Re: K&N Cold Air Intake Issue, 09' Interceptor
Intake mods are an easy way to increase performance in some engines, if the engine can benefit from additional flow in the intake. The 4.6 cylinder heads, not the intake, however, are the bottle neck in a stock engine.
It is torque, not horsepower, that accelerates a car. Horsepower is only a function of torque. You might see a small HP increase with cold air intake and larger injectors at higher RPMs, but will loose torque on the "bottom end". That's OK for a race car that spends it's life in the upper RPM ranges, but not good, in a street driven car, especially a heavy one with stock gearing. (A broader torque band, more time in the "fat range", will also accelerate a vehicle faster, but is too much to talk about in this blog).
I used a stock 4.6 intake, even the stock injectors, on a 5.4 and it made over 320 ft.lbs. of torque and a little over 300 HP on the dyno (RWHP). I didn't need larger injectors (24-30 lbs per hour) until I used a larger throttle body. This was on an engine with Trick Flow heads and cams.
Bottom line; you might impress some kid (sorry kids, not all are the same, my 13 year old knows better) when you open the hood and show off a cold air intake, but it's a waste of money and a source of problems on a stock engine, as you have discovered.
BTW, K&N makes a GREAT product when used properly. Try one of their air filters in a stock intake. You will save money over the long haul with filter replacement and a clean filter will do good things for mileage.