I have purchased a pre-owned car. After the purchase I saw there was engine oil in the coolant reservoir and also the coolant level is going down slightly. Engine is not heating up.
What do I do?
What might be the reason?
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The first thing to check for when you have oil and coolant mixing is a blown head gasket. Check compression on all of your cylinders with a compression tester (you can borrow them from AutoZone or similar) and specifically look for one or two cylinders that have significantly lower compression than the others (in the range of 30+ psi difference). Another indicator is if you also have coolant mixing with your oil (run the engine, then pull the dipstick out, if it looks like it's covered in chocolate milk, that means there's water and therefore coolant in your oil). Along with that, I suspect that by "Engine is not heating up" you mean "the temperature gauge doesn't go up", which means your thermostat is probably defunct, which could have caused the kind of overheating that can damage head gaskets, so look into getting that repaired as well.