Can't figure out P0171 Code
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Joined: Jun 2013
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Can't figure out P0171 Code
Hello, I've been on a couple different forums but am new to this one. I have a 2002 f150 4.2l xl 4x4 w/ 175k. I've had this code for a while and it recently now seems to have started affecting the performance of my truck. It just simply runs rough, seems like it's missing some power. It doesn't stall out or anything. I have yet to get it's brother code P0174, so I think it might actually only be on the passenger side.
-I've replaced the fuel filter (I think it was a Fram)
-Cleaned the MAF
-Replaced Spark Plugs + Wires (Bosch Plat +4s)
-Fuel pressure test had 30-32 PSI while running and went up to 38-40 PSI w/ ignition off
-I've shot carb cleaner around the intake, no changes in idle seems like the gasket is fine. It was replaced @ ~100k miles before.
-I've looked around for obvious vacuum leaks but came up empty.
My dad later said he thought the spark plug to cylinder 1 looked more burnt than the other plugs. Could this mean the injector for cylinder 1 is clogged or needing replacement?
Any help is appreciated.
-I've replaced the fuel filter (I think it was a Fram)
-Cleaned the MAF
-Replaced Spark Plugs + Wires (Bosch Plat +4s)
-Fuel pressure test had 30-32 PSI while running and went up to 38-40 PSI w/ ignition off
-I've shot carb cleaner around the intake, no changes in idle seems like the gasket is fine. It was replaced @ ~100k miles before.
-I've looked around for obvious vacuum leaks but came up empty.
My dad later said he thought the spark plug to cylinder 1 looked more burnt than the other plugs. Could this mean the injector for cylinder 1 is clogged or needing replacement?
Any help is appreciated.
Senior User
Joined: Feb 2009
Posts: 491
Likes: 4
From: Southern Illinois
I had to look in the noted on my phone, but I had the same codes thrown for my 5.4L in December. In my case, after doing many of the things you have done above, was to look for a vacuum leak. I sprayed with carb cleaner and nothing changed. I finally got to the point where I traced ALL rubber hoses on the intake and found that on the back of the throttle body, I had a 90* rubber elbow that was dry rotted and had a small hairline crack in it. When I applied pressure, the elbow collapsed and then my truck really ran rough.
I'm not familiar with the engine in your truck, and I'm throwing this out merely for ideas and a possible starting point, but I would think you have the signs of a small vac leak. In the case of my truck, when the check engine light first appeared, I had no immediate change in how the truck drove. However, over time it did get to the point where I had stalling out issues.
Good luck.
I'm not familiar with the engine in your truck, and I'm throwing this out merely for ideas and a possible starting point, but I would think you have the signs of a small vac leak. In the case of my truck, when the check engine light first appeared, I had no immediate change in how the truck drove. However, over time it did get to the point where I had stalling out issues.
Good luck.
New User
Joined: Sep 2010
Posts: 24
Likes: 0
Hello, I've been on a couple different forums but am new to this one. I have a 2002 f150 4.2l xl 4x4 w/ 175k. I've had this code for a while and it recently now seems to have started affecting the performance of my truck. It just simply runs rough, seems like it's missing some power. It doesn't stall out or anything. I have yet to get it's brother code P0174, so I think it might actually only be on the passenger side.
-I've replaced the fuel filter (I think it was a Fram)
-Cleaned the MAF
-Replaced Spark Plugs + Wires (Bosch Plat +4s)
-Fuel pressure test had 30-32 PSI while running and went up to 38-40 PSI w/ ignition off
-I've shot carb cleaner around the intake, no changes in idle seems like the gasket is fine. It was replaced @ ~100k miles before.
-I've looked around for obvious vacuum leaks but came up empty.
My dad later said he thought the spark plug to cylinder 1 looked more burnt than the other plugs. Could this mean the injector for cylinder 1 is clogged or needing replacement?
Any help is appreciated.
-I've replaced the fuel filter (I think it was a Fram)
-Cleaned the MAF
-Replaced Spark Plugs + Wires (Bosch Plat +4s)
-Fuel pressure test had 30-32 PSI while running and went up to 38-40 PSI w/ ignition off
-I've shot carb cleaner around the intake, no changes in idle seems like the gasket is fine. It was replaced @ ~100k miles before.
-I've looked around for obvious vacuum leaks but came up empty.
My dad later said he thought the spark plug to cylinder 1 looked more burnt than the other plugs. Could this mean the injector for cylinder 1 is clogged or needing replacement?
Any help is appreciated.
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New User
Joined: Jun 2013
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Thanks for the responses. I'm going to double check all the vacuum lines including the hard ones to look at tomorrow. If that comes up negative I think I'm gonna go ahead and replace the passenger o2 sensor and see if that fixes it. I'll update once this is done. Anymore ideas are appreciated!
Fleet Mechanic
Joined: Aug 2008
Posts: 1,446
Likes: 9
From: NE Iowa JD Country
the Po171 tends to mean a vacuum leak somewhere.??
Are you brakes spongy? or does your heating / AC ventilation deflect where it is suppose to?
check this out...does it seem similar?
https://www.ford-trucks.com/forums/1...and-p0305.html
Are you brakes spongy? or does your heating / AC ventilation deflect where it is suppose to?
check this out...does it seem similar?
https://www.ford-trucks.com/forums/1...and-p0305.html
Fleet Owner
Joined: Apr 2008
Posts: 24,316
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I have a 00 F150 4.2l and if you face the engine and reach all the way to the back on the left side, there's a small rubber hose that bends around 180deg into the back of the intake manifold from the PCV I believe. I had that problem and finally i could hear hissing.
Lead Driver
Joined: Sep 2006
Posts: 7,907
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The code 171 is set when the long term fuel trim table for bank 1 has reached it's limit for fuel correction.
I would use s Scanner to look at both trims to see if bank 2 is also shifted and by how much that has not set a code yet..
This will tend to tell you if the problem is only affecting bank 1 and narrow down the possible causes.
Look at the Ox Sensor activity for that bank as compared to bank 2.
PCV is common to all cylinders as to a possible cause.
The EGR could is some cases be a cause due to the way it adds exhaust back into the intake on 6 ports that is different than on a V8 adding it back common into the top of the intake manifold. For example if 3 EGR ports were blocked on bank 2 and only bank 1 are open this causes a large imbalance.
Exhaust does not burn the second time and can lean out the A/F mix causing the fuel trims to richen up. A tip to this is the ignition timing tends to advance as backup data indication.
You have to go beyond just replacing parts and see what the situation is, then act on that data.
Good luck.
I would use s Scanner to look at both trims to see if bank 2 is also shifted and by how much that has not set a code yet..
This will tend to tell you if the problem is only affecting bank 1 and narrow down the possible causes.
Look at the Ox Sensor activity for that bank as compared to bank 2.
PCV is common to all cylinders as to a possible cause.
The EGR could is some cases be a cause due to the way it adds exhaust back into the intake on 6 ports that is different than on a V8 adding it back common into the top of the intake manifold. For example if 3 EGR ports were blocked on bank 2 and only bank 1 are open this causes a large imbalance.
Exhaust does not burn the second time and can lean out the A/F mix causing the fuel trims to richen up. A tip to this is the ignition timing tends to advance as backup data indication.
You have to go beyond just replacing parts and see what the situation is, then act on that data.
Good luck.
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