It can be frustrating and concerning if you’re experiencing a problem where your car won’t go over 30 mph. Your vehicle may be experiencing this issue for various reasons, and it’s essential to address it promptly to ensure your safety and prevent further damage to your car.
A malfunctioning transmission is one possible and the most common reason for your car not going over 30 mph. The transmission is responsible for shifting gears and transferring power from the engine to the wheels. If it’s not working correctly, it can cause your car to struggle to reach higher speeds. I’ll explain this and other common reasons why your car may not be going over 30 mph and how to diagnose the cause.
8 Common Reasons a Car Won’t Go Above 30MPH
As a mechanic, I have come across several reasons why a car may not go above 30mph. Here are the eight most common causes that I have found:
Transmission issues
Transmission issues are a common cause of a car being unable to go above 30mph. When the transmission is damaged or worn out, the vehicle may lose power and not accelerate beyond a certain speed. Low transmission fluid levels, a worn-out clutch, broken gear linkage, or damaged gears can cause this.
For instance, in a manual transmission car, if you can’t shift out of 1st gear, you’ll have difficulty going faster than the gear allows, which is around 30mph in most vehicles. If the gear shifter is loose, the linkage is broken, which can explain the issue. Another way to confirm a faulty transmission is to listen for crunching and grating noises when changing gears, along with a strong burning smell.
Clogged fuel filter
A fuel filter’s function is to remove impurities and contaminants from the fuel before it enters the engine. If the filter is not changed per the manufacturer’s recommendations, it can become clogged with dirt, debris, and other contaminants, which can restrict fuel flow to the engine.
When the fuel filter is clogged, the engine doesn’t get enough fuel, which can cause it to lose power and not accelerate beyond a certain speed in this car, 30mph. This can be potentially dangerous, especially if you need to travel at higher speeds on the highway.
To avoid this issue, replacing the fuel filter every 30,000 miles or per the manufacturer’s recommendation is recommended. Other signs of a clogged fuel filter include reduced power or acceleration, rough idling, or stalling.
Limp mode
Modern cars are equipped with a safety feature called limp mode. This feature is designed to protect the engine and transmission from damage in case of a malfunction. If the car detects a problem, it will limit the speed to 30mph to prevent further damage and restrict power to the engine by inducing a controlled misfire.
Limp mode is electrical, as the ECU controls it, so it is only activated if a reading is sent to the ECU from a sensor outside the normal parameters. The sensors that can cause limp mode to activate include:
- Mass Airflow Sensor (MAF)
- Throttle Position Sensor (TPS)
- Engine Coolant Temperature Sensor (ECT)
- Manifold Absolute Pressure Sensor (MAP)
- Oxygen Sensor (O2)
- Vehicle Speed Sensor (VSS)
- Accelerator Pedal Position Sensor (APP)
- Fuel Pressure Sensor (FPS)
- Exhaust Gas Recirculation (EGR) Valve
- Intake Air Temperature Sensor (IAT)
As the check engine light will illuminate when a car is in limp mode, you often need to plug that into a diagnostic machine to confirm the cause.
Faulty speed sensor
Although the speed sensor is part of the transmission, I wanted to mention this separately as it has some unique signs it is failing. In a car with automatic transmission, the speed sensor sends signals to the car’s ECU to change gear. It also displays the speed at which you are traveling on the speedometer. If the sensor is not working correctly, the car may be in a higher gear, such as fifth gear, even straight after starting the car, so the vehicle will struggle to accelerate. As it tells the car to change gears, it can also cut power when getting to certain speeds, i.e., 30mph, where the transmission thinks it is changing gears.
There are telltale signs your car has a faulty speed sensor, such as the check engine light on and the incorrect speed being displayed on the speedometer; it can also act sporadically, shooting up to 100mph and then back down 0
Exhaust blockage
A clogged catalytic converter or muffler restricts the flow of exhaust gases, which can cause the engine to lose power and not accelerate beyond a certain speed. This happens because the exhaust gasses have no choice but to force themselves back into the engine, preventing the engine from taking in fresh air during combustion.
Depending on the severity of the blockage will determine at what RPMs the car will begin to struggle, thus limiting speed and power.
When the exhaust is blocked, you will notice the check engine light illuminated, a strong gas smell, decreased power, unusual noises, and poor MPGs. Also, looking under the vehicle at the exhaust, specifically the catalytic converter, you may notice a purple tint, indicating it is blocked.
Bad ignition coil
The ignition coil takes a car’s low battery voltage and converts it into the high power needed to power the spark plugs to ignite the fuel in the engine cylinders. When a car’s ignition coil is faulty, the internals break down, stopping the spark plugs from receiving the high voltage needed.
An engine misfire is one of the most noticeable symptoms of a bad ignition coil, which can prevent the vehicle from reaching speeds above 30mph if particularly bad. The check engine light will also be illuminated; however, when you plug the car into a diagnostic machine, it will just tell you the cylinder misfired. The bad news is continuing to drive with a bad ignition coil can cause damage to other engine components, so it’s essential to have it replaced as soon as possible to avoid further issues.
Faulty throttle position sensor
The throttle position sensor (TPS) sends signals to the car’s computer to give it information on the throttle position so it can change the amount of fuel being let into the engine. If the TPS is not providing the correct information, it can think the throttle is closed when wide open; in this case, the engine will not receive enough fuel, causing it to run lean and fail to accelerate past a certain speed.
With a failed TPS, there are quite a few symptoms to look out for, such as a rough idle, engine hesitation when accelerating, the engine stalling, or, in some cases, refusing to start. A failing TPS can also cause the car to go into limp mode, preventing it from exceeding 30 mph.
Fuel delivery issues
The engine requires the right amount of fuel for the speed and revs to keep the car accelerating or even holding a specific speed. If the fuel cannot get into the engine for combustion in the right quantities, the vehicle will not accelerate past speeds. This could be anything from a few mph to 30mph and above. There are many reasons fuel may not reach the engine. The most common include:
- Clogged fuel injectors
- Faulty fuel pump
- Blocked fuel lines
- Broken fuel pressure regulator
It’s often not easy to tell if you have fuel delivery issues as you don’t see the fuel when it passes into the engine. However, most of these faults will throw the engine light on and store a fault code.
Diagnostic Steps
With most of the faults described in this article, the check engine light will come on when they are present, which means a fault code is stored or the car has entered into limp mode. If the check engine light comes on, diagnosing the exact problem is relatively easy; simply plug the car into a diagnostic tool to read the fault code.
Sometimes, you may get a slightly generic fault code, but with this article and the other symptoms each fault displays, you should be able to bring to the bottom of the cause.
Is Driving Safe When My Car Won’t Go Above 30MPH?
If your car is unable to go faster than 30mph due to a mechanical issue such as a faulty transmission, it is not recommended to continue driving. Driving at such a slow speed can be dangerous, especially if you are on a busy highway or trying to merge onto a freeway. This puts you at risk from a safety standpoint.
Additionally, if your car is in limp mode and struggling to maintain a speed of 30mph, driving the vehicle can cause more harm to the engine. Therefore, you should avoid driving it from the perspective of more car damage occurring.
Frequently Asked Questions
What are the common reasons a car fails to accelerate properly?
There are several reasons why a car may fail to accelerate properly. Some common reasons include clogged fuel injectors, faulty coil packs, and a broken transmission.
What are the potential issues when a car doesn’t accelerate past a certain speed?
Several potential issues can cause a car to not accelerate past a certain speed. Some of the common reasons include a clogged fuel filter, a malfunctioning fuel pump, a faulty throttle position sensor, a dirty or clogged fuel injector, a damaged catalytic converter, or a defective transmission.
How does transmission affect a car’s ability to accelerate?
A car’s transmission helps transfer power from the engine to the wheels, and the gear ratio of the transmission can affect how quickly the car can accelerate. For example, a lower gear ratio can provide more power to the wheels, allowing the vehicle to accelerate more quickly, while a higher gear ratio can provide better fuel efficiency but may reduce the car’s acceleration.
Final Words
A car not going over 30mph can be caused by various issues; with this article, hopefully, you can identify the cause of your problems. It’s essential to address these issues promptly to ensure your safety and prevent further damage to your car. You can avoid a lot of the problems in this article by making sure you stick to regular car maintenance, such as changing the fuel filter and monitoring the check engine light. If you experience an issue like the car failing to pass 30mph, you may need to have the vehicle put onto a diagnostic machine to check the fault, which often means consulting a mechanic.
