how to check blowby

3 min read 02-02-2025

Blowby is a critical engine issue that signifies the escape of combustion gases past the piston rings into the crankcase. Ignoring blowby can lead to serious engine damage, so regular checks are essential. This comprehensive guide details how to effectively check for blowby and what to do if you find it.

Understanding Blowby

Blowby occurs when combustion pressure forces gases past worn or damaged piston rings. These gases enter the crankcase, contaminating the engine oil and increasing crankcase pressure. Excessive blowby is a clear indication of engine wear and tear, often requiring repairs.

Signs of Excessive Blowby

Before checking for blowby directly, be aware of the common symptoms:

Excessive crankcase pressure: You might notice oil leaks, bulging dipstick, or difficulty removing the oil fill cap.
Milky oil: Combustion gases contaminating the oil will create a milky, frothy appearance.
Reduced engine performance: Loss of power, decreased fuel economy, and rough running can all be symptoms.
Blue exhaust smoke: While not a direct indicator of blowby itself, it often accompanies it.

How to Check Blowby: A Step-by-Step Guide

There are several ways to assess blowby. Here's a detailed approach using a common method:

Tools you will need:

Blowby tester: This specialized tool measures the pressure escaping past the piston rings. It's crucial for accurate measurement.
Wrench: To loosen and tighten the oil fill cap.
Gloves and safety glasses: Essential for protecting yourself from hot oil and potential spills.

Steps:

Warm up the engine: Run the engine until it reaches normal operating temperature. This ensures accurate pressure readings. Cold engines will give inaccurate results.
Securely attach the blowby tester: Remove the oil filler cap and firmly attach the blowby tester to the opening. Ensure a tight seal to prevent leaks.
Run the engine at idle: Allow the engine to run at idle speed for several minutes. This will allow the tester to accurately measure the blowby pressure.
Observe the blowby pressure reading: The blowby tester will display the pressure in either PSI or CFM (cubic feet per minute). Compare this reading to the manufacturer's specifications for your particular engine.
Interpret the results: Exceeding the manufacturer's recommended blowby level indicates excessive blowby and necessitates further investigation.

Interpreting Your Blowby Test Results

The acceptable blowby levels vary greatly depending on engine size, age, and design. Consult your vehicle's service manual for the specific specifications. However, generally speaking:

Low blowby: A reading within the manufacturer's specifications indicates the engine is in good condition.
Moderate blowby: Slightly above the acceptable range might suggest minor wear. Consider regular oil changes and monitoring the situation closely.
High blowby: Significantly exceeding the acceptable range points to significant engine wear, potentially requiring repairs like piston ring replacement or engine overhaul. Ignoring this could cause catastrophic engine failure.

What to Do if You Find Excessive Blowby

If your blowby test reveals excessive blowby, address the issue promptly. Here's what you should do:

Consult a mechanic: A qualified mechanic can diagnose the underlying cause of the blowby. They can pinpoint the exact problem and recommend the necessary repairs.
Don't ignore the problem: Continued driving with excessive blowby will accelerate engine wear and lead to costly repairs down the line.

Preventing Blowby

Regular maintenance is key to preventing blowby:

Regular oil changes: Use the correct oil viscosity and change it at the recommended intervals.
Proper air filter maintenance: A clogged air filter restricts airflow and can contribute to increased engine wear.
Avoid prolonged idling: Excessive idling puts unnecessary stress on the engine.
Address any other engine problems promptly: Timely repairs prevent minor issues from escalating.

Checking for blowby is a crucial step in maintaining engine health. By following these steps and paying attention to any symptoms, you can catch problems early and prevent costly repairs. Remember to consult your vehicle's service manual for specific blowby limits and maintenance schedules.