A low pyrometer reading from one engine cylinder would most likely indicate which fault?

• A. A defective engine thermostat
• B. Restricted air inlet
• C. Defective injector
• D. Restricted exhaust system

Answer: (C)

A per-cylinder exhaust temperature reading helps assess how well each cylinder is combusting. If one cylinder shows a noticeably lower temperature than the others, it means that cylinder isn’t producing as much heat during combustion. The most likely localized cause is inadequate fuel delivery to that cylinder, such as a faulty injector. With a bad injector, that cylinder runs with too little fuel, combustion is less complete, and the exhaust gas is cooler, which a pyrometer would detect as a low reading in that cylinder.

The other faults tend to affect the engine as a whole rather than just one cylinder. A defective thermostat mainly alters overall engine cooling and would not produce a single-cylinder cool exhaust. A restricted exhaust changes back pressure system-wide and wouldn’t typically cause a single-cylinder low reading. A restricted air inlet alters air supply across all cylinders and would usually present broader, not isolated, symptoms.