Q: RPM does not change.

asked by on

When i shift from 1st gear to 2nd gear and then immediately step on the brake to stop the car completely, i notice the rpm remains. It does not goes down as it should. I would need to off the engine and restart the car again in order for the rpm to goes back to normal. Why is that so? Thanks.
My car has a manual transmission.

Hi There, It sounds like you may have a dirty or faulty idle control valve or potentially a bad MAP sensor that may not be correctly measuring intake manifold pressure. The idle air control valve monitors the air intake as it is mixed with fuel prior to being injected into the engine at low speeds and at idle. This valve is controlled by the vehicle’s computer and will adjust idle speed based upon other measurements such as engine temperature, intake air temperature and electrical system load or voltage. This is also an important function when starting the motor as it allows the motor to run and idle on it’s own once the motor fires. When you accelerate, the engine RPM increases, and as you let off the gas, the RPM slowly returns to the normal idling speed with the help of the idle air control valve making the transition from a higher RPM back down to idle speed while adjusting the air/fuel ratio constantly to allow this to happen smoothly.

The Manifold Absolute Pressure (MAP) sensor senses engine load and generates a signal that is proportional to the amount of vacuum in the intake manifold. The engine computer then uses this information to adjust ignition timing and fuel enrichment. When the engine is working hard, intake vacuum drops as the throttle opens wide. The engine takes in more air, which requires more fuel to keep the air/fuel ratio in balance. When the computer reads a heavy load signal from the [MAP sensor]((https://www.yourmechanic.com/services/manifold-absolute-pressure-sensor-map-sensor-replacement), it adjusts the fuel mixture to slightly more rich than normal so the engine can produce more power. The computer will then retard (back off) ignition timing slightly to prevent detonation that can damage the engine and decrease performance. This may also be caused by other closely related components such as a dirty or failing mass air-flow sensor, a faulty throttle position sensor or potentially a vacuum leak. I would recommend having an expert from YourMechanic come to your location to diagnose and inspect your vehicle.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1,000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
  1. Home
  2. Questions
  3. RPM does not change.

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: Car is not accelerating

Hello. The leaking air you describe is known as a vacuum leak. A vacuum leak may cause issues such as poor acceleration or limited speed. Vacuum leaks seem to be great at dodging the eye sometimes so I suggest using a...

Q: code is p0340 i change camshaft position sensor but still shows same code .any help or advice

Hi there: The problem with changing out the camshaft position sensor is that sometimes the issue is actually due to the sensor wiring harness; which might be the reason this OBD-II error code was triggered initially. The P-0340 trouble code...

Q: Suzuki Alto SB-308, model 1993. Thermostat Valve behaviour

It sounds like the thermostat you have may be the wrong temperature rating. Try to use the 91 degree thermostat. That should keep your temperature closer to the midpoint on the gauge. The air rising across the radiator while driving...

Related articles

How to Avoid Back Pain in a Car
If you have back problems, sitting in a car for an extended period of time can be excruciating. Even without back problems, you could experience discomfort and soreness from...
How Much Does a Mechanic Make in Vermont?
Automotive technician jobs in Vermont have an average mechanic salary of $37k, with some mechanics earning a salary of $53k.
P2103 OBD-II Trouble Code: Throttle Actuator Control Motor Circuit High
P2103 means there is a fault with the throttle actuator control motor circuit, likely due to a defective electrical component or part.