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Re: I have a 2003 Expedition with 110,000 miles
This is more than likely a vacuum leak, with the engine running listen under the hood for rushing air, another method to find a vacuum leak is to spray short bursts of carb cleaner in different areas (start by looking behind the throttle body, this is the most common place) and listen for a change in engine RPM, hope this helps
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This is the diagram for the 4.6 1998 Ford Expedition.( Notice the numbers 1-2-3-4 spark plugs are on the left side if you are facing the front of the vehicle.) Please rate my answer if you were helped.
It is not a fuel filter. If the left bank was lean the right would be also since the complete engine is fed from a single line to the injectors. Your O2 sensor is probably throwing the code to set the check engine light. The O2 sensors control the amount of fuel fed to the engine. Most Fords use an upstream and a down stream O2 sensor. From what you are describing I am betting your vehicle has way over 100,000 miles on it. The O2 sensors start going out after this point. They are subject to heat extremes and contamination from burned oil deposits and carbon and or fuel additives that you place into your fuel tank. If the code is reading or giving you a "lean" condition more than likely your upstream O2 sensor has bit the bullet finally.
Cam Phasers. There is an Extended Warranty for them. What happens is the Cam Phaser is suppose to automatically adjust each cylinder bank independently. When one of the Cam Phasers gums up, it can leave a Cam over-advanced.
An over-advanced Cam will be lean and generate a lean Code for that side of the engine. An engine with one bank operating at 1 timing setting and the 2nd bank operating at a different setting will run rough and stall before blowing up.
Ford has given a 7 Year, Limited mile Extended Warranty for those DOHC engines having this problem. This can be a costly repair of around $2400. A recently developed "Cheater Procedure" can cut Labor from 8 to 4 hours with savings in gaskets resulting in a $1200 to $1600 repair.
Your vehicle may develop a Diesel sound which fades at 2000RPM. So contact a Dealer with a VIN# and go from there.
Most likely the coils. Easiest way to check is swap them with another cylinder, start it, pull the plug wire connecter and if it doesn't change the engine the coil is bad. If it is the injector, then you can check it with an ohm meter. Touch the leads in the injector it should read 11 to 15 ohms. if you need help let me know.
if you have the 3.8 engine you might have a vacuum leak at the intake where the plastic intake meets the lower intake,there are rubber gaskets that fail when they get older from oil getting into intake.This will set that po171 also p0172 -3 &4,hope this helps as you may be wasting time and money to try and fix it with just the o2sensor .The maf on the ex is where the cone air filter is look for the wiring harness that goes into air filter housing,its inside there fun to remove dont brake it
I have a ford 2003 windstar that was running very rough when cold and almost stalling. engine light came on. went to autozone and they read the codes for free and told me it was running lean on Bank 2 also. i came home searched for the O2 front sensors online using the info autozone gave me. there are 2 front sensors on my car that need to be replaced at the same time. look for ones that have a direct plug in and don't need wire splicing. the ones without the wiring harness are cheaper but more of a pain, i think. they were fairly easy to replace. getting access with the wrench was the worst. reset the light by disconnecting the battery for a few minutes. seems fine now and the light hasn't come back on.
A tune up will not cure the lean code, in fact an engine miss gives a rich code from the unburned fuel. I would suggest u verify u have no vacuum leaks, also clean the inside of the MAF sensor if so equipped and replaced the bank two O2 sensor, that is the passenger side sensor before the catalytic converter. If u have the money replace both right and left sensors, it is better to do them in pairs.