* Manual Transmissions: In a manual transmission car, when you depress the clutch, the engine is disengaged from the wheels. However, when the clutch *isn't* depressed, the engine is directly connected to the wheels. Applying the brakes slows the wheels. Because the engine is connected, this slowing forces the engine to rev up. This is a normal effect of engine braking. The amount of revving depends on factors like gear selection, engine speed, and how hard you brake.
* Automatic Transmissions: In automatic transmissions, the engine is also connected to the wheels through the torque converter and transmission. However, the computer controls this connection. While less common than in manual cars, a similar effect can occur with engine braking in automatic vehicles, especially in older ones or those with less sophisticated transmission control systems. A more modern automatic might downshift to provide engine braking, again causing the revs to increase.
* Other Possibilities (Less Common):
* Vacuum Leak: A vacuum leak in the brake system can sometimes cause the engine to increase its idle speed as it tries to compensate for the loss of vacuum. This is less likely to directly correlate with brake application force.
* Throttle Cable Issue: A sticking or malfunctioning throttle cable could cause the throttle to slightly open when the brakes are applied.
* Transmission Problems: More serious transmission issues could lead to the engine revving unexpectedly.
If you're experiencing this:
* Consider the type of transmission: If it's a manual, it's likely normal engine braking. If it's an automatic and excessive or unusual, it could be a problem.
* Note the severity: Is it a slight increase in RPM, or a dramatic surge? A minor increase is often normal; a dramatic surge indicates a problem.
* Have it checked by a mechanic: If you're concerned or it's excessive, have a mechanic diagnose the problem. Ignoring a problem could lead to more serious and costly repairs later.
In short, a slight increase in engine revs when braking is often normal, especially with manual transmissions. However, any excessive or unusual increase warrants professional inspection.
