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P0171 & P0174 Codes

R

remark90

New CEG'er
Joined
Mar 13, 2006
Messages
13
Location
New Mexico
I don't post very often, however I do a lot of searching for answers to the problems I have. MY 1999 Mercury Mystique has 196,000 mi. on her. I have changed the UIM/LIM gaskets about 60,000 mi. ago and replaced all 4 O2 sensors about 30,00 mi. ago.
Car starts great and idles fine without any noticeable miss, it also runs down the highway at 75 + without any noticeable miss or drive ability issues.
I have checked the PCV and EGR hoses and function of each item as well as the vacuum hoses, and have not detected any leak or deterioration of the hoses. The car "seems" to run great and still gets 27-28 mpg after 3 tank fulls since the codes came up. What am I missing here?
Thanks for any and all the help, Kent.
 
If it's not a leak after the throttle body, I'd suspect maybe low fuel pressure, perhaps clogged fuel filter or fuel pump on the way out, sometimes a bad or dirty MAF sensor will cause this. I've pulled the MAF electronics box off the MAF body and VERY CAREFULLY sprayed off the sensor wires with Carb cleaner and made all manner of odd driveability problems go away, once it cleaned up a ragged torque curve on a chassis dyno.

If it's running well, maybe clear the codes with your scan tool and see if it comes back?
 
gmorrell, thanks for your reply. I forgot to mention I replaced the fuel filter about 300 mi. ago when these codes came up, and the codes stayed off until I let the car idle to warm up. I have cleared the codes after every time I "mess" with the car hoping I solved the problem. I also have cleaned the MAF through the intake hose off the car and sprayed both sides. If you or anyone else has suggestions I'm all ears!

Kent.
 
What I have done is use a vacuum gauge to see how much vacuum the engine is producing, then check around for leaks, especially at the heater control, which does use vacuum. The LTFT should be under +8% on both banks, typically it should be around 2.3%.
 
Generally lean codes on both banks indicate that there is a vacuum leak or a MAF that needs to be cleaned. Since the factory tune is on the rich side I would expect it to run well still with both of these codes. Can test by swapping out the MAF with another one. Also the shaft seals for the secondary butterfly plates can develop leaks too.
 
Thanks everyone for giving me suggestions, I will focusing in on the MAF again and the heater control areas, but not until the weekend when it is a little bit nicer. Please keep the ideas coming. I will report back with my findings, Kent.
 
Maybe vacuum leak small enough to show at idle only, if you still have enough idle fuel trim to cover it the engine will still idle OK but the PCM will know the difference in normal vs, artificially too rich an idle setting there. Trims may show it like said. O2 readout may show it too if they stay low too much.
 
Clean the air intake butterfly with a special cleaner you can get at oreillys. Can't remember name. Don't spray inside. Had these codes for years, reset them before emissions and went through alor of troubleshooting. Fixed now. Butterfly was not working right and filthy.
 
I use simple acetone and can even clean the butterfly without removing the TB at all.
 
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