How to Tell If You Have a Faulty Throttle Position Sensor

In days gone by, a car was relatively simple compared to today’s standards – it was fitted with an engine that used a carburetor to monitor the fuel it drank, and it was fired with a spark plug – all simple stuff, but today … we have many different sensors that work as part of the fuel injection system, and if one of those sensors fails, you could be in trouble. So what are the symptoms of a bad Throttle Position Sensor?

Fuel injection has been around for decades, you can actually go back to the seventies to find examples of it being used in production cars, but realistically, it started making an impact in the 80s, and once the benefits were understood, and manufacturers started adopting it more and more, which of course led to a price drop, it became common place on every single mass produced car, and that’s before we even get to the ‘green’ issue.

Understanding the basics

A carburetor is pretty technical, using a number of ‘jets’ to monitor and dispense fuel into an engine – it’s possible to change how much fuel or air is delivered, and although it needs to be pretty accurate to account for different loads, it’s nothing like as accurate as fuel injection, even running like for like – in other words, before we get to fancy stuff like being able to cut cylinders for fuel efficiency.

An engine has a theoretical perfect fuel state with no load, this is called ‘stoichiometric’ which equates to 14.7% Air to Fuel Ratio (AFR) – 14.7 parts air to 1 part fuel. Except … it isn’t that easy. As the speed and load increases, so must the fuel being used and burnt, so on a relatively high performance engine, with high loads (your foot buried into the firewall), you would need around 11.2% AFR – some extreme engines run as high as 7 – 8% AFR.

This isn’t the place for getting too technical, so we’ll avoid thermal dynamics and the like, but a brief note – the more fuel being delivered in to the combustion chamber, the higher the possibility to cool the charge, which can affect horsepower.

A fuel injection system uses electronic sensors to understand exactly what the engine is doing, some of these include: an air temperature sensor to monitor the air temp going in to the engine, camshaft sensor to relate exact position of the cam(s), a flywheel or crank sensor to tell the electronic control unit (ECU – essentially the brain for the system) exactly where the engine is in terms of revolution, a throttle position sensor so that the ECU knows exactly how much demand is being placed on the engine, a water temperature sensor, fuel temp sensor … a whole myriad of sensors send information to the ECU so that it can calculate the precise amount of fuel to be delivered.

An aftermarket ECU is programable, allowing you to change a number of differing factors, which means that you can tune your car for performance, rather than fuel economy, whereas a regular production car will have an ECU fitted that can’t really be tampered with, unless you have a lot of expensive equipment and extensive knowledge of the system, and even then, it allows for only small adjustments – you couldn’t fit a bigger set of injectors for example.

What is a Throttle Position Sensor?

what is a throttle position sensor

We’ve mentioned some of the other sensors, and similar to the throttle position sensor (TPS), their job is just about collating data to send to the ECU to tell it how much fuel and air to deliver to maximize the engine efficiency.

The throttle position sensor essentially monitors where the accelerator pedal is, through measuring the angle of the ‘butterfly’ in the injection system. Simple right? Depending on the car or ECU fitted, it could also be capable of altering how that pedal effects the fuel delivery – a ‘soft’ start to the fueling to make it easier to drive at low speeds, or incredibly sensitive so that as soon as your foot gets anywhere near the pedal, it’s raring to go.

But let’s keep with standard, mass production cars …

Unfortunately, we can’t give a guide on how to replace the throttle pot; there are hundreds of different sensors, with more methods of replacement and fixing, but we can give you things to look out for if you think your throttle position sensor could be faulty.

Symptoms of a bad Throttle Position Sensor

The symptoms of a faulty throttle pot can be many and varied, and just because one vehicle feels a certain way, it doesn’t necessarily translate that another will feel the same.

A slow or ‘lazy pedal’

Quite often, the first signs of a faulty TPS could be a delay in-between you pushing the accelerator and the engine actually responding, or you taking your foot off the gas and the engine slowing.

‘Missing’

A ‘missing’ engine refers to how it’s running – usually from a misfire – so the engine isn’t running smoothly, this could be accompanied by some pops and bangs through the intake or exhaust, although they’re not common.

Complete failure (inc shutdown)

Some cars (in theory) can still run when the TPS has failed completely, although only at tickover speeds, while others will fail to start due to the ECU not knowing what’s being demanded, almost like a failsafe.

Checking the TPS

Of course, in an ideal world, a little message will come up on your dash and tell you that your TPS has failed, but that’s never going to happen, the best you can hope for is that the ‘Check Engine’ light illuminates, but that gives you no further information whatsoever – a good technician may be able to diagnose or eliminate problems from that, but realistically, you need an automotive diagnostics machine or code reader.

checking the tps
Source: https://www.youtube.com/watch?v=rKK7QdochkI

Most home mechanics aren’t in the position to buy a full diagnostic machine on the off chance that they may need it once or twice – they can be very expensive, but it isn’t all bad news … basically, if your car is new enough to have a modern fuel injection system fitted, it should also have the capability for OBD – On Board Diagnostics, in which you can plug a reader in to the OBD port and it will give you some information – depending on the reader, it could be comprehensive, or just a fault code number.

You can buy an OBD fault code reader for around $15, but don’t expect a wealth of information if you choose the cheapest, there are also other readers that work in conjunction with a smartphone app.

Whenever you plug an OBD reader or diag machine in to a car, you should always check for fault codes first, then make a note of them (if the reader doesn’t store them) for reference. From that point, we would usually attempt to clear the fault(s) to see whether that sorts any of the running problems, and to see whether it’s a constant state fault or a temporary glitch – in other words – to see if the fault comes back straightaway or at some point in the future.

Electronic systems are sensitive, it could be that they’ve had a temporary fault which caused the system to log a code, even though the problem may not still exist, that’s why you need to see whether it’s permanent or not.

BONUS:

Replacement

As we’ve already stated, giving a guide on how to replace a faulty TPS is almost impossible due to the many different types of TPS and fixing methods, but … most modern era cars are designed for ease of use, this can also include maintenance and servicing procedures, so it’s entirely possible that you can buy a new throttle position sensor and swap it out for the faulty one, job done, but you may also need to set it up correctly – a little research will be needed if you aren’t sure.

Many of the modern automobiles use tamper-proof fixings for sensors like this – primarily to make it more difficult to change (so you have to use the authorized dealer network), or just so that people can’t tamper with it and accidentally cause a problem for themselves.

Another word of caution – both to do with the wiring: the plugs attaching the wiring harness to the sensor can often be held with a fastener, so that they can’t just fall off – be careful when you try to remove the plug, because if you damage the plug retainer, you’ll never get it running right until you replace the plug, which can be a job for an auto professional, and if your car is more than a couple of years old, the plastic coating and channels for the wiring can become brittle, and the wiring can suffer from we call ‘dry joints’ – so use caution when trying to move them because it is very possible that you could create further problems if you’re not careful, and chasing a dry joint in a wiring harness can cost thousands of dollars and require expert knowledge.

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