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Old 07-10-2018, 07:56 PM   #26
CaptainSlowbaru
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It can't be the cat, since you said it still wont rev with the cat removed.

Positive fuel trims mean lean, meaning the computer sees not enough fuel, so it's adding fuel to compensate. What is your A/F sensor (upstream O2) reading at idle? Ideally it should sit at 1.000 and not move much. A bad A/F sensor can screw everything up. Also, what is the MAF reading at idle? I wouldn't rule out a bad MAF (you did say it was aftermarket).
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Old 07-11-2018, 07:25 AM   #27
Elbert Bass
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Yup - ruled out the cat (at least as far as being an exhaust blockage). Not ECT, That fuel pressure should raise your trim a point or two maybe - if it makes any difference (although you do have a bad regulator if it doesn't change with vacuum hose off).

I'm bothered by the fact that it will not run with the MAF unplugged - it should start right up up and run after you unplug the MAF because it is running on a calculated map based on RPM/throttle position. It really points back to a vacuum leak. just out of curiosity - is your idle switch showing closed at idle? What is your TPS voltage? How old is the fuel in the car? Does the fuel smell like turpentine or not smell "as strong" as it should? Have you performed a cylinder leakage test (burnt/sticking/tight valves)? Although that should show in your vacuum reading - is that 23" a steady value or bouncing around?
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Old 07-11-2018, 11:47 AM   #28
Charlie-III
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Quote:
Originally Posted by CaptainSlowbaru View Post
It can't be the cat, since you said it still wont rev with the cat removed.

Positive fuel trims mean lean, meaning the computer sees not enough fuel, so it's adding fuel to compensate. What is your A/F sensor (upstream O2) reading at idle? Ideally it should sit at 1.000 and not move much. A bad A/F sensor can screw everything up. Also, what is the MAF reading at idle? I wouldn't rule out a bad MAF (you did say it was aftermarket).
I think this is still old school, thus not Lambda.
Old school should have front O2 switching between 0 and 1 volt, a second or so swing (actual is usually more like 0.1 and 0.9) LTFT's should be within about 10.
Lambda should be about 1.000 volts, vary a bit higher or lower to adjust.

Downstream should sit about 0.5 volts, minor variation.

Quote:
Originally Posted by Elbert Bass View Post
Yup - ruled out the cat (at least as far as being an exhaust blockage). Not ECT, That fuel pressure should raise your trim a point or two maybe - if it makes any difference (although you do have a bad regulator if it doesn't change with vacuum hose off).

I'm bothered by the fact that it will not run with the MAF unplugged - it should start right up up and run after you unplug the MAF because it is running on a calculated map based on RPM/throttle position. It really points back to a vacuum leak. just out of curiosity - is your idle switch showing closed at idle? What is your TPS voltage? How old is the fuel in the car? Does the fuel smell like turpentine or not smell "as strong" as it should? Have you performed a cylinder leakage test (burnt/sticking/tight valves)? Although that should show in your vacuum reading - is that 23" a steady value or bouncing around?
Agreed, interesting. With the MAF unplugged, the ECU ignores temp and airflow values and uses "canned values" in the ECU.
I can't say I have seen an engine NOT run with the MAF unplugged. Maybe not run any better (if MAF is working), but it usually runs.

Back to basics, sounds like an intake air leak.
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Old 07-11-2018, 09:05 PM   #29
Elbert Bass
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Quote:
Originally Posted by Charlie-III View Post
-SNIP-
Agreed, interesting. With the MAF unplugged, the ECU ignores temp and airflow values and uses "canned values" in the ECU.
I can't say I have seen an engine NOT run with the MAF unplugged. Maybe not run any better (if MAF is working), but it usually runs. -SNIP-
That is a go-to diagnostic step - especially if you suspect MAF. Those older MAFs like the OP has were prone to reading textbook values on both voltage and airflow but still causing a problem. I found it by accident one day when I put an oscilloscope on the airflow signal wire. Perfect waveform just like you should have but there was this jagged pulsating DC value there as well - electronic noise. That noise causes the ECM to go all kinds of crazy in closed loop.
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Old 07-11-2018, 10:53 PM   #30
Charlie-III
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Quote:
Originally Posted by Elbert Bass View Post
That is a go-to diagnostic step - especially if you suspect MAF. Those older MAFs like the OP has were prone to reading textbook values on both voltage and airflow but still causing a problem. I found it by accident one day when I put an oscilloscope on the airflow signal wire. Perfect waveform just like you should have but there was this jagged pulsating DC value there as well - electronic noise. That noise causes the ECM to go all kinds of crazy in closed loop.
Oh, trust me, not arguing with you at all, I have used the "unplugged MAF" as troubleshooting before on Subaru and other brands.
I am just sorta surprised the car won't run with the MAF unplugged.
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Old 07-12-2018, 02:04 AM   #31
CosmoTheCat
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Side bar: A cat with a broken honeycomb can very much be an intermittent clog in the exhaust. Not our problem here though.
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Old 07-12-2018, 07:26 AM   #32
Elbert Bass
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Quote:
Originally Posted by Charlie-III View Post
Oh, trust me, not arguing with you at all, I have used the "unplugged MAF" as troubleshooting before on Subaru and other brands.
I am just sorta surprised the car won't run with the MAF unplugged.
Yeah it screams vacuum leak or some other mechanical issue. I'd really like to see a vacuum gauge reading (real gauge - not MAP) at idle and part throttle...
A vacuum leak that big would respond to brake clean without a doubt.
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Old 07-13-2018, 10:01 AM   #33
lpxltt
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I may have missed it but has someone suggested testing the resistance of the plug wires? I had a BRAND NEW set of Subaru OEM have a bad connection. It would run find about 20% of the time then the rest is would just go into limp and not rev past 4k.

The issue was that the old set had the exact same problem so I ended up replacing a whole bunch of stuff before actually testing the resistance of the wires themselves - something that was suggested to me but I ignored because young and "I already replaced them".

On the plus side I never did have to change anything electrical/sensor related (besides bulbs) again on that car for the rest of my ownership...
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Old 07-13-2018, 11:03 AM   #34
Charlie-III
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Quote:
Originally Posted by lpxltt View Post
I may have missed it but has someone suggested testing the resistance of the plug wires? I had a BRAND NEW set of Subaru OEM have a bad connection. It would run find about 20% of the time then the rest is would just go into limp and not rev past 4k.

The issue was that the old set had the exact same problem so I ended up replacing a whole bunch of stuff before actually testing the resistance of the wires themselves - something that was suggested to me but I ignored because young and "I already replaced them".
Could be, he does have pending misfires. Higher engine loads makes it harder for the spark to jump on the plug, thus, "load related misfires".
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Old 07-17-2018, 02:25 PM   #35
Imfozzybear
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Sorry for the late response, we were dealing with a dying dog input family... Sucks super hard.
So...i have disconnected the evap system from the motor via the vac. Port on the intake, and like magic I have high rpms again... Still not fueling the way it should, in fact one hot day last week it quit running all together. The fuel trims wet spiking upwards of 26-40 % had random codes pop up, cam pos and a handful of other codes.. I cleared them out on the side of the road and it's been running well... I am starting to wonder if I have a bad ECM or something of the sort. I have no real history of the car, but know whoever was working on it before didn't know how to install a thermostat... So that's where I'm at right now
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