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Why do engines vibrate/shake when lugging? Is shaking a foolproof indicator to know whether an engine is lugging?

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The engine shakes when lugging because it is approaching a stall. The pistons firing downward don't have enough torque to drive other pistons upward. A sudden slowdown of a piston as it meets too much resistance traveling upward in turn decelerates another piston connected by the crankshaft. If the process keeps going, all pistons would come to a halt.

The shake, therefore, is definitely a telltale sign of lugging. Engines designed properly with enough low-end torque -- take as examples modern European luxury or classic American muscle -- can run at low rpm and relatively high gear with no shake. I can tell you from experience that they sound like they are lugging at very low rpm (1,200), but the absence of shake means that they are not, even if you ease down the gas pedal.

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Here is an answer I did a while ago pertaining to lugging and why it is bad.

Shaking in and of itself IS NOT a telltale sign of lugging, meaning, just because the engine is shaking does not mean you are lugging the engine. Shaking can occur for many different reasons, to include (but not limited to) bad motor mounts and misfire. Lugging will occur at lower RPMs and only on standard shift transmissions. The design of a torque converter in an automatic will not allow an engine to lug because when the RPM gets low enough, the torque converter basically allows the engine to run due to it having not met the torque converter stall speed.

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