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I have a 1999 Subaru Forester with an automatic transmission and a 2.5 Liter Motor. It has poor acceleration and rough shifting

I started to notice 'hesitation' on take off. It just slowly appeared. Then I realized if I was trying to pass someone at speed, 40 to 60 mph, it would actually slow down when I pressed the gas pedal harder. I increase speed by backing off the throttle. It had an 02 code that didn't affect the driving for a couple years now. I thought it might be a TPS issue so I changed that out and still the same issue persists.

I decided to swap the throttle body out and had an issue with the old gasket. So I replaced that gasket and installed the old throttle body back on just to see. My 02 code went away but it still runs like crap. its not possible to do any fast accelerating at all. It needs to slowly reach speed and if you try to go too hard on the gas pedal it will just bog down and semi stall until you back off the throttle and find its 'sweet spot' where it will start to accelerate again.

With my Snap-On scan tool plugged in I am getting an STFT of about 22% to 37% when engine is good and warmed up. The LTFT is between 25% to 30% at an easy cruising speed around 2600 RPM. When I push the throttle to the floor at speed STFT will go to a -10% area and the LTFT will go to around -5%. As soon as I let off the throttle it finds its 'sweet spot' again so it can accelerate. They both go back to what they were prior. Ignition advance is around 15 degrees and will climb to around 30 or so.

I have:

  • New NGK plugs

  • New fuel filter with 28 to 33 pounds of fuel pressure.

Any ideas before I start pulling injectors and checking and cleaning each of those today?

DucatiKiller
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Steve Mcdaniel
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3 Answers3

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I'm leaning towards the throttle position sensor being out of adjustment/bad or a bad mass airflow sensor.

Perform a volumetric efficiency test, record your IAT in °F, RPM and MAF in g/s. Use an online calculator like https://www.otctools.com/ve. You may want to do multiple pulls to get an average.

TPS voltage at idle should be around .5v and at wide open throttle should be around 4.5v. Key on engine off should be 5v. Perform a sweep test on the TPS to verify there aren't any drop outs.

Some other PIDs you should look at are MAP and BARO make sure BARO is correct for your area.

Ben
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so after pulling and checking each injector and reading all i could about this kinda thing, i found a few brief reference about the vehicle going into a 'safe mode', well that made me curious, because it sounded like what i had going on. so i just decided to unhook the battery for a awhile then try it. well it was running even rougher at start up after that, just struggling to stay running going from a real low idle to a very high one and really 'chuggin' as i am trying to keep it running by 'feathering' the throttle. then it came to me, i unhooked battery again and later when i restarted it, it was running rough, exactly the same, really. but this time i did not touch the throttle i just let it keepo corrrecting itself, then after a few minutes it actualy started to smooth out a few minutes later i was pulling out on a test drive. was it 65 mph in a blink of a eye, and i realized at that moment i had found the CURE. all it needed was a chance to fix itself lmao ya right i dont buy that crap either

anonymous2
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Steve Mcdaniel
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I had a friend have this issue and kept going into limp mode and I found her connector for the knock sensor was faulty and losing connection and when it did it would run horrible and even die out and have a hard time starting, I rewired the connector and it solved her issue.

James Herren
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