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Old 09-13-2010, 01:00 AM   #1 (permalink)
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Default OBDII Code P0171 and P0174

I have a 2004 Taurus SE. 155,000 miles. I was getting numerous codes. I replaced spark plugs, wires and air filter, that got rid of the misfire codes. (Plugs were fried). I replaced the EGR valve and that got rid of the EGR codes. My husband thought he was doing good and he adjusted the hard stop screw on the throttle. Car ran like crap and kicked out these new codes Po171 and P0174. He re-adjusted the hard stop screw and now the car idles in park at 950 rpms, and when it is put into drive with the A/C blowing it is idling at around 700-725 rpms. It is driving and idling really well. So we let the car set overnight to have a cold engine, disconnected the battery and reconnected and followed the steps to allow the car to re-learn its idle and trim. Everything was great. SES lite was off, car was purring like a kitten. Gas mileage is up to 27.2 mpg. Drove it about 80 miles and the SES light has come back on. Kicking out the same codes P0171 and Po174. Anyone got any suggestions what to do to fix this issue?
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Old 09-13-2010, 08:27 AM   #2 (permalink)
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NEVER mess with the throttle stop screw!

171/174 lean codes are likely due to a vacuum leak. Leak is often at the rubber PCV hose, but could also be intake gaskets or possibly low fuel pressure. CHeck PCV hose first.

I wouldnt be surprised if the EGR code(s) come back, as Ford EGR valves rarely go bad. Failed DPFE sensor is by far the most common cause of 401/402/1401, etc.
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Old 09-16-2010, 10:56 PM   #3 (permalink)
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The EGR codes have not come back. Replaced the EGR sensor as well just as a precaution. Took car to garage and had the trottle stop screw reset. All was fine until today and the code for lean on bank 1 has come back. This is driving me nuts. I have had one mechanic tell me the MAF may be bad. Another tell me the MAF is working just fine. Another tell me the catalytic converter at bank 1 is bad. Another mechanic told me the Autolite spark plugs we put in the car was making the problem, so he pulled all the plugs out, did compression testing, compression was just fine, (I just have no idea what this means "compression") then he installed MotorCraft spark plugs into the car. Reset the CEL and I was off and driving again. I after putting about 60 miles on the car, I was turning into the mall from a left hand turn light, and the engine just died in the middle of the turn. It started right back up, but this is the first time this has happened to me. Scared the crap outta me. After leaving the mall I drove about 40 more miles and the CEL has now come back on and throwing out same 171/174 codes. I am at wits end. I hate to spend 350 to replace the catalytic converter, and this not solve the issue. Also, I hate to spend 212 on the MAF sensor and this not be the issue. I will have hubby check the PCV hose tomorrow for signs of damage. Problem is, he doesnt know where it is at and neither do I. Can you please tell me where the PCV hose is located at on this car?
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Old 09-17-2010, 08:20 AM   #4 (permalink)
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Lean both banks means one of 3 things: 1) Vac leak that effects both banks, 2) MAF problem, 3) Low fuel pressure. Most common problem is a vac leak.


You dont say what engine you have, but if it is a Vulcan, PCV is right on the (rear I think) valve cover. Likely has an electrical connector going to it as well as a large vac line. If it is a Duratec, PCV is sticking out of the bank 1 (rear) valve cover horizontally on the drivers side.

99% chance the EGR codes were due to the DPFE sensor you replaced and not the EGR valve itself.
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Old 12-10-2011, 11:41 PM   #5 (permalink)
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did you figure out the problem?

I have a 2004 vulcan taurus, with p0171 and p0174.

the pcv valve and hose look new & i don't see any cracks.
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Old 12-11-2011, 02:01 PM   #6 (permalink)
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^ Check thoroughly for vacuum leaks. Should have an underhood vac. diagram sticker. Might spray clean the maf 2 small filament wires with crc mass air flow (maf) cleaner (gray can).
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Old 10-05-2012, 09:30 AM   #7 (permalink)
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Default fuel pressure

i have fixed a 97 taurus with p0171/p0174 codes, the car had zero acceration when rpms were over 2500 and a rough idle. Fuel pressure was @ 15psi (30-40 normal). Anyway, cut out the floor to access the fuel pump and found the screws holding the fuel lines to the pump had come loose and the line was barely connected. I put in a new pump as i had already bought one but i think it was the loose connection causing the codes.
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