STFC frequency seems high?

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The1
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Re: STFC frequency seems high?

Post by The1 »

STFT is short term fuel trims, which are pretty much realtime fuel adjustments, LTFT is Long Term fuel trim which is an average of STFT over time. So the LTFT will build over time via the STFT which keeps it in spec. So to see if something's really out check what the LTFT is doing over various driving conditions.

the millivolt fluctuations are normal, look for a variable like o2 crosscounts, this will give you the number of cycles between 0-900mv, normal o2 narrowband sensors will work on usually over 10 with some throttle, at idle maybe around 2-5. If your not getting many crosscounts even with throttle then the o2's are prob getting tired and need replacing.

Idle is the hardest thing to make smooth, check for vac leaks, give the MAF sensor wire a clean to.
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Re: STFC frequency seems high?

Post by vlad01 »

Gareth wrote:If you mark the balancer at TDC #1 and put a timing light on it, you will see the mark jump around showing timing chain slop. (at idle of course)

They are crank triggered though. The jumping you'd see will be from closed loop idle spark control.
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Re: STFC frequency seems high?

Post by JohnDee68 »

Thanks vlad01, Gareth and The1,
The1 wrote: look for a variable like o2 crosscounts, this will give you the number of cycles between 0-900mv, normal o2 narrowband sensors will work on usually over 10 with some throttle, at idle maybe around 2-5. If your not getting many crosscounts even with throttle then the o2's are prob getting tired and need replacing. Idle is the hardest thing to make smooth, check for vac leaks, give the MAF sensor wire a clean to.
I am getting what I consider a fair amount of cross counts anywhere from 0 to 15 at times, while its also cycling the O2 sensors hi / low. I feel the O2 sensors are responding as if new by what TunerProRT is showing me.
Log - Vx - 1.jpg
Al be it a backyard check, I did check for vac leaks and didn't find any. Even a Vac gauge showed vacuum was good. Though i am yet to try the smoke technique. I am also needing to check exhaust for leaks prior to O2 sensors ans it too could cause miss reads of the sensors I believe.

I should know tomorrow after my trip IF there has been any significant change to the economy. If it doesn't alter, then i will try checking further. But I feel it is likely the injectors causing the fuel usage by having the PCM adjust for inconsistent injection spray all the time.
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Re: STFC frequency seems high?

Post by Gareth »

vlad01 wrote:
Gareth wrote:If you mark the balancer at TDC #1 and put a timing light on it, you will see the mark jump around showing timing chain slop. (at idle of course)

They are crank triggered though. The jumping you'd see will be from closed loop idle spark control.
Yeah, scratch that. Then the only way to see it would be to overlay the crank and cam signals on a decent scope.
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Re: STFC frequency seems high?

Post by vlad01 »

JohnDee68 wrote:Thanks vlad01, Gareth and The1,
The1 wrote: look for a variable like o2 crosscounts, this will give you the number of cycles between 0-900mv, normal o2 narrowband sensors will work on usually over 10 with some throttle, at idle maybe around 2-5. If your not getting many crosscounts even with throttle then the o2's are prob getting tired and need replacing. Idle is the hardest thing to make smooth, check for vac leaks, give the MAF sensor wire a clean to.
I am getting what I consider a fair amount of cross counts anywhere from 0 to 15 at times, while its also cycling the O2 sensors hi / low. I feel the O2 sensors are responding as if new by what TunerProRT is showing me.
Log - Vx - 1.jpg
Al be it a backyard check, I did check for vac leaks and didn't find any. Even a Vac gauge showed vacuum was good. Though i am yet to try the smoke technique. I am also needing to check exhaust for leaks prior to O2 sensors ans it too could cause miss reads of the sensors I believe.

I should know tomorrow after my trip IF there has been any significant change to the economy. If it doesn't alter, then i will try checking further. But I feel it is likely the injectors causing the fuel usage by having the PCM adjust for inconsistent injection spray all the time.


Normally the ECU will throw an error if one bank is running richer than the other. So I doubt the one injector would be the cause. I know they also chuck an error if either or both banks run too lean/rich as well.

I would go and have a look at tyres and their pressures if you haven't, next if the engine is running at full operating temps too. Always good to check the basics.
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Re: STFC frequency seems high?

Post by JohnDee68 »

Thanks vlad01,

I don't see anything out of the ordinary with other parts of the vehicle, though i will check the basics again, even get another person to verify.

With the PCM, I wonder what sort of limits are required to throw up codes like the too lean etc.? I don't drive the car at all hard, compared to most drivers.

It drives well, at least as i can tell, all but the economy loss. I have even had friends say how it runs so smooth from a passengers perspective of course.

I will see what today's trip returns for fuel economy and see from there.

thanks again.
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Re: STFC frequency seems high?

Post by antus »

If its got poor economy due to bad fueling for any reason it'd be too rich, and your O2 sensors which look like they're working would be showing off the scale rich as they are not widebands so will easily pass their limits. But your screen shot doesnt show that. So Im leaning toward the timing theory, that if its running a bit retarded power is lost in the stroke so you have to drive it harder.
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Re: STFC frequency seems high?

Post by vlad01 »

That's what I found with my old motors as well, good AFRs, fuel trim and cross counts were normal but still using some 3L/100 more than a fresh motor.
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The1
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Re: STFC frequency seems high?

Post by The1 »

log looks fine to me. For reference our VX Wagon usualy got on average about 13L/100km with mix of town and highway, with 330,000km on it. What figures are you getting?
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Re: STFC frequency seems high?

Post by vlad01 »

thats similar to my nephews VS wagon with VX motor. Engine has about 250 on it.
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