My newly acquired 2001 WRX has an odd problem and funny enough its the exact problem I had in my Nissan 180SX,that I never did manage to fix in the 4 years I had it!, though it was much worse on the Nissan.

When the car is cold or has not been driven for a few hours ,the acceleration is very responsive/ crisp and no hesitation on take of. After driving for about 5 to 10 mins the car then becomes much more noticeably unresponsive acceleration-wise and also develops a really annoying hesitation on take of?!.

I have so far.
Cleaned the AFM= with brake cleaner.
Added Fuel Injector Cleaner.
Changed Spark Plugs( NGK Platinum PFR6G)
Changed Fuel Filter.

I'm hoping this wont be like that damn Nissan and give me years of hell trying to get it to run right!?.

fuel pump

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I am only an asshole on the internet :)

[SIGPIC][/SIGPIC]

Walbro installed last year, Maximum Motorsport installed it for the previous owner along with an $1300 ECUTEK TUNE

Water temp sensor, or AFM.

Buy new wts
Find someone to swap AFM to test

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MY99 WRX EJ2.35L 720whp 9.48@147mph
https://youtu.be/c_lYEG0_Dyo

Does it have a Brant alarm still? If it does ..

To be honest I don't know what Alarm it has??....why would that matter?do they cause running probs with the vehicle?.

The brant system is the pin code system required before starting the car.

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When its warmed up does it hunt @ idle?

Alarm shouldn't have anything to do with responsiveness of the engine

Correct, but hesitation is often Brant being a bitch.