Hi guys, just thought i would share my experience with the P0171 and Restricted performance issues.
I purchased my STR over 3 weeks ago and had a recurring Engine Malfunction light and an intermittent Restricted Performance message. I checked most vacuum hoses and tightened some connections, the malfunction light disappeared only to reappear the next day after i took car to have Air Con recharged and assumed that something had been disturbed (Vac hose).
Had the code read at the garage again as P0171 and was a little stumped so considered it to be a possible gasket. Cleaned the MAF which was quite dirty and replaced the stock Air Filter with a K&N lifetime unit but still the fault persisted. The mechanic mentioned to me that the exhaust was blowing slightly so got under and found 2 holes in the back box which i sealed with some Metal repair compound and hey presto the malfunction light has disappeared! Its been fine now for about a week.
Spoke to mechanic again and he said its possible the lack of back pressure in exhaust was causing the problem. What do you guys think?

 

I am confused?? Do you mean the muffler behind the rear wheels? This makes no sense? I have driven my STR without any mufflers at all and had no check engine light.

The car has no exhaust back pressure sensor.
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Well i am too then! When i left the garage with the advice regarding the exhaust ringing in my ears, i had the mechanic reset the codes and it was clear for a few mins then light came back on after anout a few hundred yards.
I got some of that steel putty (cant remember exact name) and filled the hole on back muffler plus a smaller one where pipe meets back muffler, left over night to harden and then reset the code again with my own recently purchased ProScan meter (£30 approx) and for the last week it has been clear with no MIL since.
Im not sure if this is the cause but took my mechanics advice as to it being a possible cause.

 

You meen you filled the drain holes in the mufflers to drain condensation????
Its purely coincidence, first because the ecu looks at the 02 sensors(upstream) for air to fuel ratios. And based on tables in ecu and what goes it and out it determines the amount of fuel to add or subtract to maintain afr's. If its adding fuel you have a vacuum leak. Subtracting and you have a dirty air filter, dirty leaking injectors, too much fuel pressure etc...

 

I dont think they were drain holes, from looking at the exhaust back box it appears that the car has had a rear collision at some time and the exhaust was damaged. the holes i am referring to were rust holes and located on the side of the muffler and at the joint where the pipe meets the back muffler not lower down where the drain holes would be located, it was clearly blowing and i could feel the escape of the exhaust gases if i held my hand in the area. (I am deaf so rely on feel and vibrations a lot more than the average person!)
Perhaps as you say it was coincidience but I'm happy now that the MIL light has finally gone for first time since i bought the car and the performance feels much more consistent.
I will add that i added some Injector cleaning fluid to the tank a few hundred miles ago so perhaps some dirt was dislodged from the injectors at the same time?
Im an Electrician by trade and used to dabble with cars years ago including rebuilding a Rover V8 and a Mini 1000 motors plus numerous Motorcycles, but for the past 20 years or so was too busy running my business and raising my family! I am quite confidient in what i do (none of the 'thats impossible' attitude! and anything is possible if I set my mind to it!.)
I would also add that the ProScan V580 scan tool that i used to reset the MIL has proved really useful as it offers the live datastream which was really useful in monitoring how the various sensors were functioning.
I have now just taken delivery of an AutoEnginuity system (for £170 approx) so i will be ready for the next problems that crop up, it has already proven its worth with identifying a Air Bag sensor fault so i look forward to the next problem (without tempting fate!)
Onwards and upwards....