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2003 Forester 2.5X 5MT & 2008 Impreza WRX 5MT & 2009 Impreza 2.5i 5MT
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106 Posts
Discussion Starter #1
I have a 2003 Forester 2.5X with 5-speed manual. It has 142,000 miles on it. I just bought it recently and I have been driving it daily. So as the title says the throttle response is sluggish.

I have cleaned the throttle body and IAC valve with CRC throttle body cleaner. that seemed to help. I then did the Subaru idle fix / idle relearn. this seems to have really helped with the stuttering I was getting when accelerating in first gear from a dead stop.

My next step is to replace the spark plug wires and spark plugs. Here's where I'm confused and need recommendations from you experts. I plan on installing NGK iridium spark plugs, but I'm confused on which part number for the plugs. Napa Auto shows the part number as NGK 3764 , but Amazon shows the part number as NGK 5464. I realize these are two different plugs and I don't want to damage my engine by installing the wrong plugs, yet I want the best possible performance I can get from my engine. In short what iridium spark plugs are you guys installing on your 2003 foresters with the 2.5 sohc engine??
 

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Administrator
2007 Forester Sports XT 4EAT
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38,941 Posts
@greengoblin68 did you go here NGK Spark plugs to find your part number? :confused:

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Bobby...

['07 FSXT MODding Journal] ['03 X MODding Journal]
 

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Administrator
2007 Forester Sports XT 4EAT
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38,941 Posts
Also, this may be of interest to you.
Waste Spark Ignitions

Like most automotive systems, the ignition system has undergone many changes. One example is the Distributorless Ignition System, or “DIS” for short. This system was first introduced in the 1980s. The distributor, cap and rotor in a conventional system were replaced by coil packs. The new DIS system proved to be more reliable and required less maintenance.

The earliest DIS featured a bank of coils; one coil for every two cylinders. Each pair of coils would provide power to two spark plugs. Each of the two paired coils would fire the paired spark plugs simultaneously, one on the compression stroke and the other on the exhaust stroke. Since one plug is fired on the exhaust stroke, serving no real purpose, the system was known as a “waste spark system.” This style of ignition has negative and positive polarity sides to the coil; meaning the spark plug also has positive or negative polarity.
It is important to know that in a waste spark ignition, both the center firing electrode and ground electrode will see gap erosion. In plugs with a positive polarity, most wear will occur at the center-firing electrode. Plugs with negative polarity will experience more wear at the ground electrode. As a result of this, vehicles equipped with waste spark ignitions often use double precious metal spark plugs, such as the OE Iridium® or OE Platinum®. These dual precious metal plugs feature either iridium or platinum on the center-firing electrode and platinum at the ground electrode. Because these electrode materials are denser, gap erosion is reduced, allowing a longer service interval. Since these materials are denser, this will reduce gap erosion, thus leading to a much longer service interval. The use of nickel-alloy or single precious metal spark plugs in vehicles equipped with waste spark ignitions will result in more aggressive gap erosion and shorter spark plug life.
We're running Iridium IX spark plugs in my son's '03 X. NOT a recommendation, just what we decided to go with. :wink:

Bobby...

['07 FSXT MODding Journal] ['03 X MODding Journal]
 

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Registered
2003 Forester 2.5X 5MT & 2008 Impreza WRX 5MT & 2009 Impreza 2.5i 5MT
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106 Posts
Discussion Starter #5
After doing some research the 2003 Forester Owner's Manual recommends NGK (2756) BKR6E-11 and NGK (6953) BKR5E-11 spark plugs. So I may end up installing one of these.
 

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Administrator
2007 Forester Sports XT 4EAT
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38,941 Posts
You can't go wrong staying with what the factory installed in the engine. Stay with NGK, which the engine comes with. We use Champion spark plugs in our '95 Neon Sport, but NOT in our Foresters. :wink:

Also... with the standard spark plugs, the Subaru recommended replacement interval is every 30K miles.

Bobby...

['07 FSXT MODding Journal] ['03 X MODding Journal]
 

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Registered
2003 Forester 2.5X 5MT & 2008 Impreza WRX 5MT & 2009 Impreza 2.5i 5MT
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106 Posts
Discussion Starter #7
I finished changing the spark plugs. I went with the NGK BKR5E-11 (6953) plugs.

Funny thing is it already had the Iridium plugs in it. I attached a pic of the plugs I pulled out of the motor.
 

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