourth is to change the oil regularly. Old oil and low oil means more heat. Fifth is to be sure nothing is blocking the radiator. A man just lost as big race over a food wrapper stuck on the radiator!! Car was over heating so he slowed down. Does the over heating happen only when you stop at a light? May be a bad fan sensor. Beyond these items you may have a bad thermostat or worst case a bad water pump.

If the oil level goes up, something has to be leaking into the engine oil pan.
That would either be gasoline from the injectors or coolant from the radiator. If you are not adding coolant, it could be a leaking injector.

Hi.

Do a compression test and look for symptoms of head gasket problems. If the leak is internal, then the most common cause is a worn head gasket. Water (or coolant) leaks internally through the head gasket reaching the cylinders where it gets vaporized. Symptoms are coolant fluid level going down (with no external leaks), bad performance, loss of compression, overheating, white smoke from exhaust because of vaporized coolant, traces of fuel in coolant reservoir etc.

If there is no head gasket problem and there is no overheating, then the leak is external. Check pump, radiator and coolant lines.

Regards.

Ginko.

depends on the type of oil in the reservoir!! Typical issues that will cause this on your 4.2L I-6 engine; 1. (usually pink/reddish oil) trans fluid leaking via faulty radiator, blown head gasket from overheating (severe), or someone accidentally added oil to your radiator!!

You did not mention what engine you have, so I assume it is a 2.4L and has any radiator stop leak been added to the coolant system lately?

If no radiator stop leak has been added, the good news is you don't have coolant in the oil.
I would speculate you have a head gasket problem. The oil pressure is higher than the coolant pressure, therefore the oil has the abiltiy to enter the cooling system. There is an oil pressure supply galley that runs between the engine block and the cyliner head to supply oil to the cam followers. That is most likly where the oil is getting in.

Unfortunatley the head gasket will have to be changed.
Let me know.
Regards,

it seems like the radiator of your car needs attention. check for coolant level in it, when its cold. if les add some coolant and then see if the problem persists.
--than there are chances of chocking in the coolant pipes, may be they not reached to engine..  
--Check the chock,, that it is not turned on. 
--May be sum loosing nut in engine.

It could be your head gasket.