I've got a 2001 4.2l v6. I've changed the coil pack, plugs, plugwires,throttle position sensor, egr valve, and other sundry parts. I continue to get a code PO 171 and PO174, mixture to lean both banks. The engine runs fine at speed but when you depress the acclerator you get several short misses until you reach your cruising speed. Also sometimes this doesn't occur when the engine is cold, although sometimes it does. I use this truck for work and drive about 1000 miles a week. I've also checked the fuel pressure and it's in limits,also the fuel filter has been changed. The engine has 166,000 miles on it.
Any ideas and help would be greatly appreciated.

 

Clogged fuel injector maybe? Or fuel pump?

 

How old are your spark plugs?

 

Sounds like the typical vacuum leak. That code combo is a prety strong indicator of one. Less likely but possible, MAF problem. try cleaning the sensor element.

Start with the PCV hose and its elbows. Most common point.

Steve

 

Exactly what he said....

 

I appreciate all of the many suggestions. I had previously changed the spark plugs, but I changed them again to Motorcraft brand as I saw several members suggest. I believe my main problem though was a $2.00 PVC rubber grommet in the valve cover. I went by the local Ford dealer and the only way the parts dept. had it listed was to buy the entire valve cover. I told them thanks but no thanks. After a stop by local parts house and installation on Sat. it hasn't missed or posted codes. Again I really appreciate the knowledge all of you are willing to share.

Trojanjoe

 

another way to check if a vacuum leak is present is to checke the IAC counts or percentage on the datastream on a good scan tool at a hot idle. will be lower then normal if you have a vacuum leak.