Once again that elusive ck engine light is coming on.
Code # 171 & 174
Bank 1 & Bank 2 running lean codes.
I have replaced the Mass Airflow sensor, the Fuel Pressure Regulator, 2 of 3 O2 sensors, cleaned the throttle plates with cleaner and new air filter.
I know there is still the option of replacing the EGR Valve, I have gone over every vacuum line on the engine to look for leaks.
I have an OBDII reader and just keep clearing the codes.
This is on my 1997 Ranger XLT SuperCab, 4.0 with 5 Speed Auto, 111,500 miles on it, I have owned it from day 1.
The only immovability issue is when I let off the gas pedal now and then the engine will surge, try to stall, you can even see the lights go bright and dim, and then when I come to a stop she idles fine.
She has plenty of power and runs perfect above 900rpm.
I dont get the surging all the time just now and then.
I'm open for suggestions.
About a year ago I installed Bosch IR Fusion spark plugs, wondering if they are doing too good of a job ? FYI

 

For one,get rid of the Bosch plugs and go with autolite/motorcraft.
I have seen posts here quite a few times regarding the intake manifold bolts coming loose and causing lean codes,may want to check them.
Do the HVAC controls work properly switching from floor to defrost and back again?
The next one I can think of but should throw a different emissions code , is to check the gas filler tube near where it enters the tank for cracks.
And don't forget to change the fuel filter. What are you running for fuel pressure at the rail?

 

I will ck the intake manifold bolts tomorrow, HVAC controls work perfect, now and then after a hot start, I get a humming sound, like a vacuum line/valve vibrating, but cannot pinpoint the cause.
I will switch back to Autolite plugs, we have many in stock here, time is plenty and easy to change, and yes I did change the fuel filter, twice, just forgot to mention it.
Dont have a pressure guage for the rail, but will ask other techs if they have one I can borrow.
Thanks for your assistance !