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Old 12-26-2013, 01:57 AM   #21
SL19302
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Default Re: short term fuel trim

Agreed, However any engine with over 200K on it will have natural wear on internal components, Compression rings included.

...
2015 Subaru Forester Limited "Prinny"
2000 SW2 Still Running Strong- 160K Traded In
2002 SL1 Still has Its Training Wheels on at 88K Traded In


CHECK YOUR OIL!

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Old 12-26-2013, 02:07 AM   #22
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1995 SL2
Default Re: short term fuel trim

Its the 02 and has 123,000 miles

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Old 12-26-2013, 02:09 AM   #23
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Default Re: short term fuel trim

Sure, but the PCV-Blowby-oil consumption connection is a bit of a stretch.

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Old 01-02-2014, 02:38 AM   #24
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Default Re: short term fuel trim

When I swapped heads the amount of carbon on the pistons and stuck to the combustion chamber could have easily raised the cranking pressure above 200. I had major detonation problems too.

I always go by the LTFT to determine the health of the system...Assuming my O2 sensor is working properly. Its the final correction. When The short term maxes out trying to trim fuel it increases or decreases the LTFT by 1 and rezeros the STFT and starts over.

I have the ScanGauge on my Satty and programmed the AFR PID. It starts cold shooting for a 8.0 AFR. As it warms up it slowly goes to 12.0 by 120 degrees. around 160 degrees its up to 14.2 AFR. and as OldNuc says somewhere between 170 and 195 it will switch to 14.7 AFR. which it considers fully warmed up......

I have not correlated anything to why it shoots for 14.7 at 170 degrees ECT and the next time I'm driving it will wait to hit 192 before it changes to 14.7 AFR.

WOT is calling for 12.3 AFR.

Around 5 or so years ago I found a great write up on how to use the LTFT to troubleshoot fuel injection problems......Never found it again.......or I wold have kept the PDF.

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Old 01-02-2014, 10:30 AM   #25
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Default Re: short term fuel trim

The A/F ratio from the fuel map programmed into the PCM considers several parameters and coolant temp is only 1 of many.

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Old 01-03-2014, 01:40 AM   #26
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Default Re: short term fuel trim

Quote:
Originally Posted by OldNuc View Post
The A/F ratio from the fuel map programmed into the PCM considers several parameters and coolant temp is only 1 of many.
I agree...........it's just interesting and thought provoking to know what it takes for the PCM to select 14.7 to 1 sometimes, I drive 10 miles before I get to 14.7. other times I'm driving along at 185 or so and its 14.2.....then I lift off the gas to coast a bit and get back on the throttle and it 14.7...........temp never changed.

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Old 01-03-2014, 09:40 AM   #27
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1998 SC2
Default Re: short term fuel trim

Yes, it is interesting to collect this data but some people will grab those numbers and then insist that at 1 degree less than 170 the system will not be fully warmed up. I have already seen this happen repeatedly.

How many times have you seen it said on here that he system does not enter closed loop until some specific temp value is reached or you are in open loop until 14.7:1 A/F is reached?

The system goes to closed loop as soon as the O2 sensor output signal commences switching(<300mv, >600mv), nominally requires a 600F tip temperature which is reached in seconds after starting the car.

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Old 01-03-2014, 04:11 PM   #28
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Location: Pacifica by San Francisco 5sp Engine rebuild with the help of people here at 177K due to 500mi/qt oil consumption. After 2yrs I am back to 40mpg+ after discovering the refirb head I used was bad. Car runs better than a top. Now at 214k.
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1996 SL2
Default Re: short term fuel trim

You can chase this issue for quite a long time and never find why your idle has the occasional hic-ups. My car used to do the same thing and I never quite pinpointed why. Possibilities are:
1) O2 sensor does not switch fast enough. I had a Bosch sensor that switched from high to low and back every 1.5 seconds. I can see how the inaccuracies of a slow sensor might periodically cause a hic-up. The Denso I have now switches much less than 1 second.
2) Vacuum leak causing the O2 sensor to periodically fuel starve the motor. The leak can be anywhere.
3) Fuel filter becoming clogged.

Note: the ST and LT information that has been posted up I see little value to. The numbers are just snapshots that change wildly and very quickly so you really cant deduce anything from them. Even the LT values can be inconsistent from starting and stopping the engine for a re-measurement. The LTs are relative measurements that may not be repeatable from one engine start time to another. The LTs are fuel delivery stair-step-ups or step-downs when the ST corrections fall out of min/max boundaries. But in general, the LTs should be fairly stable in a controlled driveway idle.

If you want to be able to utilize the ST/LT information in a more meaningful way, you will have to watch the behavior, data point by data point. (tenths of a second apart is best) Your goal is to validate the average of all STs is zero with a controlled idle in the driveway. Trying to do this while driving introduces too many moving influences.

Most likely your ST average is going to be negative (running rich). Not often is it too lean. Can you say what your fuel economy is at this point with new plugs and wires?

So in summary, make sure you have a good quality O2 (Denso/NGK) and start looking for vacuum leaks. Also you should be able to validate your old PCV runs with the same results to rule out the fixed orifice is not an issue.

The fixed PCV helps to adjust the crankcase pressure to either lower or higher pressures. If it brings the pressure down, less oil can get pushed out of the engine(leakage). If higher pressure is created, more leakage can occur.

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Old 01-29-2018, 05:24 AM   #29
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Default Re: short term fuel trim

In my opinion the brand is very cool and produces the best devices! Thank you for posting. All the best.

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