UPDATE & HAPPY ENDING:
Regarding the 2003 Subaru Forester 2.5L XS
Hesistation or stumble upon acceleration, after downshifting from 5th gear to 4th gear, or going around a curve or turn, while upshifting from 2nd to 3rd between 2,500 and 3,500 r.p.m., I finally have some good news to report. Bear in mind there was no Check Engine Light and no Codes stored. Recall that this variant and Model Year has no MAF, which is often the first thing many forum(s) first-responders suggest is the fault or they will suggest faulty Oxygen Sensors or stuffed Cat or warped and funky PCV and the list goes on.
My process of elimination revolved around, the question posed to myself, "What makes any engine run, from the most basic to the most sophisticated jet enigine?"
Air - Fuel - Ignition The ideal Air to Fuel mixture is a 14:1 ratio, which can easily be checked looking at the condition of all the Spark Plugs. That simple knowledge will eliminate a lot of guess work. If an engine starts right up and idles smoothly and after it is up to operating temperature, you can rev the engine's r.p.m. up to redline, that eliminates other faults.
Then, I considered, "What system would not render a Check Engine Light and would not cause Codes to be stored?"
Add to those questions, "What system or system fault would cause faltering/hesitation under load?"
Answer: Fuel delivery and or Vacuum
The last bit of troubleshooting I did, personally, is when I drained the fuel tanks per this thread:
https://www.subaruforester.org/vbull...nation-786687/
I returned to the Subaru Dealership, spoke with one of the owners, who put his "top mechanic" and his Manager onto the task. I had to leave the car and they would get to it and chase the problem, as they had time.
Before I left them with 3 pages of typed notes on all the troubleshooting I had done, I told the Dealership "top Mechanic", with his back resting up against the wall, and less than half my age and his Manager, "The only thing I could not check was Fuel Pressure. Could you check that for me?" I told the Manager all I had done, asking him to provide my troubleshooting to the Mechanic, and stated that I even swapped Coil Packs. He replied, "That the secondary winding or secondary stage is actually used when the car is under load." I asked, "Do you think it is possible that two different Coil Packs could both have the same fault?" He replied, "Yes." I would believe
statistically-speaking the possibility would be remote.
They had the car 5 days and drove it 136 kilometers.
They experienced the hesitation together. They had a Fuel Pressure Gauge taped to the Windshield and had other Diagnostic equipment attached while driving around.
They got some strange readings and fault codes. They contacted Subaru-Japan.
They replied that the Fuel Pump is defective. Maybe that was the source of the metallic flecks from the right fuel tank drain. The Fuel Pump was replaced with an "aftermarket" Denso, along with a new sock Filter, part of the kit. The Manager told me it is the same pump as the OEM.
Then, the Neutral Safety Switch on the 5spd Manual Transmission (Gearbox) gave a fault Code on their Diagnostic Tool.
Refer to the attached photocopy/scan of their Service.
They called me Tuesday 25 September at 4:00 p.m. to tell me they solved the problem.
The car runs like new!
Of course, I have only driven it about 75 km, since the repairs, but, I am confident it solved the unexplained Hesitation issue.
The last thing I did was to call around to all the Independent Mechanics and Shops I talked to about this to let them know, so they will know what to look for, should anyone else ever contact them about a similar hesitation issue. Each appreciated the follow-up call.
