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Discussion Starter #1
I just recently purchased my 00 Focus SE with 96,xxx mi. It was just inspected by my local Ford dealership, and everything was running smoothly. Just the other day it started bucking and sputtering around 40-45 mph while driving on the highway. After 45 mph it stops and I have no other issues. It only occurs while accelerating between 40-45 mph.

Does anyone know what could be causing this to happen?
 

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Could be several things, sorry.

Guessing you have an auto transmission. If yes, what happens at 40 to 45 mph (with light throttle) is your car is going into 4th gear. Immediately after entering 4th, at that speed, the engine is experiencing a high load/high torque/low rpm condition and this is also where the car is most likely to exhibit a performance problem... but especially when "something" is wrong. Adding additional load on the engine such as AC On or driving up a slight incline should also help increase the sputter/bucking your feeling. Conversely, AC Off and/or driving on flat or slight downhill conditions should help minimize or maybe have NO sputter/bucking.

What kind of car maintenance have you done?
I would first check the basics. Spark plug health, Air Filter clean, Fuel Filter replacement.
 

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Discussion Starter #4
Yes it is an automatic. I just recently acquired the car from a friend of the family. She kept it well maintained from what I was told. When I took ownership of it, I had the oil changed and it needed an new motor mount and front brakes to pass inspection. Nothing else has been done since I've had it. The dealership did mention that since it was need 100,xxx mi. that I should have all the fluids flushed and replaced soon.
 

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Drive it long enough that way & a CEL will appear, prob. for an ignition miss.

Start simple with a spark plug replacement, as that prob. hasn't been done yet. Old platinum plugs can still look OK yet replacing them can cure problems like this.
 

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Well, you gotta first check the basics. Spark plug health, Air Filter clean, Fuel Filter replacement. In your shoes, I would delay doing the fluid replacement stuff until after doing the basics and then continue hunting the sputter/bucking problem.

Log your gallons of gas used and mileage, and later calculate what mpg you are getting.

Edit; Sorry Sailor, did not see your post, was dealing with my Son.
 

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Discussion Starter #7
I replaced the spark plugs last night after reading your first post Marde. They definitely needed replaced, but that still didn't fix the bucking issues. It did seem to help with the cars overall performance. I'm going to look into the air and fuel filters tonight after work. If after replacing them I still have issues, I'll post back here for more possible solutions. Thanks for the suggestions so far.

One other question. As this is my daily commuter vehicle for going back and forth to work during the week, would it benefit to replace the air induction system with an after market intake, or just go with a standard air filter replacement?
 

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...would it benefit to replace the air induction system with an after market intake, or just go with a standard air filter replacement?
Standard air filter is perfectly fine.

Air induction? - I assume you are not talking about forced induction; an entirely different ball-field.

Replacing it with an after-market SRI or CAI system is a very common modification. Lots of guys do it for a TINY performance increase, but is more often done as a pacifier because the car owner wants to care for, improve, modify, and make the car cool. A pride in ownership thing. It is also done a part of multiple mods, which in the end, will add power and more fun to drive performance.
 

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Forgot to mention, older cars like yours and mine will often develop vacuum leaks within the intake system. Vac leaks can cause or contribute to the symptoms you are having.
 

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Discussion Starter #10
Standard air filter is perfectly fine.

Air induction? - I assume you are not talking about forced induction; an entirely different ball-field.
No I was just wondering if upgrading to an intake would be worth doing at this point. There's no way I'd be talking about forced induction with this car. I traveled down that road with my ZX2 back in the day, and the project was never finished. I ended up parting the whole car out in the end.

Forgot to mention, older cars like yours and mine will often develop vacuum leaks within the intake system. Vac leaks can cause or contribute to the symptoms you are having.
Is there an easy way to check for vacuum leaks that you know of? Do I need to get a specific tool, or take it somewhere to get tested?
 

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Vacuum leak will affect idle before anything else and certainly not at 40-45 mph and nowhere else.
 

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I should have not said;
"Vac leaks can cause or contribute to the symptoms you are having."
I should have said;
"Vac leaks can contribute to the symptoms you are having."

Is there an easy way to check for vacuum leaks that you know of? Do I need to get a specific tool, or take it somewhere to get tested?
There is no simple or elegant way to check for vac leaks. Do a web search and maybe watch a few youtube videos. Methods range from spraying Carb Cleaner onto all sources of vac; to Using Smoke; to Visual Inspections; to Listening. One common source of a leak on our older cars is the PCV hose.

I did not mean to ask you to start searching for a vac leak. Just wanted to make you aware of a potential problem, and to include it as part of your search.

Vacuum leak will affect idle before anything else and certainly not at 40-45 mph and nowhere else.
Yes sir. Good morning grumpy :)
 

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Discussion Starter #13
I did not mean to ask you to start searching for a vac leak. Just wanted to make you aware of a potential problem, and to include it as part of your search.
Well it's good to know if the other fixes don't get the job done. Thanks for all your help so far.
 
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