![]() Some nameplates actually specify a grams-per-second airflow rate at different engine speeds to aid in diagnosing the mass air flow (MAF) sensor itself. Entering “volumetric efficiency” into your computer’s search engine can access several volumetric-efficiency calculators. But, because calculated load values vary considerably among different engine designs, it’s always best to compare any calculated load data with known-good values.Īnother method of calculating volumetric efficiency is to compare the engine’s airflow in grams per second with the numbers generated by a volumetric efficiency calculator. In the strictest sense, the calculated load value displayed on a scan tool data stream is the most direct indicator of an engine’s volumetric efficiency. Such methods are often lacking when diagnosing modern engine management systems. An alternate method was to see if engine performance would improve by disconnecting the catalytic converter from the exhaust manifold. If the vacuum reading at a steady 2,500 rpm dropped by more than 10% from idle values, the converter was likely restricted. If both readings were nearly the same, the converter wasn’t restricted. ![]() This fatal combination would usually cause the converter to internally melt or partially collapse, which would then cause a varying degree of exhaust restriction.ĭuring that era, the standard diagnostic method for testing exhaust restriction was to test for a decrease of intake manifold vacuum between idle speed and a steady-throttle 2,500 rpm value. Raw fuel from a sticking choke or over-rich fuel injection could combine with excess air injection to overheat the catalytic converter. Early “cats” experienced frequent failure because the carbureted engines of that era were notorious for poor fuel control. To put exhaust restriction diagnostics in perspective, tire dealers and auto repair shops have been dealing with loss-of-power complaints caused by exhaust restriction since catalytic converters were popularly introduced in 1974. ![]()
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