Looking for better clues
By Francisco Aristizabal
All diesel engines need an unrestricted, constant supply of clean air to run properly and efficiently. As combustion occurs, exhaust gas forces drive a turbocharger; therefore, air intake / exhaust gas system must be operated without restrictions to avoid issues such as poor combustion, smoke, carbon build up, high operating temperatures and excessive fuel consumption or low horsepower
The OEM will establish an air restriction maximum allowance for the desired engine performance. This restriction on a system’s air flow is calculated based on maximum operating conditions at full load.
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Generally, most of the engine manufacturers suggest an intake maximum restriction value between 20-25 inches H2O. The lower limit is usually for natural aspirated engines and higher for turbocharged/supercharged engines.
Causes for low turbo boost pressures are similar between two strokes and four stroke engines. Where this can vary is when there is a gear driven blower used in addition to the turbocharger which is moved by exhaust gas. Therefore, the turbo pressure must be isolated from the blower forced air pressure to be evaluated. This is required to evaluate the seal between engine block and blower case.
All engines run with a small and desirable amount of blow by. This pressure helps to keep dirt in a suspension state flowing in the oil pan crankcase ventilation system. On the inverse, excessive blow by will introduce these harmful contaminants into the oil, leading to premature wear and tear on the engine’s internal components. This can reduce the engine’s power output, make it less efficient, and increase the risk of engine failure.
Gauges Used for Diagnosis
Air intake/exhaust systems could be checked using both H2O/Hg gauges to prove or disprove:
- Air intake restriction
- Turbo boost pressure
- Blow by
If it is necessary, a conversion between units (metric/US) is important have this conversion:
1 in H2O = 0.0375 in Hg / 0.0361 PSI
1 in Hg = 13.6 in H2O / 0.491 PSI
Air Intake Restriction
The image below shows the location where the measurement must be taken. Connect the gauge between 4 to 8 inches (approximately) from the turbocharger air intake while the engine is at full throttle. A double column gauge (U as shown in image) will display the blow by measurements.
Higher values as compared with the OEM recommended values could indicate issues with the filter or the hoses as follows:
- Air filter is dirty, has incorrect fitment, barometric pressure/humidity impact.
- Air intake hoses short diameter sized or longer as required.
Turbo Boost Pressure
Typically, the easiest way to take this measurement is on the intake manifold (see figure) or in the exit side of the turbocharger assembly. Measured lower values could be the result of air intake restriction, leak of exhaust gas on the pipes, intercooler (if is equipped) stacked with oil or carbon or a Turbocharger with internal damage. The readings must always be compared with OEM values.
Blow By
High blow by indicates compression leaks and/or pressure leaks between piston rings lands. Besides engine cylinder leaks (liner, piston, ring wear) a high blowby measurement can be related to excessive engine oil level, crankcase breather stacked/oil soaked or exhaust restriction/back pressure. Measurement can be taken at the dipstick tube or vent/ oil filling cap with the use of special adapters designed for this process as shown in the image below.
As always, we mention all this info as it is intended to serve as a general guide. Specific procedures may vary depending on the equipment manufacturer and application. Always check their literature and any other OEM service recommendations, the AERA tech line is also available to solve questions in these matters.
Read this article with all images in the digital issue of Engine Professional magazine https://engineprofessional.com/2025EPQ2/#p=86