Hi guys, I have the notorious PO171 and PO174 codes on my 2003 5.4L that come back whenever I erase them. I cannot figure out the problem. Whenever I start the truck it cranks for 3-4 seconds before it finally fires, then rpm's are very low, then they finally pick up. I have noticed a loss in power as well. I have completed a full tune-up (spark plugs, coils, boots, filters, etc.) I have checked the vacuum and PCV lines numerous times, I have yet to find a leak. MAF appears clean. I am running a K&N CAI setup. O2 sensors a good, replaced the gas cap. The only other problem I can see is a bad PCV valve? I will pick one up tonight and see if it helps. I hope it isn't a manifold gasket. The truck has been very well maintained and still looks brand-new throughout at 250,000 miles. What do you guys think I still need to check?

 

171 and 174 are vacuum leaks... I would bet that at 12 years old and 250K miles you have bad vacuum hoses. My 5.4 failed the two small 1/4 inch black hoses above the drivers side valve cover ... about 8 inches long to the EGR system. The ones closest to the exhaust are the first to go, but with 250K miles, several would be suspect. Don't look, PULL the hose off one at a time. Hard / brittle / cracked, all need replaced.

 

You cannot ignore the codes.
Look at their description.
The Ox sensors detect too much oxygen in the exhaust and keep telling the PCM to richen the fuel injection. This goes on until the long term fuel trim tables shift out of limits setting the codes for both banks.
The results is stalling, poor idle and drivability issues.
Good luck.

 

I am not trying to ignore the codes, that is why I want to fix the problem! The new PCV valve didn't help at all, I have a scan tool that gives me live data capabilities. I am new to the OBDII world, I am an expert on EEC-IV only lol. Can you tell me what I should be looking for with my fuel trims and O2 voltages. If the O2 voltage is at a certain range won't that tell me if I have a bad O2 sensor? Thanks for the help so far guys.

 

Look at your Long Term Fuel Trim values for both banks.
When they are about 25% shifted rich, they set codes 171/174.
This indicates an unaccounted for air leak into the intake system from some source.
The OX sensor voltages will not tell you much because they are working in real time and affect the short term tables quite quickly.
They will however 'react' quickly to sprays introduce at the air leak point to confirm you found the leak area.
Your EEC-iv systems has the same kind of Long Term and short term Trim Tables for the same purpose..
Good luck.