check engine light po171 & po174 problem

[solar nut]

check engine light came on,had tested these codes came up,replaced upstream o2 sensors didn't work,replaced pcv & hose,checked all vac. lines indivdually with vac.pulled intake replaced gasket,replaced fuel pump & pressure regulator,still won't go off,had engine smoke tested,found egr leaking replaced still no good.replaced mass air sensor,egr vac sol,egr sensor,purg valve sol. replaced air idle motor, after each of these i had recheked & cleared code.light keeps coming on & code has not changed.i am near the end my rope with thing. i replaced a lot of these items because they are vac related. is it possible that the computer is bad ? please help ps engine runs good in my flustration i forgot to state make & engine its a 1999 ranger 3.0 flex

 

[RonD]

Just for future reference you should not replace sensors that report a problem, the fact they reported a probem usually means they are working.

I would clear the codes again and then start cold engine, engine will run rich until warmed up so you should get no CEL, and in any case the O2 sensors are not being used for the first few minutes to report the lean condition.
If you do get a CEL then yes, computer issue.

Time how long before CEL does come on, and where the dash board temp gauge is, or if you are monitoring live what the ECT temp says.

The MAF sensor data is used by the computer to set Gross fuel/air mix and the O2 sensors for fine tuning, but only the MAF is used when engine is cold.
The MAF works by air flowing passed it, so any air entering the intake that doesn't pass thru the MAF would cause a leaner mix.
So the air plenum(tube) between MAF and intake must be air tight, any leaks here is the same as a vacuum leak, it is called a ported vacuum leak but a leak none the less.

A lean condition could be the computer misreading MAF data or bad wiring to MAF or bad MAF

Could also be dirty injectors, run some Seafoam in the gas tank see if things improve.

Easy test for vacuum leak is to warm up engine and then let it idle, should be 750rpm.
Unplug the IAC Valve, idle should drop to approx 500rpm or engine might stall, thats a good sign of no vacuum leak
If idle doesn't drop or drop alot then you probably have a vacuum leak.
Start removing lines and plugging hole with your finger, see if you can find the leak.
Include power brake and PCV lines.


And a long shot would be the Flex Fuel sensor, quite a pricey item, like $750, this sensor is on the fuel line and tells the computer the ethanol content of the fuel, now I would think an error here would cause a rich condition(if using 100% gasoline) not lean, but..........

 

[Dune Runner]

When I bought my ranger I had the same problem. I tested everything. My problem ended up being the vent line coming off the top of the fuel tank that goes to the canister was off and just laying there. I snapped it back on and that fixed it. Had a hell of a time finding the problem.