Code PO171, PO174 Help

[Lazzman]

Just got back from Autozone, they ran a code check on my vehicle and it came up PO171 & PO174.

Vehicle shows no drivability issues except when at idle. When the engine is cold the idle barely holds at 500rpm and the engine is struggling to hold idle. When the engine is warm the idle is higher than it should normaly be around 1200rpm in park and 1000rpm in drive. At times the rpm's have climbed and kept climbing on their own in park, until I turned off the engine at 3000rpm.

The guy at Autozone said it is usually a MAF sensor when both of these two codes go off. He also said it could be bad oxygen sensors.

Can someone give me some advice before I end up spending a ton of money on parts I can't afford right now.

Both heated Oxygen sensors were replaced 25k mi ago, MAF was replaced with Motorcraft unit at 100k mi.

What puzzles me is why would a bad MAF sensor cause the idle to go out of wack and not leave any drivablitiy issues?

I replaced the DPFE sensor on Sunday after I was getting surging and threw a code PO402, all was well until the newest codes came up today.

 

[2000StreetRod]

Loose vacuum line

Those codes are both banks lean. If you didn't have them before replacing the DPFE sensor I suspect that you knocked a vacuum line loose or broke one while replacing the sensor. Check the vacuum line that goes to the EGR valve to make sure its still connected. There's a vacuum line that runs next to the DPFE sensor and goes under the battery that connects to the fuel vapor management valve.

 

[Lazzman]

I'll check it out now, thanks Streetrod.

The idle problem was there even before the dpfe was replaced though.

I am thinking it is a vacuum line leak somewhere.

 

[2000StreetRod]

leaking intake manifold gaskets

If you had the problem before and you don't have any loose, broken or disconnected vacuum lines then you probably need to replace the upper and lower intake manifold gaskets. It's a common problem on the SOHC V6.

 

[Lazzman]

I replaced the upper O ring gaskets about 50k mi ago.

I did replace the fuel filter last week with a Napa fuel filter wonder if I installed something incorrectly. Have done this before with no isssues, pretty sure I installed the filter in the correct direction and put the hoses in tight enough.

How would I know if I did the fuel filter wrong?

Went over the vacuum lines thoroughly and no problems.

Autozone codes pulled said it could be caused from low fuel pressure?

 

[2000StreetRod]

flow direction

The fuel filter usually has an arrow on it showing the direction of flow. That is easily checked.

 

[Lazzman]

Ok, I will check that.


It is definately only with the idle, so to me it seems bizare. More like an IAC issue than a MAF of fuel issue.

On initial start up after sitting over night it struggles to hold idle. After it is warmed up idle is to hight at around 1200 in park and 1000 in drive???

 

[2000StreetRod]

Clean the IAC valve

Have you recently cleaned the IAC valve? If not, I suggest that you do. Sometimes cleaning will help. The IAC valve is an electro-mechanical device that eventually fails.

I've adjusted my throttle plate stop screw so the engine will idle (at a lower engine speed) when the IAC valve isn't working. Then when it fails my engine won't die until I have a chance to replace the IAC valve. There's a link to the process in My Helpful Threads.

 

[Lazzman]

Great Advice I am going to clean it today and see what happens, I know it couldn't hurt.

Appreciate the advice streetrod

 

[Lazzman]

Cleaned IAC valve and replaced MAF sensor. Ran better but still idleing a bit high.

Check engine light came on after about 60mi of driving.

I recently put in a new Screamin Demon coil, the last one I had went bad, maybe this one is bad also? Though I am not getting any misfires, the truck just seems to be running a little rich.

Other possible area's could be bad heated oxygen sensor or clogged fuel injector.

Engine runs a bit better with the new MAF but that did not solve the whole problem.

Where to turn next?

 

[Lazzman]

Went back to autozone and they pulled the same codes, PO171 & PO 174, started by changing the MAF but codes came back.

Confused as to where I should go next. The ticket stubs I got at autozone said check for clogged fuel injector or Heated oxygen sensor, the oxygen sensors were replaced two years ago and I find it hard to believe they both would go bad at the same time.

What else could cause both these lean fuel conditions to happen?

 

[2000StreetRod]

Lower intake manifold gaskets

My lean codes were mainly due to leaking lower intake manifold gaskets. The upper intake manifold gaskets were only slightly leaking.

 

[Lazzman]

Oh boy that stinks for me, bigger job than I can handle.

I am a little skeptical though because I had the 00m12 done about five years ago and had the upper intake gaskets replaced with the new Motorcraft ones about three years ago.

It seems odd that they would fail again so soon.

I did recently change my fuel filter and it is going in the correct direction, wonder if that has something to do with it? Before replacement, there were no check engine codes.

 

[BrooklynBay]

Replace the spark plugs & cables.

 

[Lazzman]

Really for a lean code? Sounds like a plan but I am not getting any misfire.


No real driving issues. The only thing that is out of the ordinary is that the truck will not idle correctly which sounds more like an IAC valve issues- but there is no code for that showing.

After sitting over night, on first start up it struggles to hold idle, then once she warms up the idle is higher than normal. It points so much to a vacuum leak but I have checked and checked again with no evidence. All vac lines are secured tight and no leaks visible or audible. Only Codes that keep coming up are PO171 & PO174.

What would cause the idle to such out of sync things? I believe if I can find what is causing the idle problem- I can solve the lean code problem.

I am going to replace the fuel filter again, as the problem seemed to occur when I first did this.

 

[cjmedina]

Do A vacuum test off the brake booster hose.

If you have that much interruption at idle you might have a vacuum leak and It's probably the lower intake gaskets.

Mine were replaced twice already. The bottom ones were flat and leaking bad.

Replace both upper and lower. The gaskets are cheap. You could do it in less then an hour.

CJ

 

[2000StreetRod]

. . . I am a little skeptical though because I had the 00m12 done about five years ago and had the upper intake gaskets replaced with the new Motorcraft ones about three years ago.

It seems odd that they would fail again so soon.

I did recently change my fuel filter and it is going in the correct direction, wonder if that has something to do with it? Before replacement, there were no check engine codes.

If you've had the 00m12 kit installed then I doubt your problem is leaking intake manifold gaskets. Also, leaking gaskets usually result in an engine flare at cold start instead of struggling to idle.

I suggest that you check your coolant level and the engine coolant temperature sensor. If the coolant is real low then the sensor won't work correctly. Or, the sensor could be faulty and reporting a warm temperature when the engine is actually cold. That would result in a lean condition. There's a resistance vs temperature table in one of BrooklynBay's useful threads.

 

[Lazzman]

That sounds good Street, I will check it out. I did have the coolant temp sensor replaced about two years ago and the coolant level is good, though I do have a small coolant leak around the water pump area.

Thanks to everyone for all of the great suggestions so far, I am going to chase this problem down eventually.

How do you check vacuum off of the break booster?

Is there any other way to check and see if I have a vacuum leak somewhere? I already checked all of the vacuum lines and listened for a vacuum leak but could not find or hear anything.

 

[cjmedina]

You'll need a vacuum gauge. Carefully pull the hose from the Brake Vacuum Booster and install the gauge at that point. Follow the instruction for the gauge. It's a quick and simple check. I had both those codes and found that the lower intake gaskets were leaking AND they had been replaced before! There are still plenty of other spots that vacuum can leak. Did you check all the hoses to the upper manifold? Did you check all the lines to the PCV?

 

[Lazzman]

Yes, I did check to see that all hoses were connected and nothing was leaking.

I agree that it could be the upper and lower gaskets leaking but also feel that the upgraded replacement gaskets should have lasted longer than 5 years. Not an expert car mechanic but I can get around ok, I would replace the gaskets if it were simple to do, but I am a little leary of taking on the job and not being able to finish it.

One thing to me that points to it being a mechanical type failure like intake gaskets is that at the very first turn of the key, after the vehicle has been sitting overnight- the engine fires up and goes into immeadiate stall type idle mode. Once it is warm the rpm's are higher than normal. I've found that with sensors and such the vehicle usually takes a few minutes to falter.

Had the leaking gaskets before and it was a little different than this, I believe I remember the engine reving up and down concstantly on first cold start, thats when I took it to Ford for the 00M12 TSB.

Thanks for the advice though.