LTFT at idle is around positive 10% - what can be the issue?

[crok]

Hi,

I have a theoretical question:
Let's say there's a VW car, naturally aspirated engine, no MAF, no turbo.. just MAP.
let's say a simple 1.4 or 1.6.. does not really matter. No codes, no driveability issues.
So the car runs normally - this is only a theoretical question.

The idle OBD2 LTFT is around 10% but it goes down to around 0% at constant load (cruising..).

How come LTFT can be 10% at idle but 0% at part throttle? Again, no MAF, only MAP.
How can I rule out possible issues?

[Rob69]

Vacuum leak at idle, leaning the mixture and ecu compensating by adding +10% when manifold depression is greatest, lessening as throttle is opened and manifold depression reduces. Just a theory though. Could be coolant temp sender thinking the engine is cold and givng rich mixture at idle, or pre cat o2 sensor exhaust leak drawing in air.

[crok]

Vacuum leak at idle, leaning the mixture and ecu compensating by adding +10% when manifold depression is greatest, lessening as throttle is opened and manifold depression reduces. Just a theory though. Could be coolant temp sender thinking the engine is cold and givng rich mixture at idle, or pre cat o2 sensor exhaust leak drawing in air.

Thank you for your answer!

I already changed the throttle body gasket and tried to identify any intake leaks by a smoke machine and a brake-cleaner spary methods (no issues found) and I thought that the MAP-only systems are less "sensitive" to thus I have no idea why it is around +10% at idle. So yes, we share the same theory anyway, an intake leak was my first thought as well but the MAP readings are exactly the same as it was 3 years ago - and in a MAP-only system the MAP should report less vacuum if there is a leak but I sprays the whole throttle body and the whole intake manifold around but not even a small hesitation has been registered during the test and smoke machine test was also negative. Just for safety's sake I already tested the EGR seals and the EGR valve as well but no luck.. yet.. it is not crucial but I just want to know the reason. (LTFT is okay (around 0+-2%) until the ECU registers coolant temp sensor values above 70°C (sensor has been changed last year).) A bit strange.

[Rob69]

I had similar LTFT issue on a 1.4 polo, ran like that for years and I never got to the bottom of it either.

[FLAPPERJACK76]

Check oil filler cap seals, if its 16v check the lower neck on the oil filler is 100% tight.

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[crok]

Check oil filler cap seals, if its 16v check the lower neck on the oil filler is 100% tight.

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Thank you for the idea! Already done. No luck. Some mechanics in the local dealership have said to me that this engine (AZD 1.6 16V) cannot let in unmetered air via the oil filler or dipstick holes, the inner architecture is different.

[FLAPPERJACK76]

Thank you for the idea! Already done. No luck. Some mechanics in the local dealership have said to me that this engine (AZD 1.6 16V) cannot let in unmetered air via the oil filler or dipstick holes, the inner architecture is different.

Im thinking that this theory is wrong. If you were to leave the oil filler cap half tightened then the ltft would increase on positive as it would be compensated for a weak mixture. I could only prove this if i had an Azd here with me. What is your stft readings. A ltft can be a fairly course measurement. Is the brake servo vacuum pipe leak free.

For a suspected vacuum leak, note the fuel trims at idle and increase engine speed to 2500 RPM and hold. If the STFT immediately decreases and moves to acceptable levels and the LTFT slowly starts to come back down, you have a vacuum leak.*
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[crok]

Im thinking that this theory is wrong. If you were to leave the oil filler cap half tightened then the ltft would increase on positive as it would be compensated for a weak mixture. I could only prove this if i had an Azd here with me. What is your stft readings. A ltft can be a fairly course measurement. Is the brake servo vacuum pipe leak free.

For a suspected vacuum leak, note the fuel trims at idle and increase engine speed to 2500 RPM and hold. If the STFT immediately decreases and moves to acceptable levels and the LTFT slowly starts to come back down, you have a vacuum leak.*
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I tried to spray brake cleaner everywhere, literally everywhere, even around the dipstick and tried to "inject" butane from a butane pen all around the whole engine, every single part, intake manifold, dipstick, oil filler, brake booster and it's hose, the EVAP canister and all it's hoses.. everywhere.. RPM is steady, not even a little bit of change (to make sure it is really butane I let the engine breathe some via the throttle body and that place is the only place where the RPM changed.. STFT is at idle is moving up and down +-5. The signs you described are clear to me (classic signs of a vacuum leak) and yes it seems somewhere I have a leak but I have no more idea where.. : |

[FLAPPERJACK76]

I hear you. Used to hate these faults year's ago way back when we used vas pc and guided fault finding. The test plan was huge. We used to have vag leak detector spray at £45 a can and it was no bigger than a can of deodorant. Once you have exhausted yourself looking for an air leak consider fuel delivery, injector spray patterns, exhaust restrictions o2 sensors and catalysts. High positive fuel corrections can be traced to MAF and O2 sensor faults, vacuum leaks from intake gaskets/hoses, unmetered air (intake snorkel leak), clogged or dirty fuel injectors, fuel delivery issues, and exhaust restrictions such as a clogged catalytic converter.

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[crok]

Thank you, I will go through the list. Again.. Except the MAF, this engine has no MAF (: lot's of stuff are already new, but who knows (: