Car hesitation is the event caused by something wrong with the engine such as misfiring, exerting less torque than it should or infrequent stumbling. In general, those are the symptoms of an engine that doesn't make the best use of its fuel and they often happen when you go down a hill, change the gear or accelerate.

To be more specific, when you step on the pedal, it doesn't respond immediately or it does, but with much less power exerted than usual because the engine misfired. The problem could be the ignition system itself where it fails to combust the fuel-air mixture with supposed frequency, or maybe the air-fuel mixture is either too rich or too lean.

It could be caused by an aged manifold pressure sensor, the mass airflow sensor and/or a bad throttle sensor, a dirty injector. Also, a low fuel pressure or contaminations in the fuel could contribute to this too. What's more, a vacuum, a wet plug, a bad ignition coil or bad wires leak can also be the culprit here.

A wide range of reasons could contribute to a lean fuel-air mixture

How to diagnose car hesitation
First and foremost, the ignition system is located pretty deep into the system and therefore literally actually getting your hands on it is not the best choice here. Normally, we would need a "computer diagnosis", i.e using a scan tool to plug in the car to see if there are any issues underneath. It can be able to do so by a set of sensors placed almost everywhere in the system. And you should know a problem is present if the value recorded is not within the acceptable value range. What's more, the scan could unveil other kinds of problems too such as low fuel pressure, vacuum leakages or spark plug.

With vacuum leaks problems, there are two ways you can approach it. With a car that has an EFI (Electronic Fuel Injecting) system, you can detect the issue by looking at the airflow signal given out by the airflow sensor and/or looking at the throttle sensor and MAP sensor if there's any. You should have no problem detecting this issue with a car that uses EFI system since they're particularly sensitive to it. With cars that don't have a mass airflow sensor ( the speed density type) the system will read the information by the RPM of the transmission, the manifold pressure sensor, and the air temp sensor and from there calculate the amount of airflow, thus adjusting the fuel will be injected to the mixture.

The EFI system is less sensitive to vacuum leak than a normal system that uses Mass Airflow Sensor

But those things should not be fully responsible for the engine's misfiring. Sometimes a dirty fuel injector or a lowered fuel pressure could be the culprit here. In this case, the fuel stream will be trimmed to fit the new output, so that the fuel-air mixture could be sufficiently maintained. But no matter how efficient it is, the "cure" often comes several seconds late, thus the lag in the movement. In most cases, vacuum leakage will make the content of fuel in the mixture much richer than usual because the sensor tries to balance out the mixture, know that there's more air in it, by adding more fuel in.

Now the scan tool can be useful again. Though there's no button in the tool that says "lean" or "rich, it could be used to check those characteristics in a fuel-air mixture. This could be done by measuring the mixture when the car is running at idle mode since there's little air getting inside the chamber from the first place so that even the small amount of air sucked in from the leak could be noticeable.

Having one of these things in your car all the time would be a good idea

The process could be done by seeing if the values measured are within the acceptable range. Simply plug the tool in the plug and read these two values: STFT and LTFT and notice the differences between them. The difference should be a negative result, somewhere around -8. The problem is there when the value is greater than 10, indicating the mixture is too lean (too much air). Now there're two ways to bring the test further. When you rev the engine to around 2000 RPM and the values come back to normal, there's definitely a vacuum leak. If it doesn't, the culprit might be the bad fuel pump, a damaged filter, a malfunctioned fuel injector or a failed fuel pressure regulator.

Also, if your scan tool only displays the Short-term fuel trim -STFT you can still know if the mixture is too lean by seeing whether the number is bigger than 12. Next, to know what caused it, rev it to around 2000rpm, if the value drops by about 4 points, it is definitely a vacuum leak. If it remains unchanged, some of the sensors are broken or the pressure of the fuel is too low.

Causes of a lean mixture
It can be caused by a variety of reasons such as a leak in the EGR valve or PCV Valve, and/or hose, maybe you don't clean your injectors as frequent as you're supposed to. A vacuum leaks at the intake manifold, the hose connections or throttle could be the reason too. It is notable though, that when the problems are with the Mass airflow sensor or any kind of sensor, you should not attempt to clean it with regular cleaning liquid but electronics cleaner or else it will break down and you'll have to purchase a new one. Also, keep your hands off the wire if you don't want to get electrocuted, badly.

Also check out this tutorial by Scotty Kilmer on How to Stop Car Hesitation:

https://www.youtube.com/watch?v=yqZo6K468to

Note:
On some Ford models, the codes P0171 and P0174 appeared on the scan tool indicates that there's a lean problem with the fuel mixture and the cause might be the differential pressure sensor ( DPFE) since it commands the EGR flow which has an effect on the fuel mixture. In this case, you should have it replaced.

In case you find yourself in this situation, the best course of action to do is to replace any parts necessary instead if fixing it, because fixing isn't that financially beneficial and also it cost much more time to just replace it.
https://naijauto.com/car-maintenance/causes-and-solutions-of-a-stumbled-engine-1090

plagiarism at its peak