Coolant in the engine oil is a serious issue that suggests there is a breach somewhere in your engine where the coolant is leaking into the oil system. This can lead to severe engine damage if not addressed promptly. Here are the most common causes and potential solutions:

### Common Causes:

1. **Head Gasket Failure**:
- **Cause:** The head gasket seals the cylinder head to the engine block. If it fails, it can allow coolant to leak into the oil passages.
- **Solution:** The head gasket needs to be replaced. This is a labor-intensive job that involves removing the cylinder head, cleaning the surfaces, and installing a new gasket.

2. **Cracked Engine Block or Cylinder Head**:
- **Cause:** Cracks in the engine block or cylinder head can allow coolant to seep into the oil system.
- **Solution:** Depending on the severity of the crack, the solution can range from using a sealant designed to fill in minor cracks to replacing the cracked component. Severe cracks often require engine block or cylinder head replacement or a complete engine rebuild.

3. **Intake Manifold Gasket Failure**:
- **Cause:** Some vehicle designs route coolant through the intake manifold. If the gasket fails, coolant can enter the oil passage.
- **Solution:** Replace the intake manifold gasket. This involves removing the intake manifold, replacing the damaged gasket, and reassembling the manifold.

### Diagnosis Steps:

1. **Check for Milkshake-Like Oil**:
- Open the oil filler cap and dipstick to inspect the oil. If it looks milky or frothy, it's a clear indication that coolant is mixing with the oil.

2. **Perform a Compression Test**:
- A compression test can help determine if the head gasket is compromised or if there's a crack in the cylinder head or block. Low compression in one or more cylinders can suggest these issues.

3. **Pressure Test the Cooling System**:
- A cooling system pressure test can help identify leaks. If the pressure drops, it indicates a leak somewhere in the system which could be into the oil passages.

4. **Use a Block Tester**:
- A combustion leak test (block tester) can detect the presence of exhaust gases in the coolant, which would indicate a head gasket failure or crack in the head/block.

### Steps to Fix:

1. **Drain the Engine Oil and Coolant**:
- Before starting any repairs, drain the current oil and coolant. The contaminated oil cannot be reused.

2. **Disassemble as Needed**:
- Depending on the diagnosis, you will need to disassemble parts of the engine. For a head gasket repair, this will involve removing the cylinder heads. For an intake manifold gasket, it will involve removing the intake manifold.

3. **Inspect and Clean Surfaces**:
- Once disassembled, inspect the surfaces for warping or damage. Clean the mating surfaces thoroughly before installing new gaskets to ensure a proper seal.

4. **Reassemble with New Gaskets**:
- Install new, high-quality gaskets. Follow the manufacturer's torque specifications and sequence to ensure proper sealing.

5. **Refill Engine Oil and Coolant**:
- After reassembly, refill the engine with new oil and coolant.

6. **Test Run and Monitor**:
- Run the engine and monitor for leaks. Check the oil and coolant levels frequently to ensure the repair was successful.

Addressing this issue promptly is crucial to prevent extensive engine damage. If the damage is too severe, like a cracked engine block, you might need to consider more extensive repairs or even engine replacement.