The lack of compression after changing the water pump suggests a problem unrelated to the pump itself. Here's why:
* Overheating damage is not instantaneous: Overheating typically takes time to severely damage a cylinder head. If the engine overheated *because* of the water pump failure, you might have other issues, but it's not certain the cylinder head is the culprit.
* Other possible causes of no compression: There are many reasons for a lack of compression, including:
* Blown head gasket: This is a common cause of loss of compression and is often associated with overheating, but it's not necessarily a *direct* result of water pump failure.
* Valve issues: Burnt or bent valves, or problems with valve seals, will lead to compression loss.
* Piston rings: Worn or broken piston rings prevent proper compression.
* Holes in the pistons: Less common, but possible.
What you need to do:
1. Check for coolant in the engine oil or vice-versa: A blown head gasket is a strong suspect if you find coolant mixing with the oil.
2. Perform a compression test: This is crucial. A compression test will tell you precisely which cylinder(s) lack compression and how much. This helps pinpoint the problem.
3. Leak down test: This test helps determine *where* the compression is escaping (valves, rings, head gasket).
Only after performing a compression test and possibly a leak-down test, can you diagnose the root cause of the lack of compression. The water pump replacement might have revealed an existing, unrelated problem, or an unrelated problem may have occurred during the repair process.
