Car Won’t Downshift When Accelerating (Slow Acceleration)

Everyone knows the basic rules; you shift the gear up as the speed goes up and shift it down when you require more power from the engine and less speed. An automatic transmission takes care of shifting the gears automatically but the transmission dynamics remain the same.

What happens when the car’s accelerating but the transmission won’t downshift?

On this page, you’ll find some possible reasons that could cause this issue.

Why Does My Car Not Downshift During Acceleration?

A vehicle that experiences slow acceleration is not only infuriating but extremely dangerous, especially when the vehicle is crossing an intersection.

This problem usually arises in automatic transmissions.

In an automatic transmission, the driver can’t control when to change the gears, so they have to rely on the transmission system. Sometimes the downshift is either erratic or very sluggish, which makes driving extremely uncomfortable and dangerous.

Since the downshift solenoid is responsible for shifting the higher gear to lower when the vehicle is slowing down to a stop, there’s a probability that the issue lies with the solenoid functioning.

A solenoid problem can also make the vehicle undrivable to an extent. Here’s how you can tell if the solenoid is failing.

  • If the vehicle behaves erratically when downshifting.
  • The transmission stays engaged in a higher gear for a longer period than usual when it should downshift. Hence, delayed gear shifting occurs.
  • The check engine light is turned on.

A clogged filter or a loose plug can cause a fluid leak in the vehicle. The most basic sign is a puddle beneath the engine.

To ensure it’s a transmission fluid leakage, check if the color is bright red and has a slightly sweet or pleasant smell to it.

Transmission fluid doesn’t leak very often but it could be the reason behind the vehicle’s downshifting problem.

Here’s how the leakage happens:

  • In automatic transmissions, a leaking fuel pan gasket is what leads to transmission fluid leakage. The gasket must be replaced with a new one every time the transmission pan is removed. Using the old one over and over again can cause a transmission fluid leak.
  • A cracked or rusty transmission pan is another reason. The transmission pan can crack if it’s made out of aluminum. It mostly happens if the pan makes contact with a hard object during driving.
  • Some automatic transmissions also have a drain plug for the transmission fluid which can suffer a leakage. If you replace the O-ring sealing on the drain plug every time you replace the fluid, it won’t leak.
  • A bent transmission pan can also be the reason behind the leakage. The bend makes it harder to remove when replacing the transmission fluid of the vehicle.
  • Not all automatic transmissions have fluid cooling, but those who do have fluid lines go to the transmission cooler. These pipelines can get rust very quickly due to their steel element. The rust creates holes in the pipes and causes them to leak.
  • Many vehicles contain open transmission ventilation on the top of the transmission itself to protect it from building up too much pressure. When these ventilations get clogged, there are leaks everywhere.

If any of these transmission parts are causing the issue, get them replaced. If the fluid is low, refill it to the factory-recommended level. It should make your car downshift smoothly again.

What Car Parts Should Be Checked?

Here are a few additional vehicle parts you can check if it experiences slow acceleration.

1 An Oxygen Sensor Problem

The oxygen sensors of a vehicle track the air-to-fuel ratio, determining how much fuel is needed by the internal combustion chamber. It keeps track of the vehicle’s exhaust emissions.

A malfunction in the oxygen sensor interferes with the amount of fuel that is burned during the combustion process.

If the wrong amount of fuel gets burned, it creates a highly rich fuel mixture, resulting in slow acceleration as you step on the gas pedal.

An oxygen sensor malfunction can also reduce the gas mileage on the car, resulting in slow or sluggish acceleration.

2 There’s A Problem With The Air Flow Meter

The airflow meter is attached to the air intake cleaner in the vehicle. As air flows through the cleaner, the mass of the air is calculated by the airflow meter.

The meter sends this information to the engine control unit. With this information, the ECU calculates the proper air-to-fuel mixture.

If the airflow meter isn’t working properly, it can disturb the air-to-fuel mixture calculation by the ECU, resulting in slow acceleration of the vehicle.

3 The Timing Belt Is Worn Out

The timing belt synchronizes the rotation of the crankshaft and the camshaft. If the belt has a single tooth worn out or loose, it could result in slow acceleration and multiple other engine problems.

The timing belt is an expensive replacement, especially when you leave it long enough to break entirely or get worn out. It could snap altogether, which would be the worst-case scenario for a car with an interference engine.

An interference engine is when the valves of the engine make contact with the pistons or other valves when properly timed with the timing belt.

Hence, a timing belt by all means can result in very slow acceleration.

A worn timing belt can also cause the car to not accelerate well at high speed.

4 A Dirty Fuel Filter

The fuel filter lets fuel pass to enter the vehicle’s engine. If the filter is clogged with dirt or debris, the fuel insertion is stopped.

A clogged fuel filter makes it harder to accelerate when the gas pedal is pressed. It could also result in rough idling.

A simple fuel filter replacement can fix the slow acceleration issue.

5 Foul Throttle Position Sensor

The opening angle of the throttle valve is analyzed through the position sensor. The position sensor sends this information to the ECU.

If there’s an issue with the position sensor, the gas pedal can no longer control the acceleration as it manages the throttle valve.

Hence, the car may go into slow acceleration.

How Much Does It Cost To Fix It?

A solenoid replacement might be able to fix the downshifting problem in automatic transmissions. It will also prevent late or erratic shifting of the gears.

But, the downshift solenoid replacement is not part of the routine maintenance, hence it can be a little more expensive than usual.

A single solenoid is about $150-400 per replacement. However, the cost increases if there is further subsequent solenoid damage that needs to be replaced.

Not replacing the damaged downshift solenoid to save cost will do more harm than good, because bad solenoids can cause serious transmission issues.

A single solenoid replacement will save you a lot of money on further repairs that come after the damaged solenoid remains intact.

The parts of each solenoid cost from around $15-$100, which doesn’t sound so bad if the parts have a good warranty for them.

Most of the parts found online have a limited warranty. To get a good replacement, take your car to a mechanic for advice or suggestions on where to get the best solenoid parts from.

On the other hand, if you want to change the transmission parts of your car, it can cost around $1500 to $4000. Depending on the make and model of the vehicle, the transmission prices may vary.

It costs around $80 to $200 to change the transmission fluid because it contains a new filter and 15 quarts of fluid.

Will Changing The Transmission Fluid Help The Shifting?

Usually, when the transmission fluid appears dark or smells like it’s burnt, changing the fluid and the filter help fix the downshifting problem.

Having the vehicle checked for any diagnostic codes that could lead to the fault also fixes the problem to an extent. Many auto parts suppliers offer free check engine light diagnosis which could be to your advantage considering the expensive replacements.

Many times you can check transmission issues and replace the fluid yourself, but in case of a severe shifting problem, take the vehicle to a technician for inspection.

On the outside of the transmission, there’s a shift control module. It lets the computer know what gear the vehicle is in. You can check there to address the issue.

Fixing a transmission fluid leak can sometimes fix the shifting issues in the vehicle. Replacing the fluid with a fresh one can help clutch discs and steel discs bond better. It helps them hold without slipping. There are seal conditioners in the new fluid that help soften the clutch piston lip seals.

It helps them seal better.

But in most cases, changing the transmission fluid does not help the shifting problem because it’s too late for that. An internal seal leakage can cause the shifting problem, but mainly changing the fluid won’t help.

But there are some things that simply changing the transmission fluid might not fix, including:

  • Cracks in clutch pack seals.
  • Worn clutch discs
  • Worn bands
  • Servo seals that wear out.
  • Valve body passages and valves.
  • Excessive clearances
  • Torque converter wear
  • Transmission pump issues
  • Worn bushings, bearings, or gears.
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