The Fix-It Club can help you understand and diagnose common engine repairs include disassembling, replacing or repairing, and reinstalling primary parts such as pistons, connecting rods, crankshaft, camshaft, valves, and related parts. Unfortunately, modern engines require precise parts and assembly so most consumers don’t repair engine components. Depending on what’s needed, it may be less work to buy a new, rebuilt, or used engine and replace the old one. In many cases, that’s what professional mechanics do. (The photo shows components of a typical engine, including a cutaway of two cylinders.)
What could go wrong with an engine that would require major repair?
- If the engine lacks power, the valves or piston rings could be worn.
- If the engine uses excessive oil, the piston rings or valve guides could be worn. (Note: first check underneath the car to see if there’s an easy-to-fix leak.)
- If the engine makes a light clicking noise, the valves may require repair. (Note: first check to make sure that all spark plug wires are working properly.)
- If the engine will start, but won’t run, the timing chain may be loose or broken.
- If the engine makes a sharp metallic knock when running, the cause could be the bearings at either end of the connecting rods.
Fortunately, you can test the engine with a compression tester (photo on right). It measures how much pressure a specific cylinder can generate in a single rotation of the crankshaft. You can buy or rent a compression tester. Here are the steps:
1. Remove a single spark plug and insert the tester in the spark plug hole per the manufacturer’s directions.
2. Remove the spark plug coil main wire so the engine won’t start when you perform step 3.
3. Use the ignition to carefully rotate the engine at least four revolutions (about one second).
4. Read and write down the compression test results.
5. Replace the spark plug, remove another, and repeat the test.
6. When done, compare the compression test results to the engine manufacturer’s data to determine normal operating range and for suggestions of causes and solutions.
Compression ratio is the ratio between the space in a cylinder when the piston is at the bottom of travel and at the top of travel. A 7:1 compression ratio means the space in the cylinder is seven times larger when the piston is fully down than when it is fully up, called top dead center or TDC. A compression tester measures the actual compression within a cylinder. If the engine is designed for 7:1 but the tester shows 6:1 compression ratio, there is a leak of compression within the cylinder, usually around the piston.