I read all the P0171 related threads, but no one really gave root cause of the problem.
P0171 is not easy to detect the cause because it is the mix of various factors.
I have been chasing my P0171 case almost 6 months now with step by step actions, and finally I caught something interesting in action.
In my case P0171 is coming back almost every time when I turn on engine.
My symptom at idle;
water temp 88c.
Short Term Fuel Trim (STFT) +29.5%.
+29.5% stays during idle. Idle is stable at around 870 - 900 rpm. STFT is definitely high at idle, but when I drive, STFT seems OK within +/-17% (according to Lotus service book, if it goes beyond +/-17% it throws P0171 engine check lamp) , but some time it jumps up/down beyond +/-17%.
When I leave the car with idle for 20 min. check lamp comes on, - this is annoying. So I thought there must be a intake leak.
But I am not 100% convinced this could be the root cause of my P0171 case.
Appreciate any of your comments and advise.
this is my intake leak inspection set up.
see the video.
I sprayed intake butterfly area, supercharger gaskets, intake manifold gasket and female thread that holds intercooler mounting bracket. (before this video, I sprayed all over whatever intake area, including hoses. all hose are OK without crack.)
As you can see when I sprayed the thread hole, engine stalled and finally engine stopped.
But this thread hole is sealed with bolt and intercooler mounting bracket with intercooler installed.
Actually I screwed a bolt to it, I could not detect same kind of stall, but detected smaller idle instability, though....
Exige S MY2006 (originally 220HP)
ECU upgraded to CUP255 with BOE reflashed last year December (I don't think ECU is the cause , as I had same problem before CEU swap)
Reverie air box
Larini sports manifold
Larini sports cat
Larini 6 inch side exit
Catch can: smaller diameter> Radium >after intake butterfly, lager diameter > Mishimoto > before butterfly, both original hose set up.
What I have done
1) changed Pre Cat O2 sensor to new
2) cleaned Post Cat O2 sensor with brake cleaner
3) Cannot detect exhaust gas leak before Pre cat O2 sensor
4) MAF sensor cleaned with carburetor cleaner
the result after above action is STFT is +29.5% at idle.
ECU is trying to inject more fuel to maintain 14.7:1 at idle.
Again, when run, STFT seems OK within +/-17%.
Shall I try to change gaskets of intake manifold and supercharger?
Any comments, suggestions welcome!!