Short term fuel trim

[petervivian]

Hi, everyone. Happy holiday!

My 05 Base got the P0171 code again. The temperature has been around 34 F recently. When the engine is started, the STFT will gradually go up to +20% while the engine idling around 2000 rpm. Several minutes later, the engine speed drops to 900 rpm and the STFT goes down to -5%.

I have clean the mass airflow sensor and throttle body, changed the PCV valve, and inspected the PCV hose recently. I am going to change the intake manifold gusket. I haven't checked the two oxygen sensors yet.

Is there anyone kind enough to explain the mechanical reason behind my observation? Thank you very much.

[korax234]

When the engine is cold it needs more fuel, that's why it's adding 20% more when it's cold and
at 2000rpms. That sounds a little high to me. but the -5% means it has to much fuel and it's taking
some out. But -5% isn't even that bad.......

Typically the code P0171 is set when either short term fuel trim (STFT) or long term fuel trim (LTFT) adjustment exceeds a certain value (usually +25%).

You probably have a vacuum leak somewhere. Check all the hoses on your intake. Or spray down your intake / intake manifold / and
other hoses with WD40 and see if your RPMS go up if they do that's where your leak is.