Your 1994 Lincoln town car has a coolant leak and it seems like its coming from little round cap under the engine next to transmission what could be?

That little round cap under the engine, near the transmission, is most likely the expansion tank cap (or sometimes called the coolant reservoir cap). A leak from there suggests a problem with either the cap itself or the pressure within the cooling system.

Here are some possibilities:

* Faulty Expansion Tank Cap: This is the most common cause. The cap seals the system and regulates pressure. If it's cracked, warped, or the pressure valve is malfunctioning, coolant will leak. This is an inexpensive and easy part to replace.

* Cracked or Damaged Expansion Tank: The tank itself might have a crack, especially if it's aged and brittle. This is less likely if the leak is only from the cap area, but inspect the tank carefully for any cracks or damage.

* Excessive System Pressure: A problem elsewhere in the cooling system (radiator cap, water pump, thermostat, head gasket, etc.) could cause excessive pressure buildup, forcing coolant past a slightly weakened expansion tank cap. This needs further investigation. Overheating is a common symptom of this.

* Loose or damaged hose connection: While less likely if it's clearly coming from the cap area, double check that all hoses connected to the expansion tank are securely fastened and not damaged.

What you should do:

1. Check the cap: Carefully inspect the expansion tank cap for cracks or damage. You might be able to gently wiggle it to check for looseness.

2. Check the tank: Inspect the expansion tank itself for cracks or damage. Look closely, even with a flashlight.

3. Check the coolant level: With the engine cold, check the coolant level in the expansion tank. Low coolant confirms a leak.

4. Top off the coolant (carefully): Only add coolant if the engine is cold. Never open a hot system – you'll risk serious burns. Use the correct 50/50 mix of coolant and distilled water.

5. Replace the cap: The easiest and cheapest thing to try first is replacing the expansion tank cap. This is a relatively inexpensive part and easy to replace.

6. Monitor: After replacing the cap, monitor the coolant level for a few days to see if the leak persists.

7. Professional Inspection: If the leak continues, it's time for a professional mechanic to diagnose the underlying cause. A pressure test of the cooling system is a good next step.

Ignoring a coolant leak can lead to overheating and serious engine damage. Address this problem promptly.