If you’re filling your car up regularly and still getting crummy mileage, there are a few things that could be to blame. In this post, we’ll explore some of the most common reasons your Volkswagen Jetta is getting bad gas mileage.
Before diving in, it’s important to note whether or not your Jetta’s check engine light is on. If it is on, there are likely diagnostic trouble codes stored in your vehicle’s PCM that will shed light on why it’s not running smoothly.
The most common reason a vehicle gets bad MPG is it needs a tune up.
A good place to start is figuring out what your Jetta’s fuel mileage is supposed to be. You can use this chart from fueleconomy.gov to get a baseline.
Table of Contents
| Bad MPG Causes | DTC Codes? | Symptoms |
|---|---|---|
| Misfiring Engine | P0300, P030X (where X = the misfiring cylinder) | Sputtering engine, rough idle |
| Air Filter | No | Lack of power, sometimes a hissing or whining sound. |
| Low Tire Pressure | TPMS Light | The vehicle will roll more in corners, the ride is bouncier. |
| Brakes Sticking | No | Hot wheel or brake, vehicle pulls left or right, slows down when letting off the gas. |
| Idling | No | None |
Misfiring Engine
Typical Causes: Bad coil packs, spark plugs, or fuel injectors
If your Volkswagen Jetta’s engine is misfiring, it should throw the check engine light. The plugs may not be burning the fuel quite as effectively as when they were new, which can decrease your car’s gas mileage.
You may notice the RPM fluctuating on the tachometer when your cartortruck is not in gear. Your Jetta’s RPM may go up and down when in Park.
Your Jetta’s engine in your car relies on a precise combination of fuel, air, and spark to properly ignite.
A misfiring engine should throw a code, but even if it doesn’t, you should still look at your Jetta’s spark plugs. If they stop burning fuel efficiently, it’ll decrease your car’s fuel mileage, and are the most common reason for a misfire.
Clogged Air Filter
Usually, there is no code associated with a bad air filter. You may hear a hissing sound as air tries to bypass it.
The air filter’s primary purpose is to filter the incoming air before it enters the intake manifold. If you drive your Volkswagen Jetta on a dirt road regularly or have been off-roading, the air filter can clog up well before the manufacturer’s recommended intervals.
Rodents are known to find their way into a car’s air inlet. A day’s worth of nesting work is enough for them to completely block an air filter. It happens more than you’d think.
If left unchecked, a clogged air filter can restrict airflow and lower your Jetta’s fuel economy, as well as cause potentially expensive repair bills due to particulate build-up in sensitive engine components.
Looking at the air filter only takes a moment, and replacing it is inexpensive.
Low Tire Pressure
Under-inflated tires can lower gas mileage by about 0.2% for every 1 psi drop in the average pressure of all tires…
per fueleconomy.gov
Low tire pressure will reduce the fuel efficiency of your Volkswagen Jetta as more energy is needed for acceleration.
As you can see above there is a .2% reduction in fuel economy for every PSI the tires are down. Even if you have a vehicle with a Tire Pressure Monitoring System, the TPMS light (pictured above) typically won’t come on until the tires are 25% below the recommended pressure level.
Example
Let’s say your Jetta needs 40 PSI per tire, which means 30 PSI is when the tire light comes on. This leaves 9 lbs of pressure you can miss without the tire pressure light.
9 PSI X .2% = a 1.8% reduction in fuel economy.
Where to Find How Much Air to Put in Your Volkswagen Jetta’s Tires
There is a sticker inside your car’s driver-side door that shows exactly how much air pressure goes into each tire, as well as what size tires go on each wheel.
Low Tire Pressure is Dangerous
Low tire pressure can cause a number of issues if it is not corrected. A tire with low pressure will have increased wear and tear on the sides, resulting in the treads wearing out quicker and shortening the life of your Jetta’s tires.
In extreme cases, it could even lead to a tire blowout due to uneven pressure across the face of the tire from contact with the road.
For safety reasons, it is advised that you regularly check your tire Jetta’s tire pressure with a gauge from time to time. Don’t rely on the tire pressure light.
Brakes Sticking
Sticking brakes usually smell. They also produce a lot of heat.
Brake sticking can be a very dangerous driving experience, your Jetta may pull to the left or right. The engine has to work harder to move your car or truck. It can even lead to a fire if they get hot enough.
Sticking brakes usually occur due to a build-up of dust or rust inside the brakes, a failure in the rubber brake lines, a stuck caliper or wheel cylinder, or a crimped hard brake line.
Sticking brakes do not fix themselves. It is dangerous to drive if your Jetta has sticking brakes.
How to Tell if Your Brakes are Sticking
It’s relatively easy to tell if your car’s brakes are sticking. Obviously, if the vehicle pulls in one direction, that indicates the brakes are sticking. You could also just really need an alignment. There’s more you can do to verify they’re stuck.
- Is there a strong smell coming from a wheel?
- Wave your hand in front of all four wheels without touching them. If you can feel heat radiating from one wheel, that indicates a brake is stuck.
- Use an infrared temperature gauge to verify what you felt.
- Look at the brake pads. A pad that’s wearing unevenly usually means there’s a stuck caliper.
If your Volkswagen Jetta’s brakes ever begin to feel sticky or unresponsive, getting your vehicle into an auto shop as soon as possible is essential for maintaining safety and reliability on the road.
Idling
Idling can cause diminished MPG if you’ve been in a lot of traffic recently.
Idling is letting an engine run without going anywhere for a long period of time, even when it’s not in use. It doesn’t burn gas fast, but it does burn gas. So if you’ve been waiting to pick the kids up from school with your Volkswagen Jetta running, idling is likely to blame for your decrease in MPG.
Other Causes
Here’s a quick list of other potential causes of a fuel mileage drop. They aren’t as common as the ones listed above or are super obvious.
- Excess Weight– Leaving a vehicle full can weigh it down and decrease the average fuel economy.
- Transmission issues– If your transmission isn’t shifting into overdrive, is slipping, or the torque converter isn’t locking, fuel mileage will suffer.
- Vacuum Leak– A vacuum leak can lean out the air-fuel mixture by allowing unmetered air into the engine. You’ll likely get the check engine light and P0174/P0171 too.
- Oxygen Sensor– O2 sensors check the exhaust gasses and help the powertrain control module (PCM) control the air fuel mixture. There’s a myriad of codes that come with a faulty O2 sensor, and they rarely fail without one turning the check engine light on.
- MAF Sensor- The mass airflow (MAF) sensor is responsible for telling the PCM how much air is entering the intake manifold. Like the O2 sensors, it would almost certainly throw a code.
- Clogged Catalytic Converter– If your Volkswagen Jetta’s catalytic converter gets clogged, it’ll cause the engine to choke on its own exhaust. It’ll also increase the backpressure to the point that combustion is suffering.