Choking (shudder, losing power) for a half second at low speeds

Thank you Ken. Will try it and let you know. I really appreciate this, thank you for your time.
Satriana

Hi Ken, the situation had evolved over the last two days. On Sunday I drove around on some country roads and recorded ECU and TCU parameters, graphed some of them and those I looked at seemed right.
But the car moves like it is towing a big load! Accelerates very slow, it is like something is holding it. Checked the wheels after a longer trip, not hot, it is not the brakes.
Car has this instant fuel consumption gauge, and there is a big change there, before when pushing the gas pedal hard used to go to 40 to 50 l/100km, now barely touches 18 when pushing hard. Generally the readings now are way lower than they used to be. That makes me think that gauge actually meters the flow through the fuel lines and that flow decreased big time now. So either fuel pump cannot push enough or injectors are clogged. I should add, at idle engine runs perfect.
So I realized the hesitation, the shudder I have described initially are just symptoms of this low engine output. Now it is quite obvious, but it got here slowly over two months. So looking back, I guess it this this shudder because it did not have enough torque to continue in that gear and did not downshift because program said should stay, felt like driving manual in a high gear uphill and not downshifting when rpm's went too low.
There were several different symptoms and it felt like transmission problems at times.
I will check the fuel pump pressure and I will report back once I have found the root cause.

I did check the catalytic converters and they are definitely fine. Brought the car to mechanic, I had it hauled on a flatbed as I was not sure I would make it there. Anyway, went on a test drive with the mechanic, and, unbelievable, car runs fine! I have no explanation whatsoever. Left it there, he is going to drive it around, check/clean throttle body, gas regulator (he said it may be pushing too much gas back into tank for some reason).
So the only thing that happened is that the car was tilted at about thirty degrees while loading/unloading on the flatbed. Could it be some sediment in the gas tank? That must be a lot, not likely. Consumption gauge back to normal parameters, so seems to be getting enough gas.
I will keep you posted.

There was not much development, except that I had to replace some bushings on the front suspension. When leaving the shop, the car performed excellent, By the time I got home power decreased noticeably. I put some STP fuel system cleaner in last tank, engine seems to run a better, but cannot say it is because of that. While running better now, it is still not at full power, and it is also inconsistent, sometimes runs fine, some days not, sometimes within same day you notice the difference.
So I am using it every day but my commute is quite short, trying to find a pattern that I could use.
I had also mention, I have replaced ATF fluid end of February, used Valvoline, compatible with Subaru ATF. I guess that could not be problem. Another little clue is that I had two burnt front light bulbs around the time it developed the loss of power. I have found some post about how the stop bulbs on some older Outback that caused the ECU to malfunction, not sure it applies since they were Australian, but I will try it. Also, in same threads, some people say it is pulling the fuses and putting them back in have helped. I will do it this week end.

I have managed to improve the situation a little bit. I know this will sound weird, but I have tried that fix I have mentioned, replacing the stop bulbs, just because it was easy.

So I have replaced the sway bar bushings (unrelated) and after that I have replaced the stop bulbs although they were fine. The new ones different brand, but same type and wattage. Replaced them while my wife pressed the brake pedal. After that I went on a test drive on some country roads around here. I could not believe how well the car ran, plenty of power, no shudder, no vibration, steering felt light, everything was perfect. Needless to say I was very happy, thinking I have got the fix.

Anyway, the second day, it was not that good anymore. Better, but clearly not the same, meaning not as peppy, had to press the gas pedal more. So I thought it's clearly electrical. I have cleaned and tied two grounding straps, checked the two underneath the engine, re-seated all the fuses in the fuse box in the engine bay, cleaned the battery terminal and connectors, reset the ECU in the process. Again seemed to run better after the ECU learning cycle, but not as it was the day before. I have replaced the battery as well, did not seem to make a difference.

I still experience the occasional shudder when going light on gas, I think it is when shifting from second to third. Today again stutter when taking off, but this is rare. Not horrible, but enough to scare my wife. And sometimes I get that vibration like going over strips.

I still feel I have to press the gas pedal more than normal, although much improved.
My feeling is that I have got an electrical problem, but cannot say what it is and do not know what to do next. Please let me know what you think.

Hi Ken,
I think I got a bit closer to diagnosing the problem, but still missing the key piece. :-(
I have used RomRaider to log the running engine parameters, did that a few times, but it took me time and some online education to figure out what to do with them. Finally I have noticed that fuel pump duty never reaches 100%, no matter how hard I press the gas pedal. It looks like it is at 33% at idle, which seems normal, but as the load goes up, the fuel pump duty doesn't follow, so fuel pump does not deliver enough gas under load. It looks like this is very likely the cause of reduced torque, basically fuel starvation. I guess this explains why sometimes it hesitates on take off. Please see the graph attached to see how fuel pump behaves.
The max value I have recorded for fuel pump duty is 66.67%, that is probably not enough on higher load. The fuel pump duty and pulse modulation is controlled by ECU, my understanding is that it uses the MAF sensor(I have cleaned that, but was very clean already, and I have graphed the variation and I am pretty sure it is normal. The service manual does not specify what voltage should deliver, it only says to check that voltage varies with air delivered). There is also the MAP (Manifold Absolute Pressure) sensor that contributes to computing the input for fuel pump, but less important than MAF sensor. This one I could check by measuring the output voltage. Looked at the values received by ECU (only says pressure, not voltage) and seems normal. Show absolute pressure and differential, also records atmospheric pressure, the differential is negative and when adding them they match fine.
I think some parameter(s) cause the computer to give wrong input to fuel pump, but cannot seem to figure out which one. :-(
Thank you

Hi guys,

I can now post my findings here with confidence, my car works fine for some time now. I hope you won't find my post too long :-)
So basically back in February 2018 when that hesitation started, I thought it could have been transmission acting up. Lack of experience, it wasn't the transmission. But I did go to a shop to replace the transmission fluid and they put Valvoline in it, they said it's compatible. Big mistake, that had caused shuddering on torque converter lock and some gear shifts. As I still had the hesitation problem it was difficult to figure out what was going on.
In July 2018, my wife has found some Subaru specialist in the city here and insisted I take the car there. Called them and got an appointment in a month. In the mean time, I read about this fuel system cleaner called Techron(from Chevron). Contains PEA (polyether amine) as the active substance. Being in Canada, that's hard to find so I've tried Gumout All-in-one fuel system cleaner. You need to pour it in an empty tank, then fill up.
After that the hesitation had virtually disappeared. One or two times again after couple of weeks, another bottle, things got way better. No more hesitation. Still went to the Subaru specialists, but they couldn't say what was wrong and definitely dismissed my story of how Gumout had improved the power.
I still had the shuddering from transmission. Took me a while to figure out it was the friction factor of that ATF that caused the shuddering(clutch was grabbing due to higher friction of that ATF) and that I need to flush that Valvoline ATF and put in one recommended by Subaru. Found Idemitsu HP in US@7USD/quart, had to drive to Niagara Falls to pick it up :-( Anyway, it was definitely worth the effort, all shuddering is gone! So must stick to Subaru or Idemitsu HP ATF no matter what! It shifts and locks TQC super smooth.
So things were pretty good and was thinking how to write a last post to close this thread.
Then in November 2018 the hesitation and lower engine torque started again. Right after a fill-up at Costco. Can't say gas may be the culprit but I've decided to fill up elsewhere just in case.
Tried that Gumout fuel system cleaner trick again, clear improvement in just a few days. Stopped filling up at Costco. Used ethanol free since then (I think it was ethanol free, used mostly 91 octane from stations that say it's ethanol free). Also filled up twice with super 94 at petro Canada, that's guaranteed ethanol free.
Looking back, there's a lesson here, I guess. Things are not always what they appear to be, an engine problem seemed like a transmission problem, replacing the ATF with the wrong type only made it functionally worse and more confusing and more difficult to diagnose.
Cannot explain what that fuel system cleaner actually cleaned. And can't say what got dirty in about three months. By the way, that engine is not a direct injection engine, it's multi-port injection.
Thank you for your help, much appreciated!