What do you mean by K&N? They have more than one product. If you're referring to the K&N Typhoon, then it is the absolute worst intake for the Evo - it is terrible and should never be used. Take it off the car immediately, and re-install your stock intake.
If you're talking about the drop-in filter, then that is not normal. The drop-in filter can't cause bad gas mileage and doesn't need the ECU reset.
That's interesting. There must've been some other issue at play. They only way I could see you losing MPG is if there was a leak between the MAF and Turbo causing the ECU to read it as lean and dumping more fuel to compensate. I ran a K&N cone on my stock intake snorkle and never had any issues with it.
Hmm, so you bought a cone filter separately and installed it on your stock, rubber MAF pipe (hose going from intake to turbo)? If so, then that's an ok mod as long as it wasn't the Typhoon with the hard MAF pipe. However, you aren't supposed to add a cone filter without a tune. I haven't seen an untuned intake kill gas mileage, though. They typically cause the car to run leaner with erratic lean spots, not richer.
PS. The "intake snorkel" is the inlet that sends air to the airbox from outside the engine bay, not the MAF pipe that sends air to the turbo. Just FYI...
I only mentioned it, because I often refer to the intake snorkel when diagnosing the common problem of not fastening the snorkel with both clips. I don't want people being confused. We refer to the hose going to the turbo as the MAF pipe. Just need to be clear on the nomenclature...
I wouldn't say "shouldn't" be done. I started out with a K&N cone just like he had and didn't have any problems with the way the car ran. I've seen this done with quite a few Evo's. You really only run into idle and drive issues when you replace the intake pipe.
Unless you can duplicate the same occurances by putting the cone filter back on I wouldn't worry about a tune for it. However if you have a lot of mods planned I'd try to plan to have a tune done shortly after installing them.
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