Long Term Fuel Trim = -7% in idle?

[SpaceSputnik]

Hi All

I am a new member from toronto. I drive a 08 Yaris 4 door liftback (auto).

I have a question. I recently bought a bluetooth OBD adapter and stated playing with it.
First thing I see is that on idle, my long term fuel trim is consistently negative. -7% or so. That's with the engine fully warmed up. -7% seems within specs, but on the other hand it's not insignificant a number.

That's been consistent and filling up doesn't seem to change it. I let go of the throttle (moving or standing, doesn't matter) it instantly goes back to -7%.

My mpg numbers have been pretty disappointing too. Not saying it's related but can't help wondering.
I checked it on our other car, LTFT is not moving much past a couple of % up and down.

So, my question is whether it's characteristic of the ECM programming on that engine. I'd love to test another yaris, but can't really find one that I can get my hands on...

Thank you.

[denman]

I can't answer your questions as I'm way low-tech side of your issues, but what ARE your fuel economy figures? I'm running in around 40 (+ -) mi/Imp-gal in the cold, but stay closer to 50 in summer. Some of our intrepid techies will chime in shortly on your data, ...

[SpaceSputnik]

Originally Posted by denman

I can't answer your questions as I'm way low-tech side of your issues, but what ARE your fuel economy figures? I'm running in around 40 (+ -) mi/Imp-gal in the cold, but stay closer to 50 in summer. Some of our intrepid techies will chime in shortly on your data, ...

I do mostly city and it never gets better than 27-28 mpg (US) or 8.5 l/100 km. At times it goes as low as 24. My personal best is 33 mpg and that's when I do mostly highway. I understand about the winter gas and whatnot, but still, there are people even around my neck of the woods that get way better numbers.

I don't have a lead foot, but drive rather conservatively.

At this point, I have done the following:
1. cleaned the tb
2. cleaned the injectors...i mean really cleaned, took them out and brought to an specialized shop for ultrasonic cleaning and flow-testing. The guy observed a slight improvement of a spray pattern, but I don't see any mileage improvement. New o-rings/insulators.
3. air filter, of course
4. oil changes are at the scheduled intervals
5. new PCV valve
6. tranny fluid looks clean and pink
7. Cleaned MAF..it was pretty clean though.
8. tires are stock and are consistently at 30-32 psi
9. alignment is in the green on all angles. the rear right toe is borderline though, but the combined toe is decent.

there's barely any mileage on the car, it's below 30.000 km (20.000 mi) and it's a 2008 auto.

There are no codes, the air/fuel mixture holds at 14.7 (both measured and commanded).

one thing I noticed...before the injector service when I started the car there was none of the white steam coming out of the tail pipe. I see that on about every car in winter. But after the injector cleaning I started getting that during warmup. Kinda weird.

as far as the original question goes - I really would like to know that what I am seeing is a normal behavior for this ECM... although I don't have much hope that this line of thought will bring any results.

thank you.

[SpaceSputnik]

now, after the injector service I actually see idle LTFT even lower at -11%. seems the gas mileage is getting even worse, but I need more time to confirm that...

[ericlp]

Maybe you cleaned it too much? The last thing you could try is resetting the computer by removing the battery negative side for an hour or so. I dunno, for some reason I'd be kinda worried about taking injectors to someone that flushes them out ... maybe too much of a flush could make them go bad?

When was the last time you changed your plugs? I'd pull em and have a look to see if it running too rich...

[SpaceSputnik]

Originally Posted by ericlp

Maybe you cleaned it too much? The last thing you could try is resetting the computer by removing the battery negative side for an hour or so. I dunno, for some reason I'd be kinda worried about taking injectors to someone that flushes them out ... maybe too much of a flush could make them go bad?

When was the last time you changed your plugs? I'd pull em and have a look to see if it running too rich...

No, ltft was like this before and after.

Btw, this is a reputable shop that specializes in fuel systems. They flow test before and after to ensure the correct function.

Plugs are new. Here is the picture of the original ones (all 4 look like on the picture):



what does this tell you? the base ring is black, does that indicate rich condition?

[denman]

The old plug looks pretty good. I looked at mine, saw they were iridium type and stuck'em back in. Did you pay a ton for the replacement plugs? I read somewhere since that the original type are to be replaced exactly. Might be important, ....

[SpaceSputnik]

The new ones are NGK irridium, supposed to be a direct replacement. 50$ shipped, hardly a ton. Could've gotten Densos (oem), but I don't think it matters. Densos were about the same price.

Just for laughies, might stick the old ones back and see it changes the reading. Kinda doubt that though.

[ericlp]

after reading up on plugs I'm going with OEM's.

http://www.amazon.com/Denso-SK16R11-...owViewpoints=1

For Denso SK16R11 for a set of 4 30 bucks shipped seems like a pretty good deal for me, since napa / autozone wanted 11 bucks a plug plus tax.

My echo is pushing 80K miles, so....... I think it's time to change the plugs. Probably won't hurt. I also bought Permatex 81343 Anti-Seize Lubricant.

That pic of your plug looks good. You could probably clean it up a bit and throw it back into your car. I guess the next step would be to check the mass flow sensor (test) and if that was working ok check your fuel pump for correct outflow? I'm just guessing here..

Good Luck!

[SpaceSputnik]

MAF appears to be good...I tried it with another one, no change at all.

[Jason.MZW20]

Your plugs look pretty good. Usually the base ring is black with carbon.

Look at your O2 sensor waveforms if your OBD2 software supports it. That'll tell you if you have an O2 sensor issue. Typically, the MAF determines the initial injector pulse widths. If there's too little oxygen (rich) in the exhaust stream as detected by the O2 sensor, the ECU will command a pulse width reduction.

Generally, engines run much richer in the winter because of the lower temperatures and increased air density. So, the MAF reports this to the ECU and the ECU commands a pulse width increase. Once again, the O2 sensor detects the amount of oxygen in the stream and the ECU re-adjusts fuel trims based on that.

So, either there's really some amount of fuel that's going unburned and passing into the exhaust, or your O2 sensor has slowed from contamination and wear (or maybe a bit of both). As engines wear, they tend to lose compression and can't burn the fuel as efficiently as before. A simple vacuum gauge will tell you a host of things about the health of your engine, and I recommend doing that before throwing any more money at the issue. -7% isn't something to be overly concerned with. Yes, 0-5% is more of an acceptable range, but honestly, you shouldn't be too worried, yet.

Also, here are my 1MZ plugs for reference:


Left: NGK iridiums with heat range 6 (stock; Denso 20); right: Denso K16TR11s (OE side discharge; one step hotter for smog purposes only, hence the relative newness)

[SpaceSputnik]

Thanks Jason.

I can read only O2 1x2 voltages, the only thing I get from the ecm. It seems to be the post cat sensor, it's readings are somewhat delayed and not in line with fuel trims, i.e. i can be idling at 0.8V than snap rev it up and within a couple of seconds it falls down to 0.1V. I can however read the AFR values, they stay at 14.7 except on decel when it sometimes goes up to 18V

Someone suggested that due to my driving cycle I am getting oil contaminated with gas and because oil often does not get hot enough that gas is never evaporated. But, even after a couple of 30 min highway drives (within a couple of hours) there's no change.

I would assume that an engine that new shouldn't have compression issues, so I will the test that during the next go round. For now, I want to change the oil and while the car is on the hoist examine O2 sensors. Does that make sense to you?
In regards to that - do O2 get dirty normally and can it be visually determined?

EDIT: -7% is pretty much the best I see. I've seen ltlf at -8.6% to -11% quite a few times.

[ericlp]

hmmm, maybe you should check your tires? What PSI do you normally run? I normally get around 42-45 highway. I set my tires at 37PSI. I know some people go as high as 40 to get a few more MPG's.

Sounds like your engine is operating ok. I'd check other things. Maybe you have a sticky brake? Hows the alignment? Gotta be something but it doesn't sound like an engine problem.

[SpaceSputnik]

tires are at 32 psi, alignment is in the green. haven't checked the brakes yet.

[Jason.MZW20]

Originally Posted by SpaceSputnik

Thanks Jason.

I can read only O2 1x2 voltages, the only thing I get from the ecm. It seems to be the post cat sensor, it's readings are somewhat delayed and not in line with fuel trims, i.e. i can be idling at 0.8V than snap rev it up and within a couple of seconds it falls down to 0.1V. I can however read the AFR values, they stay at 14.7 except on decel when it sometimes goes up to 18V

Someone suggested that due to my driving cycle I am getting oil contaminated with gas and because oil often does not get hot enough that gas is never evaporated. But, even after a couple of 30 min highway drives (within a couple of hours) there's no change.

I would assume that an engine that new shouldn't have compression issues, so I will the test that during the next go round. For now, I want to change the oil and while the car is on the hoist examine O2 sensors. Does that make sense to you?
In regards to that - do O2 get dirty normally and can it be visually determined?

EDIT: -7% is pretty much the best I see. I've seen ltlf at -8.6% to -11% quite a few times.

The post-cat sensor will always be delayed so long as your catalyst is in good condition. Once the post-cat sensor starts reading the same voltage as the upstream primary O2 sensor, your catalyst is toast and you'll likely set a code.

That said, you need to be able to read the primary O2 sensor's voltages to know if it's correlating properly to the amount of air being drawn or to enable an O2 waveform. The fuel trims are a reading of the O2 sensor's voltage. Typically, 0.5v is stoich (14.7:1) while 0.6-0.9v is rich and 0.1-0.4v is lean. The O2 sensors will always read lean on deceleration because the ECU cuts the fuel injectors.

Gasoline shouldn't be contaminating the oil unless it's getting past your piston rings or you're running so rich it's being pulled down into the crankcase. The latter isn't the case based on your fuel trims (-73% would be the maximum adjustment to lean out a super rich mixture).

Unfortunately, there is no visual diagnosis for O2 sensors because the actual element is behind a shield and has no servicing elements. However, if the O2 sensor tip cover (the part that enters the exhaust) is coated with black carbon, you are running richer than normal. If it's coated with oil, some amount of oil is not being controlled by the oil control rings and is entering the combustion chamber and exiting the exhaust (could be bad rings or worn valve stem seals).

Over time, the O2 sensing element becomes contaminated and begins to read slower than normal. There is no way to fix this other than replacing the affected sensor.

Toyota uses a rich/lean, lean/rich switching rate of 8 times per 10 seconds at 2500rpm. If the O2 sensor fails that, it must be replaced. A shop with a more feature laden OBD2 software could test this.

Another thing is that many Toyota ECUs are tuned for E10 gasoline as their primary programming, so using non-ethanol gas can also cause richer readings as less fuel is needed than an E10 enriched fuel.