How to measure engine blow-by ?

Engine Blow-By: An Overview

A combustion engine operates by burning fuel in a cylinder, where a piston moves up and down. The piston is sealed towards the cylinder by piston rings. However, a certain amount of exhaust gas escapes from the combustion chamber into the crankcase housing, which is referred to as “blow-by.” A visual representation of this can be found at

Checking the Engine for Blow-By

When inspecting a used machine, it is essential to assess the engine for the amount of blow-by. By measuring the amount of escaping gas, an experienced inspector can estimate the engine wear. Typically, this can be done by placing a hand on top of the oil filler tube and checking for the amount of blow. The inspector will also examine the smell of the gas for any signs of burned oil or coolant in the engine lubrication system.

Engine blow-by measurement

On larger engines, it may be necessary to measure the blow-by and compare it with the manufacturer’s specifications. Some inspectors may be equipped with a gauge, called an anemometer, for this purpose. However, it is important for the inspector to have a good understanding of the engine configuration as not all gas escapes through the oil filler tube. Some engines may have a certain amount of blow-by even when new. Inspectors opening filler tubes on Perkins engines can tell stories about it.

Examining Engine Condition through Exhaust Fumes

Experienced technicians can use the appearance and smell of the exhaust fumes to determine the condition of the engine. For example, if the engine is burning oil, the technician may detect a burnt oil odour. The colour and amount of smoke can also provide insights into the engine’s condition. Engine smoke can appear white, grey, grey-blue, or black. If the smoke appears abnormal, the inspector may need to examine the engine systems closely for issues such as malfunctioning injectors or coolant entering the engine lubrication.

Engine oil sampling

Last but not least it is an option to send an oil sample for analysis. A certified lab will provide info about oil condition and containment. Engineers can read a lot from sample as long as it is known how long the engine oil was in use.

Using an inspection app

We are often approached by developers of machinery inspection apps, who believe that anyone can perform an inspection with the right application and guidelines. However, from our perspective, it is still necessary for an inspector to have several years of experience with machines and engines to accurately assess the engine’s condition.

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