WHAT IS A THROTTLE BODY?
Before we start mentioning the symptoms of a bad throttle body, a brief explanation about this car part would perhaps be useful.
The throttle body is a part of the air intake system. It is essentially a big valve that regulates the airflow into the intake manifold and engine.
When you press the accelerator pedal, the metal plate inside (aka the butterfly valve) moves and lets more air inside. More air means more acceleration.
Modern throttle bodies are almost all electronic. This means they have a small electric motor on them that moves the butterfly valve when you press the accelerator pedal. The connection between the electric motor on the throttle body and the gas pedal is with wires. Sort of a fly-by-wire system.
The other option is if the throttle body is mechanical, so to say. In this case, the gas pedal is connected to the throttle body with a metal cable. This is mostly found on older type cars but is perhaps somewhat more durable and simple to maintain.
The common location of the throttle body is after the air filter housing and tubes, mounted on the air intake manifold.
If you’re interested in a more in-depth explanation provided by Wikipedia, click here.
10 SYMPTOMS OF A BAD THROTTLE BODY
1. ROUGH OR HIGH IDLE
This is perhaps the symptom you’ll notice first. Instead of smooth idling when you start the engine, it will idle poorly and in a rough manner. When the situation gets worse, the engine may want to cut off completely.
Besides this, the engine may occasionally run on higher revs.
At first, these symptoms will be barely noticeable while in the later stages they will be present all the time.
2. ROUGH ENGINE RUNNING
One more very obvious symptom of a bad throttle body.
The engine doesn’t run as smoothly as it should regardless of the rev count.
The difference is that in lower revs it might be barely noticeable while at higher revs there might be more drastic manifestations.
3. ENGINE STALLING
When you press the gas pedal, instead of getting the usual instantaneous response, you’ll get a sluggish one.
Another easy way of recognizing this symptom is when starting from a standstill. You’ll feel a brief but uncomfortable hesitation when pressing the gas pedal.
At moments, it will seem as the gas pedal isn’t connected to the throttle body at all.
As the problem progresses, the engine stalling becomes worse. When you notice this, take action as soon as possible. No need to say how dangerous it is to drive around like this.
4. ENGINE MISFIRES
This is a more serious symptom of a bad throttle body and usually appears in the later stage of the problem.
It happens because there is a disturbance in the air-fuel mixture ratio since the throttle body isn’t working properly.
You’ll notice it as some serious sputtering or it may seem that the engine isn’t working on all cylinders.
One more symptom, just like engine stalling, where you should react as soon as possible so the engine doesn’t get damaged.
For more information on engine misfire, click here for a separate article on that topic.
5. POOR ACCELERATION
All of the mentioned symptoms of a bad throttle body will surely be accompanied by poor acceleration.
Lots of times this may be the first symptom, besides rough idling.
In the beginning, it will be barely noticeable. There might be a short hesitation when you press the gas pedal.
But as the problem progresses, it will take a lot more time to gain speed and you’ll start noticing a serious power shortage.
6. POOR FUEL ECONOMY
Poor fuel economy, in this case, is caused by a bad air-fuel mixture which leads to the engine not working properly.
This symptom is best noticed by the fast decline of the fuel gauge needle.
Don’t be surprised if the fuel consumption goes up thirty to even fifty percent in more drastic cases.
If you want to learn more on how to calculate the fuel consumption for your car and notice the problem on time, click here.
7. CHECK ENGINE LIGHT COMES UP
By the time all of these symptoms of a bad throttle body appear, so will the check engine light.
Sometimes it may appear at the very beginning of the problem. In this case, it may come up and then go away after a few engine starts only to come back again in the near future.
When the problem becomes more serious, the check engine light will be on all the time. When it is, it would be good to hook up an OBD2 scanner tool and see if there are any error codes.
If you would like to learn how to use an OBD2 scanner tool, click here for a video tutorial.
Besides this, if you’re thinking of buying one, you can see a video review of a good one by clicking here.
For more information on the possible reasons for a check engine light coming up, click here for a separate article on this topic.
8. EPC LIGHT COMING UP
I must mention this symptom although this warning light is typical for Volkswagen group cars but it may perhaps help with other car types.
The EPC light (Electronic Power Control) might come up in a combination with the check engine light.
Sometimes it comes up separately.
It will start lighting up more and more as the problem gets worse. Of course, it can be a sign of other trouble, but the throttle body is the first place to look.
More on this topic in a separate article that you can read by clicking here.
9. SUDDEN HARD JOLTS WHILE DRIVING AND COMPLETE ENGINE CUT OFF
This symptom is the most drastic one and may appear at the final phase of the problem.
The bad part is that it can start at a second’s notice, in the middle of driving, without any previous warning.
It will manifest itself as an abrupt, violent jolting. After a few hard jolts, the engine may cut off completely and you may not be able to start until a few minutes have passed.
The good part is that all of the previous, less harsh symptoms tend to appear before this one. If you react on time it will probably never get to this.
10. GRIME BUILD-UP INSIDE THE THROTTLE BODY
Instead of a shiny and clean throttle body, you might encounter a filthy one with grime build-up.
Problems begin when the grime and filth cause the butterfly valve to get stuck causing an irregular airflow and poor response time.
Of course, in order to notice this symptom, you must make an inspection from time to time, like when changing the air filter.
If you do, you might prevent this problem from happening altogether. Some simple cleaning on time may mean the difference between a big problem and one not happening at all.
If you want to learn how to clean the throttle body, click here for a separate article on that topic.
In the end, I must emphasize how important it is to notice and react on time when any of these symptoms of a bad throttle body appear.
Your car’s reliability and your safety on the road depend on it.
A slightly rougher idle, a bit higher fuel consumption, or the check engine light comes up (although everything seems fine) are good enough signs to check the throttle body.
One more reason for doing it is that on most cars it’s pretty accessible and the cleaning procedure isn’t that complicated, expensive nor time-consuming.
Another car problem that’s, in most cases, easy to fix but if not tended to on time, can cause a lot of problems.
Written by: Sibin Spasojevic
Former car technician, life-long car and DIY enthusiast, author for Despairrepair.com