Why Replaced MAF Sensor Still Rough Idle [Reasons + Solutions]

The major reasons behind replaced MAF sensor still rough idle are low fuel pressure, dirty throttle body, vacuum leak, cracked air duct, incomplete MAF learning procedure, faulty BARO sensor, and bad PCV valve.

More often than not, this problem is caused by vacuum leaks or cracked air ducts. Depending on the source of the leak, you can either use a sealant or replace the leaking component altogether.

It doesn’t mean that you should take the other reasons that could cause this problem lightly. Keep reading this article to find out all about these potential issues in detail and learn how you can fix them. Let’s dive in!

Why Replaced MAF Sensor Still Rough Idle [Reasons + Solutions]

If your car runs worse with a new MAF sensor and still idles rough after replacing the old one, it’s understandable to be frustrated. Let’s look at the main reasons behind that and find out how you can fix them.

1. Low Fuel Pressure

If your car is getting sufficient fuel pressure, there will be rough idle even after you replace the MAF sensor. The usual symptoms of having a lack of fuel pressure are unresponsive throttle, trouble starting the car, stalling the engine, misfiring, and low performance.

Unfortunately, there are many potential reasons that could lead to insufficient low pressure. It could be due to using contaminated fuel or not having enough of it. Clogged fuel lines and fuel filters, bad fuel pumps, and malfunctioning fuel pressure regulators can all cause this issue.

Solution:

You need to diagnose the main reason why you have low fuel pressure. Once you identify the faulty parts, you can replace them with new ones and solve this problem.

2. Dirty Throttle Body


The build-up of carbon deposits in the throttle body can lead to several problems including rough idling. It controls how much air goes into your intake manifold and then, the combustion chamber. The throttle body opens up at many different angles to let the correct amount of air in.

When the dirty throttle body gets dirty, the required amount of air can’t get into the combustion chamber. That causes the check engine light to come on, rough idling, engine stalling, rough acceleration, and bad fuel economy.

Solution:

The first thing you should try if you have a dirty throttle body is to clean it. There are many throttle body cleaners available in the market like Berryman’s B-12 Chemtool, CRC Throttle Body and Air-Intake Cleaner, and so on. You can choose the one that works well for your car.

cleaning the throttle body

We’ll share some general instructions you can use to clean it. But it can vary depending on your specific vehicle. While you’re better off following your owner’s manual, you can use these steps as a general guideline:

  • Locate the throttle body by having a person press the accelerator when the engine is turned off.
  • Disconnect the negative terminal of the battery before you start cleaning it.
  • Label the hoses and air ducts attached to the throttle body before removing them so you can put them back together later on.
  • Use a screwdriver if you need it to remove the air duct connected to the throttle body.
  • Don’t continue with the project if you’re not being able to remove the air duct.
  • Put on some rubber gloves and protective eyewear before applying the spray when the throttle body is exposed.
  • Spray the throttle body cleaner to the air duct and remove the loose dirt and other deposits using a small brush.
  • Keep repeating the steps until the throttle body is completely clean.
  • Add a little bit of oil to the throttle shaft using a cotton swab to smoothen the rotation of the throttle blades.
  • Use paper towels to clean off any remaining grime residue around the throttle body area.
  • Reconnect the components you removed and put everything back together.

The problem should now be solved if the throttle body being dirty was causing the issue. Don’t smoke while you clean it as the cleaners can irritate the skin. But if cleaning it doesn’t do the trick and you’re sure that you have a faulty throttle body, you’ll need to replace it. It can cost you $400 – $600 to do so.

3. Vacuum Leak

Normally, all the air that an engine takes in passes through the throttle body. The MAF sensor measures this quantity of air and signals the ECU about the quantity of air getting through the engine.

But when there’s a vacuum leak, the air goes to the intake manifold without necessarily passing through the throttle body. The MAF sensor or the oxygen sensors don’t account for this extra air. Meanwhile, the ECU only injects enough fuel based on the readings it gets from the sensors.

testing a vacuum leak


So, the fuel isn’t enough to match the quantity of air that’s in the engine. A cracked intake hose or a leak in any of the rubber tubes leading up to the intake manifold are the usual culprits behind a vacuum leak. If you’re getting lean codes, chances are you have a leak somewhere after the MAF sensor.

There are times when you’d be able to hear it. You can easily diagnose if you have a vacuum leak if you can listen to a light whistling or hissing noise. For confirming whether you have a vacuum leak or not, you can take it to a shop and perform a smoke test.

Solution:

Once you’ve located the source of the leak, it should be easy to fix. If the leak is happening because of a hose, you can easily repair it on your own. Sadly, a leak in the intake manifold can also cause this issue. So, you’ll need to spend big bucks to fix it if it’s the faulty part in your case.

Keep in mind that you’ll have to reset the ECU after fixing the leak. The engine will automatically be running rich to compensate for the extra air that got through because of the leak.

Even after you fix it, the engine will continue to inject more fuel than necessary. It’ll be alright on its own if you drive for a few weeks. But it’s much better to reset the ECU to drive your car properly right away.

4. Cracked Air Duct

There’s an air duct between the MAF sensor and the throttle body. If there are cracks or splits in it, unmetered air will also go through it. In that case, it’s common to see issues like rough idling, lower fuel efficiency, and an increase in carbon emissions.

The worse that can come from a cracked air duct is that the engine can catch on fire. It can happen as the engine will run too hot due to the presence of extra air. You can have a professional mechanic take a look at it if you’re unsure whether it’s cracked.

But if the crack is big enough, you’ll most likely see it yourself and you may also hear a howling sound from it.

Solution:

Having a cracked air duct is generally an easy problem to deal with and fix. Once you have identified the source of the leak, you need to figure out how you can seal it. If the leak is small, you can use a sealant and solve the issue. There are many types of sealants but we recommend epoxy putty for this job.

For bigger leaks, you’d have to replace that section of the hose. You can also do it on your own if you have access to some basic tools. Get a good-quality silicone hose to prevent this issue from happening again. Inspect your hoses at regular intervals so that you don’t get any unwanted surprises.

5. Incomplete MAF Learning Procedure

Your ECU doesn’t automatically know if you’ve changed the MAF sensor in your car. If you’re still having problems after replacing the MAF sensor, then there was no issue with it in the first place or your car hasn’t had the time to adapt to the new MAF sensor.

As you’d know, by now the ECU will send extra fuel to compensate for the extra air getting into the system. Since it didn’t have the time to adjust to the new sensor, it causes the rough idling issue.

Solution:

You need to follow the MAF learning procedure so that your ECU knows that there’s a new MAF sensor. There are many ways you can go about it. The exact process for it depends on your vehicle. So, how long does it take for a new MAF sensor to work?

There are both fast and slow ways to complete the MAF sensor relearn procedure. One of the easiest ways is to disconnect it, start the engine, and let the check engine light turn on. After that, you can attach the MAF sensor, clean the codes, and let the vehicle idle for 20 minutes or so.

You can also disconnect the battery and leave your car in that condition overnight. Doing that can force the computer to reset to the factory settings and relearn the parameters of the new MAF sensor. Otherwise, you can just drive your car for a few weeks until the ECU gets used to it.

6. Faulty BARO Sensor

If you’re still having rough idle after replacing the mass air flow sensor, another potential reason behind it is a faulty BARO sensor. It measures the atmospheric pressure of the environment in which you’re driving the car.

baro sensor

When you’re driving in high altitudes, there’ll be thinner air and the engine will get less oxygen. The barometric sensor passes this information along to the ECU so that it can make adjustments and send the right amount of fuel.

But with time, the sensors can become dirty or completely damaged due to exposure to other elements. The common symptoms of a bad BARO sensor are rough idling, poor engine performance, and stalling.

The check engine light will also come on when you have a bad BARO sensor. If you have access to a scan tool, you can confirm whether you have a bad BARO sensor as it’ll show the DTC P0105 code if you do.

Solution:

If you have a faulty BARO sensor, you’ll have to replace it. Depending on the vehicle, it can be located in the firewall, inside the fender skirt, or incorporated into the MAP sensor and mounted on the intake manifold. Replacing it will cost you anywhere between $150 – $300 including labor charges.

7. Bad PCV Valve

The PCV valve eliminates emissions from the crankcase and sends them to the air intake. Its purpose is to make the emissions of the engine more effective and cleaner. There’ll be a plethora of engine problems that you could face if the PCV valve goes bad and you notice it leaking.

damaged pcv valve

The main symptoms of a bad PCV valve include the check engine light turning on, engine idling rough, lean or rich fuel mixtures, engine misfires, and rough acceleration. You may also see oil leaks or unusual colors of smoke coming from the exhaust pipes.

Solution:

You need to replace the PCV valve if it’s damaged. It’s usually located on the valve cover on top of the engine. But you may also find it on a hose that’s between the air intake filter and a hose. The PCV valve itself costs $20 – $50 and that’s all you’d be spending on it if you replace it yourself.

FAQs:

How long does a throttle body normally last?

If you take care of your throttle body, you can expect it to last for 150,000 or 200,000 miles. You can only expect this result if you clean it after driving for every 75,000 miles or so.

Can you use glue or electrical tape to seal the air intake hose?

Glue might be a good option depending on how strong it is and if it’s able to handle the heat and pressure inside the engine. But electrical tape is an insulator, it won’t allow air to flow and make the engine overheat.

Do I need to disconnect the battery to change the MAF sensor?

Yes, it’s a good idea to disconnect one of the terminals of the battery before you replace the MAF sensor. Otherwise, it can lead to an accident.

Is it okay to drive a car with an unplugged MAF sensor?

You can drive the car with an unplugged MAF sensor but it’s not recommended that you do it for more than a week or so. If you do, it could damage the spark plugs or other engine parts.

Conclusion

Now you know why a replaced MAF sensor still rough idle issue can be caused. If this happens to you, keep your cool and follow this guide. Hopefully, you’ll be able to find out what’s causing the issue in your case and fix it.

Make sure you follow safety precautions when doing sensitive things like cleaning a dirty throttle body. Leave a comment if you have any questions about why the engine idles rough even after replacing the MAF sensor.

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