2002 Mustang GT running lean

a good start would be to make sure all your vacuum lines are connected. i remember i forgot to reinstall the hose on the back of the plenum when i installed my intake.

You may have a vacuum leak. If you've got all the vacuum lines connected and in the right locations, you can use a spray, such as throttle body cleaner to spray around the intake, throttle body and intake ducting to try and pinpoint a leak. While the engine is running, if the idle changes when sprayed in an area, you can usually pinpoint the leak.

it well run like that if you have a bad coil to. i think there is a code for that though.

Yes, a vacuum leak could definately cause these kind of issues. In fact, a leak between the MAF and throttle body could cause this sort of problem too.

I also have an 02 GT and had the same problem you are describing.

I was throwing lean codes at tiems and have had a few fixes that might help you.

1.) I don't remember the code number but the diagnosis was EVAP PURGE VALVE FAILURE. I ended up replacing the Evap Purge Valve that is located in the passenger front fender well. Problem fixed. Here's the part: MOTORCRAFT PART F8ZZ9C915CA (long) or F8ZE-9C915DA (long) or CX1692 (short).

2.) I was getting another code for LEFT BANK SENSOR LEAN. I found a cracked and rotten hose that ran from the Throttle body (coming out next to the EGR valve) to the passenger side bank of the engine, attaching to the PCV valve. I have the funky "twist-on" PCV valve that none of the parts houses had, I had to order it form ford but its NOT listed in their parts database for our 02 GTs. It is NOT the heated PCV, it has no connectors other than a 1/2" 90 degree rubber neck. Look here: http://forums.stangnet.com/779936-pcv-tb-hose-collapsed-why.html I also replaced COP wires and plugs in Summer 2006.

I bought the hose from Ford in Summer of 2006 for $35, replaced it. Now in January 2009 and 30k miles later, that same hose has collapsed under vacuum as indicated in the other link. I am quite sure that this collapsed vacuum hose is responsible for my occasionally high idle and a low RPM hesitation, stumbling, and hitching tendency.

I have a 2000 GT that is converted to a Cobra-R and was getting the same exact codes. If you've changed out the TPS & MAFS check for a vaccum leak. If it's chugging or idling really low at a stop, but in gear this is exactly what happened to mine. Mine was the MAFS & filter combo.

Here's a pic of the engine. It is a 4.6 DO and 90% of the parts have been replaced from the original to make up the HP to meet the Cobra-R requirements. I wish I had the 5.4, but they are hard to find in decent condition

Yea, we found the importantance of gauges, its very simple to hook a vacum gauge, water temp, and oil gauge. These are the basic 3, your car has a very decent volt meter. A/F gauges are just for craps and giggles.
Your vacum should be between 19-20 hgs at idle, your oil depending on the viscosity, should warm up to 35- 40 Psi and 70 to 80 cold at idle. Your stock thermostat is 195 degrees and the fan turns on at 210, provided that you have not reprogrammed your CPU. If you cant find in Vacum leaks, I suggest you raise your octane level. P.s. Your should really think about getting rid of PVC set-up and get your self a couple of catch cans, that car dosn't need to burn all that guck from the valve covers, that will give you headaches in the future.