My 2001 beetle is stuck on third gear after the oil was mixing with coolant ( they flushed it already)

More than likely you need new head gaskets.

Try the thermostat. Very common on these models.

An oil leak mixing with coolant is an internal leak, which is usually the result of a faulty or damaged head gasket, or cracked head. This is a major repair job, because the entire top of the engine has to be removed to do the repair. You can't see the leak because it's inside the engine, not outside of it. White smoke (steam) should be coming out of the exhaust pipe if this is true.

2001 is an older vehicle - so I can suggest some typical things which might cause overheating on an older vehicle.
- Thermostat stuck / not working? (typical to change thermostat with a new one during other cooling system service - they get old and stop working properly sometimes. They're not very expensive and it's just worth it to put a new one in - they perform a very important function - so typically just put a new one in.)
[might be able to do yourself - but not sure if that is your problem]
- Radiator Electric Fans not coming on? (controls, wiring, or fan motors broken or not working properly)
[Do they come on when the vehicle gets very hot? Try and check.]
- Time for complete cooling system Flush and Refill ? Ever done? This is important service which many people neglect to ever do - which they say to do every 2 or 3 years on any vehicle. Ensure Heater Core inside the car is also given a separate "flush", and ensure it is refilled with the correct stuff. ( ? distilled water + coolant in the proper mix/strength so it won't freeze given your local temperature conditions)
[Could try and do yourself - but can be a messy job. Typically, best to do when it's still warm out / not going below freezing. Drain, refill with "flush" mix, run for awhile like that, drain again, refill with correct coolant mix for your vehicle. If your vehicle uses water + antifreeze use distilled water or de-ionized water in the final coolant mix]
Typically these things are not easy to do yourself, unless you have knowledge, tools and experience. And the job is a little messy + you are left with old coolant & flush mix that has to be disposed of properly so you don't pollute the environment (hazardous waste).
Take to expert RAD Shop / cooling system specialists. There may be other issues like: bad sensors etc in the cooling system. They may need to troubleshoot and look for any error codes in the car's computer control systems - to help diagnose, find the problem quickly, and fix the correct problems.
Otherwise: Get service manuals yourself - start reading and learning about your specific car's cooling system - and see if you are able to troubleshoot, find the issue, or do anything yourself.
An expert "System Flush" at a Rad Shop with their special machine will do a better job than you can do yourself. If never done, I would recommend that.
Best of luck.

Duplicate Question

Technically speaking, the cooling system uses a 50-50 mix of coolant and water. You can buy coolant already mixed, or full strength.
There are hydrometers that you can buy to test the condition of the coolant. I would suggest draining the radiator and refilling with full strength, then test it. You would still have some water in the block and heater core to mix with the new coolant.

could becoming from ur oil cooler lines that go into rad,,,could have internal crack in rad,,,pull dip stick out & ck to see if motor oil is low or a grayish color,,if ok ck trans fluid to see if is low,,have to find out what kind of oil residua is in the over flow jug

either you havent flushed the cooling system out properly ,or the oil cooler has packed up ,their is a oil cooler radiator of sorts that the oil filter screws onto ,these often leak ,just remove the water pipes and join them together with a small piece of copper pipe ,then if you see oil coming out of the water pipes fit a new cooler

have you checked your engine oil if it looks the same?? If your engine oil is mixed/milky colored, then its your head gasket!!hopefully this helps!!

yes it does because you should not mix the two even though there is coolant on the market says mix with any color but I am old school I plan on getting the pink out of my car and flushing the system to change to the green coolant