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Negative Fuel Trims at Idle


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  • Member For: 17y 10m 3d
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  • Location: Central Queensland

I had a similar issue, which turned out to be a damaged fuel reg diaphragm. Fuel was leaking through while under vacuum (Idle) and entering the plenum through the vacuum line.

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  • Puff
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  • Member For: 9y 1m 27d
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  • Location: South Australia

I think it's worth pointing out that the title of the thread is incorrect. The car isn't actually rich at idle, it's pulling fuel out via the trims and it's only 10%, not over 20/pegged. In the original post op says that the afr fluctuates between 0.96 and 1.04 at idle.

 

A busted reg would mean that it goes lean under boost, as there is no increase in fuel pressure.

 

It'll still be interesting to see the whole picture once El Andrew goes over it all.

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  • Member For: 8y 11m 5d
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I think it's worth pointing out that the title of the thread is incorrect. The car isn't actually rich at idle, it's pulling fuel out via the trims and it's only 10%, not over 20/pegged. In the original post op says that the afr fluctuates between 0.96 and 1.04 at idle.
 
A busted reg would mean that it goes lean under boost, as there is no increase in fuel pressure.
 
It'll still be interesting to see the whole picture once El Andrew goes over it all.
It is a good point. Keith/Rab if you can be bothered, negative fuel trims at idle word be a better description for the title.

What pisses me off is than I/mechanic/Toyota can't find the positive LTFT cause in the Kluger!!

I think I should just average the trims of both cars and be happy.
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topic title changed
Thanks - will be more useful now if someone else has the same issue. Thanks for removing our friend's post too!
@Puffwagon yeah tried that - smoke test says no. Lean fault codes on both banks, electrics tested, upstream O2 sensors changed, vac lines replaced. The intank fuel filter is due for replacement (which requires seat removal) so planning to check the fuel system out fully then. Is a dead head setup so reg is in the tank as well.
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  • Puff
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  • Member For: 9y 1m 27d
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20 minutes ago, El Andrew said:

The intank fuel filter is due for replacement (which requires seat removal) so planning to check the fuel system out fully then. Is a dead head setup so reg is in the tank as well.

That's where I'd be looking.

 

53 minutes ago, El Andrew said:

I think I should just average the trims of both cars and be happy.

There is every chance that this will fix the issue!

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  • 2 weeks later...
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Well, I'm going to cautiously call this one fixed. Fuel pressure tested fine on Tuesday, but the mechanic did a very thorough check over the car and a bunch of electrical testing.

Found the downstream O2 sensor voltage was all over the shop. He reckons that most modern cars do use the downstream one to either a) correlate with the upstream sensor and adjust if there are variations and b) make adjustments to keep the cat operating conditions correct. He suggested replacing the sensor.

Checked afterwards and the second O2 sensor is still active in the custom tune that was in the car when I bought it. Given the aftermarket cat probably fluffs the readings I've just switched it off as most people seem to do anyway.

Fuel trims have been almost perfect ever since. Obviously fluctuates a bit as you drive but settles down close to zero at steady cruise and idle. Also I'd noticed that when stationary the car would sometimes just catch a near stall, or would actually stall when moving from N to D. That has gone now.

Also the mechanic had a HiAce with a turbo 1JZ.

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  • less WHY; more WOT
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  • Member For: 15y 11m 18d
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haha well done... figured there was a good reason most tuners turn off the downstream sensor :couch3:

 

pics of hiace with 1jz-t?

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Sorry no pics!

Yeah I assumed the switch off was always to prevent a CEL due to cat behaving differently. I didn't think the sensor actually played an active role in engine management. Doing some reading though suggests I was believing a common myth.

Proof is in the pudding though I guess as it definitely made a difference!

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