How To Clean Your Subaru Forester's Throttle Body?

By Nick Marchenko, PhD

Updated: August 5, 2023

The Subaru Forester's throttle valve is an integral piece of the puzzle when it comes to delivering air to the engine's combustion chambers. It's all about the perfect balance of fuel and air. When you flex your foot on the accelerator, that throttle opens wide, letting a river of fuel flow into the cylinders. The throttle valve's performance is intimately tied to the fuel-air mixture ratio. 

Through regular use, the throttle tends to gather its share of grime and dust that comes in with the air.

All this starts to build up on the throttle body, which can throw a wrench into the smooth operations of this crucial component. When the throttle gets heavily clogged, your Subaru Forester starts sending distress signals. Signs that your throttle body might be chock full of gunk include:

• Erratic idle RPM;
• Stutters when pressing the throttle pedal;
• Lagging acceleration;
• Increased fuel consumption; diminished power output

If these symptoms sound all too familiar, and you can't recall the last time your throttle valve had a good scrubbing, perhaps it's time to roll up your sleeves and get to work.

What do you need to clean your Subaru Forester's throttle body?

The shopping list for this chore isn't a long one and you'll probably find most of the items hanging around your garage. 

Here's what you need:

• Throttle body cleaner, or Throttle body and Air Intake cleaner (which are virtually the same thing);
• Carburetor cleaner - can also be useful;
• A good old brush;
• Cleaning rag(s);
• A trusty screwdriver

It's a good practice to give the throttle valve a cleaning every 75,000 miles (121,000 km). There isn't a set-in-stone schedule, but regular throttle body cleanings can help keep your Subaru Forester running like a dream. If you start to notice odd symptoms, that's probably your cue to start the cleaning operation.

How to clean your Subaru Forester's throttle body?

You've got two options for cleaning that throttle valve - either leave it in the Forester or take it out. We'll focus on the method that leaves the throttle body in the car for this article. 

This approach is just as effective as the other, but it saves you from doing some heavy lifting. However, if your throttle valve is looking more like a grime museum, it might be worth it to remove and clean each part individually.

Disconnect the negative battery terminal

Start by disconnecting the negative terminal from the battery. This step is crucial to ensure your Subaru Forester is safely powered down and prevent any potential electric shock.

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Detach the air duct from the throttle body

Next up, disconnect the air supply hose linking the air filter to the throttle. Most times, it's held in place by a clamp - but if not, just unhook it using the appropriate method before you remove it.

Disconnect the hoses and sensors that are connected to the throttle

The throttle valve has an array of wires and pipes connected to it - some are sensors, and others channel coolant. When you're disconnecting these pipes, ensure you fix them vertically and plug them with a spark plug to keep the liquid from seeping out.

Label all sensors and hoses

After you've given your Subaru Forester's throttle a thorough clean, you're going to have to reconnect everything back in its proper place. Now, if you've got a bird's nest of wires to deal with, things can get confusing real quick. So, do yourself a favor and label and mark everything you disconnect - it'll make it a cinch to put everything back where it belongs after you've cleaned the throttle.

Apply throttle cleaner to the surface

Time to whip out the throttle body cleaner. Give the throttle body a good spray and let the cleaner sit and do its magic. If you're dealing with stubborn grime, a good 3-5 minutes should be ample time for the cleaner to dissolve it.

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Get down and dirty with your Subaru Forester's throttle body

Now, start scrubbing the throttle with a brush (not too stiff, we don't want any scratches), rag, or even a cotton swab.

If at first, you don't succeed, try, try again! If the grime's still there, hit it with some more cleaner and repeat the cleaning process. A spick and span throttle valve should have a near mirror-like shine and show no signs of leaks or any residual dirt.

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Put everything back together

Once you're satisfied with your throttle body cleaning, it's time to reassemble the whole system. After everything's back in its rightful place, give it a quick check to ensure everything's functioning as it should and you're not dealing with any odd noises or other symptoms.

How to adapt your Subaru Forester's throttle body

In recent models of the Subaru Forester that feature an electronic throttle (activated by an electronic signal rather than a cable), there's a designated throttle position sensor that monitors the current position of the throttle blade. When the engine isn't running, this blade aligns perfectly straight at a 90-degree angle.

It seals flawlessly against the top and bottom walls, preventing excess air from sneaking in. However, as time marches on, dirt starts to accumulate on the walls of the air channel or the blade itself. This grime keeps the throttle from completely closing and sticking to the same position. 

The sensor then reads this new position, leading your Subaru Forester's computer system to "believe" this is now the reference value.

So, the neutral position of the throttle blade, taking into account the layer of dirt, becomes its reference (factory-set) value. This is a normal occurrence, as it's factory-planned and crucial for maintaining the engine's operating condition.

Nevertheless, after we've scrubbed off the dirt from the throttle body and restored the surface to its original state, the Forester's computer won't exactly "comprehend" this change.

With no indicators to inform it that nothing is obstructing the throttle blade from fully closing - the vehicle's computer will continue to "presume" that there's dirt present and, thus, hold the blade in the remembered position. 

This causes a gap to form between the throttle walls and the blade, allowing at least a few millimeters of air to pass through.

Consequently, your Forester's computer will "think" that the throttle pedal is always pressed, even when it's not, and allow more air to pass into the cylinders via this newly formed gap. 

This situation isn't ideal for the engine; it continuously experiences a load, which will be particularly evident at idle, where instead of running at 800 rpm, it'll rev at 1500–2000 rpm. This can lead to overheating and reduce the engine's lifespan.

Thus, after cleaning the throttle valve, it's necessary to recalibrate the blade back to its factory position.

The throttle valve adaptation algorithm applies universally to many Subaru Forester models.

Heat the engine to its operating temperature, then switch it off and wait 5-10 seconds. Turn on the ignition and wait for 3 seconds. Next, you'll need to press and release the throttle pedal 5 times over 5 seconds.

If this procedure doesn't yield results, it means that your Subaru Forester's system follows a different programming. You'll find the correct throttle valve adaptation algorithm in your Forester's owner's manual.

To Wrap Everything Up

Cleaning your Forester's throttle body is a vital task as it prevents dirt from sticking to the blade or forming gaps between the blade and the body itself.

While performing this cleaning operation, it's important to be patient and meticulous to ensure the best results.