How to Check for Blow-by in a Car

Checking an engine for blow by requires expertise not usually found in a do it yourself set-up. Certain steps will require precision instruments and expert knowledge to read the instruments and do the testing.

Things You'll Need

• 0-50 engine compression gauge
• End fitting matching spark plug size
• 1/2-inch drive spark plug socket
• 1/2-inch drive extension
• 1/2-inch drive ratchet
• Fender cover
• Service manual for the year model of the vehicle

Instructions

1. Identify the year model of the vehicle and purchase the service manual that includes the procedure. Sometimes this procedure is not available in a general service manual and must be located by your mechanic via computer.

2. Identify the size of the spark plugs. Attach the matching 1/2-inch drive spark plug socket to the 1/2-inch drive extension. (The extension should be long enough to bring the ratchet level with the top of the engine). Attach the extension to the 1/2-inch drive ratchet and push the on/off lever on the ratchet to the off position. (Counterclockwise).

3. Position fender cover over the fender side you are starting on.

4. Look up the firing order for the engine you are working on and remove the first spark plug in that order using your 1/2-inch drive set-up. Look for carbon build-up on each spark plug. This is a symptom of a blow-by condition.

5. Disconnect the battery and all spark plug wires

6. Attach the appropriate size fitting to the compression gauge, and screw it clockwise into the spark plug hole until hand tight.

7. Have a helper turn over the engine while you are observing the compression gauge reading. You are looking for a steady rise from 0 to the maximum compression for the engine. If the pressure does not hold at a steady position, there is blow-by (loss of cylinder compression) in this cylinder.

8. Repeat this procedure at every cylinder, and note all cylinders that are not maintaining steady pressure. These cylinders will require a complicated procedure to be repaired before the vehicle is returned to acceptable running condition.

Tips & Warnings

• Make sure your spark plug socket still has the rubber fitting inside. This will make it much easier to remove the spark plug.

• Make sure your compression gauge is properly calibrated, and you are using an up to date manual for the procedure

• Safety glasses should be worn throughout the procedure.

• Make sure to disconnect all the spark plug wires so that the engine does not start up and injure you.

• Make sure the battery is disconnected to avoid a potential electric shock.