Looking for better clues
By Francisco Aristizabal
Engine diagnostics procedures are often the greatest challenge when trying to solve a diesel engine failure. When hard parts are involved, i.e., head, block, etc., sometimes some symptoms can be misleading, and cause much confusion as to what path to take to solve the problem.
When a cylinder head failure (or the associated components) failure is involved, the symptoms could be low engine power, oil consumption, or other details, like fluid leakage or sealing issues. This is why heads are often removed without any idea of what the problem truly is.
The standard/mechanical tests are a compression and leak down test, these are useful as long as we have the OEM values to rely on. Before the compression test, the engine must be at operating temperature. It is important to check for an intake air restriction and exhaust back pressure. You may need to install a false injector/gauge assembly where the compression gauge is installed. The engine must be rotated between 6 to 10 times until the gauge stops at the highest recorded value to capture accurate readings. Some engines will require a fuel bypass to the return line before the test, or even a complete fuel drain. Both the values of each cylinder, and the variation between each cylinder are important to review and compare with OEM standards.
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When doing a cylinder leak down test, some of the previous requirements are different. Adding to the engine operation temperature and drained fuel, all the ducts/hoses/tubes to intake manifold must be removed, as well as the exhaust tubes to the turbocharger. Each cylinder evaluated must be at TDC and assure a regulated air supply. 80 psi (regulated) is the normally accepted pressure in; however, OEM values may vary.
Listening Points
Engine function knowledge and experience are essential to interpret the results. Results such as escaping air from intake/exhaust manifolds could be generated by damage to intake/exhaust valves and/or seats, from injector tubes/holes or related to a head-gasket or general sealing issues. It is possible that bubbles in the radiator or expansion tank could appear.
More recently other techniques have been applied to diagnostics, like oscilloscopes with pressure transducers. The transducers measure pressure or vacuum in the intake and exhaust manifolds as well as compression and pressure in the combustion chamber. When the diesel engine is running, these tests allow for detecting airflow pulsation from the combustion chamber as well as determining the valve timing of the valve train and fuel system.
Unfortunately, just a few OEMs include this kind of test information in their shop manuals.
Regardless of the test method that is used, the key is the knowledge, expertise and how the technician interprets the results based on the OEM values and info.
Common sense advice is to go from the known to the unknown, and from the easiest to perform to the hardest to perform tasks to determine failure. Primary information is essential, so the more logical the approach the more likely the diagnosis will be a success.
Sometimes, all the tests mentioned are not enough to understand the problem. Issues such as tiny head cracks, injector sleeves leaking, and others only could be detected by removing the cylinder head and performing a head pressure test. You may need to further examine the extent of the issue with tools such as Magnaflux, penetrating dye or even a combination of all these techniques would be necessary.
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 / specifically 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/2025EPQ1/#p=96