I have a 2005 Focus 2.0 ZX5 manual. 131,000 miles. I believe this has been an ongoing intermittent issue until now. Occasionally while driving before at 45-75 mph I would sometimes get a stutter like the throttle closed for a split second. Now, after installing a K&N CAI the dang thing is about undriveable. At idle no load the engine will not rev, step on the gas and all I get is a groan, pop pop groan slooooow climb in revs to @ 2000 rpm then quickly up to the fuel cut off. In gear , 1st or 2nd, trying to roll out off idle I get nothing more groan sloooow climb up to 2000 rpm then accelerates normally. At speed in any gear when RPMs drop below 2000 rpm it will not accelerate no matter how hard I press on pedal. Up above 2000 rpm mostly normal unless I get out of throttle, then I get a bucking sensation from engine. I can pull away from stop if I give lots of revs and slip the clutch…not so good for clutch life…The MIL is on, P0113 IAT high bypass. Cleaned old MAF no difference. Replaced MAF no difference. Got a too rich code also but cannot remember DTC. I have also checked for air leaks after the MAF and it all seems rather buttoned up. Forum searches yield solutions from plugs to pumps…hoping someone out here has run into this before.
What is a K&N CAI? Is that a replacement engine air filter? If so, I expect you disconnected something or forgot to reconnect something or crossed something up in the process of doing the filter replacement. There’s probably an intake air temp sensor in that area. Check to make sure it is in the correct place and the connector isn’t loose. There are probably weird looking gadgets called air baffles in the air path from the filter to the engine. Make sure those are all connected and positioned correctly and air tight, and you didn’t leave a screwdriver in them or something. They have to be doing their job to make the air flow profile match what is assumed, or it will confuse the MAF. There’s also a chance there’s something wrong with that particular air filter. Did you try putting the old air filter back in, see if that returned the car to pre-K&N-filter-change status?
K&N Cold Air Intake.