Is your car knocking, sputtering, or stalling out at the lights? Are you thinking about buying an older car but aren’t too sure about the health of its engine? Automotive pro Scotty Kilmer helps you to determine if you are looking at some serious repair bills in this quick and concise video.
What does #savagescotty do to test if an engine is bad?
- Remove the spark plugs
- Insert a compression test nozzle
- Run a dry compression test
- Run a wet compression test
- Perform a test on all cylinders
That might sound simple, but how does he do each step? Is it something that you can handle on your own? We’ve broken down the video into five easy steps for quick reference so you can check back as you complete your own compression test.
Step 1: Removing the Spark Plug
You will be testing how much pressure is created inside each cylinder while the car is running. To access the engine, remove the spark plug wires or coils. Next, use the correct size spark plug socket wrench and twist counter-clockwise to remove the plugs. Clean up around the plug sockets to ensure no debris falls into the engine.
Take a good look at the spark plugs. The presence of excess gummed up oil or carbon build up around the electrode and tip indicate that the engine may not be burning fuel cleanly. This is the first indication of a bad engine.
Step 2: Screwing in the Test Nozzle that Fits
You can pick up an engine compression test kit at any automotive supply store for under $50. Some parts stores even loan these kits out at no cost! Most of these kits come with a variety of spark plug socket adapters to fit most makes and models. Screw on the adapter sized right for your car to the empty spark plug socket. Connect the adapter to the gauge.
Step 3: Running the Dry Compression Test
Turn the engine over. Let it crank up to four times, enough for the gauge to repeat the highest reading. Log the pounds per square inch reading from the compression test kit gauge. Remove the test kit.
Step 4: Add Oil for a Wet Compression Test
Next, you will want to add just a teaspoon of oil to the cylinder you are testing through the spark plug hole for the wet compression test. Use a long funnel with a flexible spout to prevent pouring oil over the rest of the engine. Reinstall the compression tester. Turn the engine over again letting it crank up to four times to wait for the pressure reading to normalize. Record the number.
Pro Tip: use a grease pencil or marker to write the readings on the top of each valve.
Step 5: Check all the Cylinders
Run this same test on all cylinders. There is one spark plug for each cylinder. If one cylinder has a significantly different result from the others, you may only need to repair that one valve.
What the Numbers Mean
Check the owner’s manual for the engine that you are testing. If it is an older vehicle, the engine may not be the same one that came with the car when sold as new. Make sure you are getting the right specs for the right engine.
For most engines, if the dry test is running below 100 psi, you likely have an engine in need of repair. If the wet reading jumps significantly, that is an indicator that the piston rings are worn. Otherwise, you may be replacing valves, gaskets, springs, or having the engine re-bored.
Is the Engine Good on the Car You Want to Buy?
If you are looking at a pre-owned car to purchase and want to know if it has a history of engine trouble, you might want to download its maintenance history before doing any compression tests.
All you need is the vehicle identification number, then drop it into VehicleHistory.com. Get a report that includes any engine repairs or chronic repair shop visits.
