Hello everyone, failed my emissions and got the following codes po171 po174.. I have tried installing a new pcv valve, maf sensor,unplugged the iac and it slowed the rpm down some but didn't cut out,searched for air and vacuum leaks with carb cleaner no leaks found, at my wits end, check engine light still on and codes are still reading the same.God bless

 

Check fuel pressure, should be 60-65psi on '99
Low fuel pressure cause lean codes.

If you haven't changed it recently, change fuel filter, $10-$15, dirty filter can lower flow but pressure stays high.

Also DPFE sensor usually fails after 60,000 miles, $35-$45 for non-Ford DPFE sensors, replace that if it hasn't been replaced before, make sure it's hoses are good.
Incorrect DPFE sensor data to computer will cause too much exhaust gas to be sent to the intake via EGR valve, this causes lean codes 171/174


And often overlooked is the "ported" vacuum leak.
MAF sensor reports the air flow/volume to the computer, computer bases air/fuel mix of 14:1 on this data, so ALL the air going into the engine must pass thru the MAF sensor.
On some engines the MAF sensor is near the air cleaner and connected to the intake manifold by a long hard plastic air plenum/tube.
If that air plenum cracks or has a leak somewhere, that is a "ported" vacuum leak.
So all the air coming in to the intake is not passing thru the MAF sensor, so MAF sensor is under-reporting the amount of air flow.
If you have an air plenum, remove it and inspect it closely.

 

RonD, whenever I read your responses, I learn! I had to go off and Google what a DPFE sensor was (Differential Pressure Feedback EGR). This sensor, as I read on it, allows the EGR to allow hot exhaust gases back into the intake to be reburned, allowing for "cleaner" exhaust. The DPFE gets pressurized by the exhaust gas, and informs the ECU. The ECU tells the EGR actuator to open the valve and allow exhaust through tothe intake to be burned.
So now I can see how this would lean out the mixture if it's not functioning properly.

 

Thanks

Yes, the DPFE sensor has one hose connected near the EGR valve and one hose connected farther away from the EGR valve on the exhaust system, the pressure difference between the two hoses 'tells' the computer IF the EGR valve is open and if open, how far it is open, computer then adjusts EGR flow based on that info.

Hot exhaust gases "cool" the cylinder temperatures, odd as that sounds, lol.
EGR system is only used when engine is under load because that is when cylinder temps start to rise.
High cylinder temps cause high NOx emissions, so EGR system was added to reduce NOx.
By product of lower cylinder temps is less or no "pinging/knocking", which is caused by high cylinder temps pre-igniting air/fuel mix.
If you start to get pinging when accelerating or going uphill then EGR system probably needs some attention, or you got some very low octane fuel, lol.

 

Ron d, your a huge help,thanks again!

 

Did that sensor take care of your problem? Researching the same codes and possible causes. My truck runs fine, I have already changed out the upper and lower intake gaskets last spring as they had gone bad and also changed out the O-2 sensors at that time. I know I have a bad filler tube on my gas tank that is pretty rusty and crusty and may have sprung a leak so I am replacing it as well. May go ahead and order the DPFE sensor and hoses since they are original to the truck, checked pcv and hose, they are fine. My lite just popped on a couple of days ago and I pulled the codes with my scanner, probably only the 3rd time my truck has flashed check engine in the 17 years I have owned it with just over 95000 miles on it.