I just got a 2006 explorer 4.0 for a realy great price with what i thought a simple problem. The car idles rough like its got plugs missing. It struggles badly when accelerating and if you stomp on the gas a little it boggs down an shutters. If i can manage to gain speed its only mabey 45 50 mph down hill. Now going up hill, its almost crawling at a stand still. If i give it gas it just shifts down winds out an goes slower untill i let off an let it shift up, but then its rpms drop way down an go even slower. It does start right up without strougle and engine is very quiet. Iv had computer an mechanic look at it and so far thats only gotten me is part after part for things that arent the problem. Parts Replaced: spark plugs and wires, along with an ignition coil, and a mass air flow sensor. Could it be 02 sensors or a plugged catalytic converter?

 

Welcome to the site.

How many miles on the clock?
Being the plugs and a few ignition items have been changed and it 'still' continues to idle very rough, I'd be suspect of large vacuum leaks, PCV, manifold etc, or possibly cylinder comp issues or fuel injection problems. If higher mileage ? and unknown prior issues or repairs, with the plugs being replaced,,, a cylinder compression test would have been a given to check for any base engine issues and a general health check.

Being it starts right up,, fuel supply or delivery (system pressure and fuel volume) probably isn't an issue. Always good to check though. Lack of needed fuel supply could certainly cause a flat throttle and misfires if off spec, but most times when fuel pressure/volume are off, they generally have hard/no start issues. A restricted fuel filter most times won't affect idle.

Restricted cats most times will affect top end/RPM's (like a restricted fuel filter would) and not so much idle conditions.. It certainly must have set some DTC's (check engine light ON=codes) and they'd be very helpful moving forwards.'Individual' cylinders failing to contribute could point to more direct cylinder performance problems like Comp, injectors, ignition related. Random misfires, generally follow vacuum leaks, fuel delivery issues, ignition component break down (plugs etc) MAF, EGR and such.

With all new ignition parts, a comp test would be a starting point to prove out 'what it is, or what's not a problem. Sometimes you have to make sure what it's not before replacing what you think it is. If the CEL is on, if you can take it to your local chain parts house, most will check for codes at no cost. Checking trouble codes is always a first and starting point..