Negative Fuel Trim Reading at Idle, what can that mean? (2008 Toyota Yaris) - Drive Accord Honda Forums | radio-pro.ru
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post #1 of 28 Old 01-23-2012, 06:35 PM Thread Starter
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Question Negative Fuel Trim Reading at Idle, what can that mean? (2008 Toyota Yaris)

Hi All

A question to all gearheads of radio-pro.ru. Been banging my head for a while. This is not about a honda vehicle, but none of the car specific forums have any kind of answer, so I will ask it here. Please forgive

The car in question yields consistent negative long term fuel trim numbers on idle. Not enough for a cel. Doesn't seem to matter if the car is cold or warm. Under any kind of load the numbers are smaller and jump from negative to positive, which is something I'd expect.
And this is not the injectors. I took them off and had them cleaned and flow-tested. No leaks, only minor improvement in a spray pattern (as per the technician's observation).

Before the cleaning the LTFT was hanging around -7%. After the cleaning it's at -11%. Under load it's more or less within +-3% range.

Bad MAF? I am saying "bad" because I cleaned it already. Coincidentally the fuel mileage is not stellar. Not horrible, but I'd say 10-15% below expected. Air filter is clean, throttle body is lean, new plugs, new pcv valve...The car is very low mileage, but has always been driven short distances in the city

This article here seems to make this diagnosis solely based on negative LTFT on idle. Since I am pretty much out of ideas I am thinking about getting a refurb MAF from rockauto.

Anybody seen anything like this?

PS The car in question is a 08 Yaris.


1999 Honda Prelude, base 5 speed.
2008 Yaris Liftback, auto.

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post #2 of 28 Old 01-23-2012, 10:00 PM
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Does it have drivability issues or why do those LTFT numbers bother you? I bet with city driving the fuel economy is pretty variable, is this all of a sudden or gradually trending to poorer mileage? Sorry I don't know anything about the Yaris fuel system setup or O2 sensors but I bet there are people on this forum that do..


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post #3 of 28 Old 01-24-2012, 05:41 AM Thread Starter
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Quote:
Originally Posted by DonutLimo View Post
Does it have drivability issues or why do those LTFT numbers bother you? I bet with city driving the fuel economy is pretty variable, is this all of a sudden or gradually trending to poorer mileage? Sorry I don't know anything about the Yaris fuel system setup or O2 sensors but I bet there are people on this forum that do..
Fuel mileage is the issue, everybody and their dog seem to pull better numbers. City or not but when a yaris drop to 24 mpg you start suspecting something is not right. Now, after reading enough about fuel trims I see that consistent double digits are not right.
I tested our other car that runs in the same conditions and it shows much healthier numbers. The long and short of it i know something is off. Always been like that, but i only had the car for a few months.

1999 Honda Prelude, base 5 speed.
2008 Yaris Liftback, auto.

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post #4 of 28 Old 01-24-2012, 06:14 AM Thread Starter
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forgot to mention. i cleaned the throttle body and there were deposits...at less than 20.000 miles? base ring on all spark plugs was quite black too.

1999 Honda Prelude, base 5 speed.
2008 Yaris Liftback, auto.

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post #5 of 28 Old 01-26-2012, 05:55 PM
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You can't clean Toyota MAF sensors! They are of a film type that are ruined by cleaning.

You need a NEW one from Toyota, remans are worthless.

Air filter has NOTHING to do with fuel mileage in an EFI feedback fuel system.

What are tire pressures? Brake drag? Wheel alignment readings? How badly fuel contaminated is the engine oil?

If it's constant stop-go city driving with no constant over 50 mph running, 24 mpg isn't that bad. -7% isn't the greatest. But within fuel control. What are ST readings when it learns LT?

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post #6 of 28 Old 01-26-2012, 06:07 PM Thread Starter
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Fred, Yaris has a hot wire type maf sensor.

Tire pressures are fine, alignment is all in green (albeit rear right toe is borderline, but on the green side). Oil looks fine, level wise and composition wise.

Mostly city, yes, but it is a Yaris...I know of many people who do the same type of driving with better results. 24 just doen't cut it (I drive conservatively..heck my mother is more aggressive).

I have a nagging feeling that ltft has something to do with it...Whether it's a maf sensor or any of the 02 sensors or something else, i am not sure. I am getting the reman maf tomorrow, I will see if it changes anything (at all), if not I will return it.

STFT numbers dance around 0 and are maybe within 2-3% percent.

1999 Honda Prelude, base 5 speed.
2008 Yaris Liftback, auto.

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post #7 of 28 Old 01-26-2012, 06:45 PM
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the car is in proper fuel control if it was at -7 with trims at +2 or 3.

There's nothing wrong with it.

How long is the car run per trip/per day?

Sapphire blue '04 V6 6 speed w/ full HFP, comptech ss, momo shadow, TL-S 27.2h/20 bars, intake spacer, retro chrome MH1 HIR2 highs, hella DE fogs, EBC Ultimax, subwoofer/amp, infinity rears, HD Radio, aux adapter, navi conversion.

Vermilion 1999 SVT F150 Lightning, Sylvania/Visteon Xenarc HID headlamps, '01 L turn/marker lamps, PIAA fogs, '01 up Bilstein shocks, all Pioneer speakers, Clarion subwoofer. All Redline fluids.

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post #8 of 28 Old 01-26-2012, 06:52 PM Thread Starter
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sometimes I see ltft at as low as -11%. i understand that it is within specs...but those are +-20% which seems pretty wide. Like you said, it's not the greatest (so is the mileage ).
I suppose wrong is a relative term....I am presuming 'in-spec' and 'optimal' is not quite the same thing.

lately it's been run about every other day, i'd say 30 min per trip.

another thing....I do see a slight difference in how car acts after off-car injector cleaning. It's ever so slight, but I think I can hear more air ****ed in the intake. The way I know - the intake on these cars is quite bendy and has a tendency to sound a bit raspy under accel. I noted the point where it happens on an average and now this sound comes in a bit sooner. So, the thing was 'in spec' before and now, and yet I can feel a small difference.

1999 Honda Prelude, base 5 speed.
2008 Yaris Liftback, auto.

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post #9 of 28 Old 01-26-2012, 07:05 PM
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I just saw where you are. I'm assuming that Toronto also uses winter blended fuel, expect at least a 5% mileage loss just from the fuel.

30 minutes city driving is not enough to get oil temps up to operating temp. The oil will not burn off excess fuel and moisture in those conditions. Unless of course you're WOT for a good part of it making the engine work.

have the oil sampled, see how much fuel is in it.

What's valve lash? All Toyotas use shim adjustment.

Sapphire blue '04 V6 6 speed w/ full HFP, comptech ss, momo shadow, TL-S 27.2h/20 bars, intake spacer, retro chrome MH1 HIR2 highs, hella DE fogs, EBC Ultimax, subwoofer/amp, infinity rears, HD Radio, aux adapter, navi conversion.

Vermilion 1999 SVT F150 Lightning, Sylvania/Visteon Xenarc HID headlamps, '01 L turn/marker lamps, PIAA fogs, '01 up Bilstein shocks, all Pioneer speakers, Clarion subwoofer. All Redline fluids.

Silverstone 2003 Honda S2000, bone stock
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post #10 of 28 Old 01-26-2012, 07:26 PM Thread Starter
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yes, we definitely use winter blended gas. but I think I am losing more than 5% of mileage.

can you elaborate on fuel in oil...not sure I have a good grip on this.
don't know about valve lash either, another thing I have to get a better grasp on.

1999 Honda Prelude, base 5 speed.
2008 Yaris Liftback, auto.

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post #11 of 28 Old 01-27-2012, 06:33 AM Thread Starter
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Checked the oil this morning, it smells like gas..the level is fine, same as a while ago.

1999 Honda Prelude, base 5 speed.
2008 Yaris Liftback, auto.

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post #12 of 28 Old 01-27-2012, 11:38 AM Thread Starter
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Received a reman sensor,it acts the same as the original, so it goes back.
However, today I noticed something different (with both sensors) - under load the ltft gets positive (+7% or so). I checked my data from yesterday and it was not going that high, the max was 2.4. What's up with that...

1999 Honda Prelude, base 5 speed.
2008 Yaris Liftback, auto.

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post #13 of 28 Old 01-27-2012, 04:52 PM
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Fuel in oil, this occurs during engine operating conditions where the engine doesn't reach full operating temp. Not coolant temp, but OIL temp. If the oil doesn't get to a sump temp of at least 180F (other places will be higher) the moisture and fuel that is put into the oil via blowby past the rings isn't boiled off.

All engines have blowby to some extent, once it gets excessive, it'll show up as oil coming out a vent, usually into the intake ducting before the throttle body.

First, change the oil. And reduce oil change intervals, especially in cold conditions where the engine won't heat up fully. What oil are you running in the Yaris?

Any fuel vapors being drawn into the pcv system, during normal engine operation will cause a negative trim. Under loads greater than say 50% throttle, it should go +, some will show fixed 15% + by design or others will show open loop.

I have seen severely fuel contaminated oil cause an engine to run so poorly because the PCM pulls pulse width off the injectors due to the perceived rich running condition.

All Toyota engines iirc, have shim over bucket valve adjustment. They have no way to automatically have zero valve lash, it must be manually done. However, I've rarely ever seen a Toyota with radically out of adjustment valves, if at all. They aren't easy to do, as all clearances must be measured, and then recorded. The cams HAVE to come out. Then all shims are measured and values recorded, and NEW shims are installed in cylinders that are out of spec, with shims of the proper thickness.

It makes Honda's simple rocker arm lash screw/locknut set up work so much better.

Sapphire blue '04 V6 6 speed w/ full HFP, comptech ss, momo shadow, TL-S 27.2h/20 bars, intake spacer, retro chrome MH1 HIR2 highs, hella DE fogs, EBC Ultimax, subwoofer/amp, infinity rears, HD Radio, aux adapter, navi conversion.

Vermilion 1999 SVT F150 Lightning, Sylvania/Visteon Xenarc HID headlamps, '01 L turn/marker lamps, PIAA fogs, '01 up Bilstein shocks, all Pioneer speakers, Clarion subwoofer. All Redline fluids.

Silverstone 2003 Honda S2000, bone stock
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post #14 of 28 Old 01-27-2012, 07:31 PM Thread Starter
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Thanks Fred, a lot of good info.
I am reasonably sure Yaris takes 5w30.

The fuel in oil sounds like as a possible explanation for the trim numbers. Let me see if I fully understand it though. If the it gets pulled into the pcv system, does it end up in the mixture and is an actual rich condition? And if fuel control correctly adjusting for it what bearing it may have on mpg numbers?

Btw, the intake has no oil in it.

1999 Honda Prelude, base 5 speed.
2008 Yaris Liftback, auto.

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post #15 of 28 Old 01-27-2012, 08:07 PM
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thread moved due to not relating to honda....

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