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Discussion Starter · #1 ·
Hey Guys,

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.
IMG_5413.jpg


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....

My car:
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!!
 

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07 Exige S
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Gosh man, I would seal that hole up somehow and see if that code comes back. You were spraying directly in it and it was immediate the first time. Not saying that is your main or only culprit but it is certainly one of them!

If your going to replace that intake gasket, maybe look into one of those phenolyic barriers
( hopefully spelling right)

Also you will get your tshirt in the mail when you get that pesky back, I don't remember if it was bolt or nut off the manifold, but hard to get to that little bastad.
 

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In all of my years of working on cars (and airplanes), I've found that if a vacuum leak can cause whatever problem the car has, a vacuum leak, typically, is the cause.

That being said, I noticed in the service manual that it checks for the fault continuously when engine speed is > 1100 rpm (and other criteria are met). In your description, you mentioned that it set the fault when you idled the engine for about 20 minutes. During the 20 minute "idle" is the engine staying under 1100 rpm or were you just generically saying idle and maybe had some excursions above 1100 rpm? I'm just checking to see if the fault is tripping properly.

You don't mention any other symptoms but have you checked your idle fuel pressure? Typically low fuel pressure problems have many symptoms besides a lean trending fault, but checking fuel pressure is a lot easier than changing intake gaskets. How long has the fuel filter been installed?

Just my thoughts.
 

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Discussion Starter · #4 ·
Hi Motard and Jetblast,
Thank you for your response.

Motard, Ok, I sealed the thread hole for intercooler mounting bracket and tried again.

Jetblast, My OBD reader does not have fuel pressure gauge (iphone version of Dashcommand), but I had uprated fuel pump and uprated fuel injector when I installed BOE reflashed CUP255 ECU. So I presume my fuel lines are OK. But How can I check fuel pressure is OK, you said it is easier. Do I need special tool for fuel pressure check?

You are right, Service book says the criteria is 1100 RPM MAF 6g/sec, Inlet air temp -10C (?) >almost impossible to create such

see this video, I tried maintain RPM 1000 above, STFT goes up and down and go beyond +17%.
In my case, at idle STFT sticks to 29.7%, occasionally up and down. This symptom became obvious after after I drove Spa Francorchamps and Nurburgring last week.



This result I got was this again, great!! :clap:
IMG_6429 s.jpg


this is the video that I sealed the thread hole for intercooler bracket. see at around 3.50, engine stalled when I sprayed around SC outlet manifold....


In any case, my engine is reacting when I spray, so there must be somewhere air intake leak.
I suspect in the below, No. 8 O ring is damaged or sealing between SC and upper outlet (No16) or could be intake manifold gasket.

SC.jpg

I think next step would be talking to local Lotus shop.... change all the inlet gaskets and seals....

any comments, suggestions, questions are welcome!
 

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Honestly man, I would just tear into it. You can have it all off, cleaned , sealed and back on in a weekend easy, and that is being ocd about it. I say this, but then I would be taking the clam off to do all that myself with more leisure.

Will give you an opportunity to look at... blah blah blah. heh
 

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I had been chasing P0171 for about a year. Smoked tested, replace O2 sensors, swapped throttle and idle valve. It ended up being a bad mass airflow sensor. The readings were fine at idle but were off when throttle opened.
 

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Discussion Starter · #7 ·
I had been chasing P0171 for about a year. Smoked tested, replace O2 sensors, swapped throttle and idle valve. It ended up being a bad mass airflow sensor. The readings were fine at idle but were off when throttle opened.
Mine is opposite, when idle STFT is off, but when run it is within the correct range.
 

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I agreed with your diagnosis @exigeams, there's a leak in the intake somewhere. I'm happy that I can tilt my clam on a hinge so doing a repair like this would be pretty easy, but sadly mine is NA for now.

Good luck and let us know what they find when it's repaired.
 

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Discussion Starter · #9 ·
I agreed with your diagnosis @exigeams, there's a leak in the intake somewhere. I'm happy that I can tilt my clam on a hinge so doing a repair like this would be pretty easy, but sadly mine is NA for now.

Good luck and let us know what they find when it's repaired.

Yes, I will report again once repaired. Thanks!:smile2:
 

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Discussion Starter · #10 ·
I finally found the root cause of my P0171 case!
Local Lotus garage founds a crack on my intake manifold, from this crack engine was sacking extra air.
The Lotus garage said one of my SC bracket bolt is gone and made a new thread for it.

I think this happened because of stiffened e/g mounts, and Nurburgring bumpy track where I go a few times a year in the past 5 years....

I installed new intake manifold and my engine idle and the short term fuel trim is stable -no more +/-17%.
Now my engine running fine and feeling so good!!

My Exige is a track dedicated car and everything is tighten up including Innovative race grade e/g mounts.
So if you guys have SC cars and stiffen your e/g mounts and track often, be aware of manifold cracks.

i
IMG_6101[1].JPG


IMG_6112[1].JPG

IMG_6108[1].JPG
 
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