Here are some likely culprits:
* Connecting Rod Bearing Failure: This is a high probability given the knocking noise and low compression. A connecting rod bearing allows the piston to move up and down; if it fails, the rod can knock against the crankshaft, causing significant damage. This is a major repair.
* Main Bearing Failure: Similar to connecting rod bearings, but these support the crankshaft itself. Failure leads to crankshaft movement and knocking, impacting compression and idle. Another major repair.
* Piston Slap: While less serious than bearing failure, piston slap is still a problem where the piston moves loosely in the cylinder, creating a knocking sound. Worn piston rings contribute to this and also lead to low compression.
* Worn Camshaft: A worn camshaft can cause a knocking sound and affect valve timing, impacting idle and compression.
* Valve Train Issues: Worn or damaged lifters, rocker arms, or pushrods could create a knocking noise and affect valve operation, leading to poor compression and idle.
* Loose Timing Chain/Belt: While less likely to cause a knock, a loose or damaged timing chain or belt can lead to poor timing and uneven compression, manifesting as a poor idle.
What to do:
1. Do NOT drive the car: Continued use will likely cause significantly more damage.
2. Have it towed to a mechanic: A proper diagnosis requires compression testing, a visual inspection (possibly requiring engine disassembly), and potentially further diagnostic tests.
3. Get multiple quotes: Engine rebuilds or replacements can be expensive. Getting multiple opinions is wise.
Given the age of the vehicle, a complete engine rebuild or replacement might be more cost-effective than trying to pinpoint and fix individual components, depending on the extent of the damage. The mechanic will assess this once they've inspected the engine. Expect to pay a substantial amount for the repair.
