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Restricted Performance and how I cured it


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I have a 2003 v6 3.0 l s type.

I got the dreaded Restricted Performance message come up. At first it seemed to make very little difference to the performance except if I tried to accelerate too fast.

I then had to go on a fairly long journey on the motorway and then the warning went away. It later returned the next day and stayed on all journeys of short duration.

I started to research the fault and came across a U Tube video which explained all about STFT and LTFT. That is 'short term fuel trim' and 'long term fuel trim'. It explained about how unmetered air getting into the engine will, via the O2 sensors, change the fuel/air ratio. It will first change the STFT and then after time change the LTFT. It's this change that will trigger the Restricted Performance warning.

It then went on to explain about how the amount of unmetered air getting in, at low engine speeds, is a high percentage of total air flow but at high engine speeds is a relatively low percentage of total air flow. Thus at higher engine speeds over time, the LTFT will drop and the warning will go away.

What I then needed to look for was a relatively small air leak into the engine after the mass air flow sensor.

It took about half an hour but I found a small hole in a rubber hose on the idle air system. I replaced the hose and the warning went away. 

Isn't the interweb thingy wonderful.

I hope this helps somebody else



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air leaks is one of the most common faults as cars get older

through either pipes spitting or perishing with age

you can get cheap smoke machines now that you hook up to the inlet and see where smoke leaks from to find the problem area

well done on finding it



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