Many new automotive vehicle designs incorporate an Electronic Throttle Control (ETC) system on the gasoline internal combustion engine to manipulate volumetric efficiency and control engine speed. These engines have the possibility of developing various mechanical problems at some point in the vehicle’s lifespan. Compression testing is a common procedure used to diagnose certain types of engine mechanical problems. Compression testing procedures have traditionally been performed with a fully open throttle. However, ETC systems may not allow the throttle to open fully, if at all, during conventional engine compression test procedures. Technicians in the repair industry, students in vocational programs, as well as educators and trainers need to be aware of the effects of the ETC system on engine compression test procedures and make accommodations for these effects to reliably diagnose engine mechanical problems in these vehicles.

A sample of twenty-two vehicles equipped with an ETC system were gathered for testing. Compression testing procedures were performed on these vehicles to determine the throttle opening and cylinder pressures. The procedure was performed on each vehicle in three variations: (1) a conventional compression test, (2) Electronic Throttle Body (ETB) unplugged, and (3) ETB blocked open. The results of these testing procedures were analyzed to determine to what extent the throttle was opened by the ETC system during a traditional compression test. In addition, the variation in cylinder pressure among the tests was analyzed to determine if a statistically significant result existed. The results of the testing showed a wide variation in throttle openings among tested vehicles as well as a weak correlation between throttle opening and cylinder pressure. The conclusion of the study is that a conventional compression test is a valid testing procedure on ETC equipped vehicles.