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-100% fuel trim super lean


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Gooood morning lupo people! 1st post!

To the nitty gritty.

Cars mot is due, CEL is on due to a lambda heater issue (diagnosed) so, purchased a replacement NGK one and before fitting checked the codes, threw up a couple MAP, EGR and such so decided to give the map a clean.  Come out in a million bits didnt it so new one of those. Replaced lambda and the map sensor and broken vacuum lines, fired it up, runs like bag of sh**!  So check codes, red lambda heater so clear that and it doesn't come back.  Still running badly check over everything find new vacuum leak so tape that as a temp job, go back to the diagnostic and theres a new cel, p0171 system lean lambda voltage is 0.03v so yeah its lean, spent 3 hours hunting everywhere for any leaks and can't hear a hiss, feel a leak or anything.  

So, now I have fixed all the vacuum leaks and given it new sensors its telling me its lean af, its also telling me that the short term fuel trim is -100% at idle... so the ecu is telling me that its yanking out all the fuel?

I changed the plugs before fixing vacuum leaks and changing everything and they were normal, the lambda was a little sooty but not super black indicative that its super rich..  so fixing vacuum leaks and giving it new sensors has broken it essentially.

Any ideas?  Going to pull the EGR tonight clean that out and give the throttle body new gaskets.  After that I am lost!

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What readings are you getting from the temp sensor? Sounds like multiple faults TBH.

Assuming all the sensors are ok, the ECU is simply delivering the amount of fuel it should. So could be something else, like fuel pump, fuel pressure regulator, fuel filter, that sort of thing. Easy to prove though- does the car pull ok on the road at high revs in high gear? That'll eliminate a fuel supply problem. If it's only lean at idle, then something else... VCDS will give you all the measurements. Do it cold then hot.

Welcome to club lupo :)

 

Edited by mk2
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3 hours ago, mk2 said:

What readings are you getting from the temp sensor? Sounds like multiple faults TBH.

Assuming all the sensors are ok, the ECU is simply delivering the amount of fuel it should. So could be something else, like fuel pump, fuel pressure regulator, fuel filter, that sort of thing. Easy to prove though- does the car pull ok on the road at high revs in high gear? That'll eliminate a fuel supply problem. If it's only lean at idle, then something else... VCDS will give you all the measurements. Do it cold then hot.

Welcome to club lupo :)

 

Hey thanks for the welcome. 

I'm using torque Pro for this.   But the coolant temps seem correct (fan clicks on at 90deg idles around 80-85) the intake air temps seem ok as well.

This is what I find weird, short term fuel trim is -100% yet it shows a fuel lean code?  It knows its lean but it isn't adding more fuel?  Could it be possible the long term fuel tables have been so screwed up by the fact the vacuum system was essentially cheese cloth?

Quick edit: forgot to mention that the car has no mot but revs up pretty good, the lambda voltage raises when its being reved to a more normal level (1v) but drops back down at idle.  Im going to pull apart the egr system tonight and blank it off see if that helps

Edited by Capt.Perky
forgot info
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