Gen 2 V8: Cold weather lean codes


Dedicated LVC Member
Aug 5, 2017
Reaction score
Camarillo, CA
As the weather just starts to get colder (under 60), I get a CEL pop up with P0171/P0174 codes. What happens is that when I start the car up and monitor my short term and long term fuel trims (STFT,LTFT), they go up to 25 but only when I am idling, when I am accelerating and driving, the STFT and LTFT are +/- 5%. Once the car is fully warmed up, about 20 mins or so, both fuel trims are right where they should be...under 10%. When my CEL went on the other day, it went off again later in the day after 4-5 engine starts as the day warmed up.

I have had this problem before and can read all about what I have done in these posts from fuel filter change, all the vacuum lines, and many other replacements:

'04 V8 P0171/P0174 codes
P0171/P0174: The codes of frustration!

I also got some input in the recent Gen2 Intake Manifold gasket thread related to the gaskets between the throttle body and the intake manifold. I haven't replaced that gasket and the part number on 06 workshop manual is 9F809 but 9E936 comes up on line.

Let me know if anyone has any epiphanies and I'll keep you all posted. What's most frustrating is that the car is actually running fantastic other than running a bit lean.
I think I may have found the issue. This morning on first start up, I pulled out my propane bottle and pointed it around all the vacuum areas. I monitored my STFT and LTFT as I moved it around. I put it right up next to the intake manifold on the drivers side by the fire wall. After a few seconds, the STFT went from +25 down to - 10 - 15.

So, it appears I have a slight vacuum leak back there at the intake manifold. I was pointing it right where the plastic part meets the metal part of the manifold.

At least now I have some idea of where the problem is. Now, it is getting those small gaskets!!!

Members online