Hello,
I've been getting the P0171 & P0174 code for a couple of weeks now. I have checked the PCV line that goes to the rear of the intake and not found any cracks in the line or holes in the fittings. The truck get's poor mileage, but otherwise runs well. I'm assuming it's a vacuum leak, but can't seem to find the source of it. Are there any other areas prone to this?

 

Any rubber parts connected to the intake manifold, intake tract between the mass air meter and throttle body, the dip stick tube or stick seating, loose bolts at any point, intake gaskets etc.
It's there place.
Good luck.

 

You can also try to replace the fuel filter and clean the MAF.

 

You said you checked the PCV line - did you actually pull the rubber elbow off the back of the intake and check it carefully?

 

The fuel filter has been on my list for a while and cleaning MAF won't hurt. I'll check those out tonight.

I did remove the PCV assembly from the vehicle and inspec. I didn't detect any holes or cracks in the plastic tubing.

I'm not sure i follow your points. What is stick seating?

I'll also note that since it has gotten cold here in Chicago, I do have the "whistle" sound usually when my truck is warming up in the first 5mi. I read that this is the rear bearing in the alternator, but I just had it rebuilt and rechecked with no issues found. Could this be an vacuum leak somewhere?

 

It's not the plastic tubing that goes bad, it's the RUBBER elbow that connects the tubing to the back of the intake.

Dipstick seating - in the tube.

If you are getting lean both banks AND hear a whistle, you definitely have a vacuum leak somewhere.

 

When this happened to mine, it turned out to be the DPFE sensor, the driver side of the engine, 2 rubber hoses and black box (sensor) connected to the EGR tube. It is something to look into along with the PCV's as mentioned.

 

I meant holes in elbow or crack in tube. I removed assembly and did not see issues. Rear elbow was pliable.

 

Sounds like it's time for a smoke test.

 

I did a fuel filter, MAF and replaced PCV line tonight. We will see how mileage is tomorrow.

 

Only made it 1 day before the P0171/P0174 codes came back. MPG does appear to be poor, but I haven't checked it yet by refuelling. Otherwise the truck runs smooth. What would the symptoms of a bad DPFE or plugged EGR be?

When I was replacing components last Monday, I did notice that the dipstick tube was up and I reseated it.

 

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I will check tonight, but I recall between 650 & 700 RPM

 

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The EGR system would set a code in the 4xx range.
If you don't have that code range , you don't have EGR system issues.
If there is an EGR code, it to will affect fuel milege.
.
The 171/174 codes (both banks) are the result of the fuel tables being shifted 'rich' out of limits due to an 'unaccounted for air' leak and does impair fuel mileage.
A look at the fuel tables with a Scanner would verify the tables are out of limits. 'Only' when these tables are out of limits are the 171/174 codes set.
.
Normal idle is 750 +/- 50 rpm.
Only reason for idle to go out of range high is either the IAC stuck open or so much air from a leak that even with the IAC closed down, the extra air causes the idle to rise uncontrollably.
.
Low idle is a inoperable IAC stuck closed.
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The throttle plate 'stop' setting is part of the idle circuit and is never to be adjusted.
If gummed or blocked around the bore to plate area, idle can be affected.
You cannot 'set' idle othwerwise because it is set in softwere tables and is fixed except for internal shifting from cold starts, A/C , power steering loads and alternator load corrections as they occurr dynamically during operation.
You have an air leak yet to be found.
Good luck.