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Hard shifting issue & loss of acceleration 1999 Subaru Legacy

1999 Subaru Legacy Wagon 2.2L Automatic
152K miles

Hoping someone has experienced this issue and may have some advice on the route of repair.

So far three mechanics (including the Subaru dealership) have been unable to resolve the issue or pinpoint the source of this issue. The issue is intermittent and random but very scary as it results in a total loss of acceleration and jolting hard shift. The tricky part is that it doesn't happen all the time and we can go up and down the interstate on long road trips with no issues at all.

In town driving is when the issue usually occurs. I have yet to have a mechanic be able to experience the issue when they road test - most likely because they do not have the time to drive it for long amounts of in town driving time.

This is an automatic transmission and the issue happens when you are in town driving at around 35-45 mph.

Conditions in which hard shift occurs:
Start and stop traffic i.e. between stoplights where you can get up to 35-45 mph. Also tends to happen when accelerating and then going down a hill or steep grade between 35-45 mph. For example accelerating up to a hill and letting off the acceleration (no brake just foot off accelerator) when going down the hill and then give it good amount of gas once it get to a slower rate of speed.

Most times this issue happens when going up a bit of a hill and giving the car gas or going down a hill while in gear but not giving the car gas and not braking (where the car has to regulate its own speed on a downhill grade).

When the issue happens you notice first a loss in acceleration and then the AT Oil light will start flashing and a very noticeable, jolting, and hard knocking shift where you won't be able to accelerate above 20 mph (like its stuck RPMs will rev with foot on accelerator but you cannot get above 20mph). The check engine light usually comes on AFTER the hard shift.

Sometimes however NEITHER the AT oil light nor Check Engine light comes on. The codes I have been able to pull when the AT Oil light has been flashing and the check engine light comes on are PO731 and PO122.

The only way we have found to get it "unstuck" from not being able to accelerate after hard shift is to let the car idle for 20 minutes or more OR turning the car off and on sometimes once but sometimes 2 or 3 times. This gets it back to normal and if AT oil light was flashing, this will go away and check engine remains.

On recommendation from our mechanic, we took it to a transmission shop to confirm that it was not transmission related or transmission computer related and they told us they could find no issue and suggested that it was something electrical not transmission related.

Since transmission was ruled out, we had our mechanic replace the throttle position sensor and that has not resolved the issue.

As a result and our our mechanics suggestion, we decided to take it to the Subaru dealership and the dealership mechanics said the voltage was set incorrectly on the Throttle Position Sensor (this voltage was actually set correctly by my mechanic and I suspect whatever is causing the issue is throwing the voltage out of whack).

The dealership mechanic readjusted the Throttle Position Sensor to spec voltage. The hard shifting accompanied by all the above mentioned symptoms happened again within a day after the voltage was set back to spec voltage by dealership. Took it back to dealership and they cleaned conductive grease of the Throttle Position Sensor's plug contacts - they said was put on there incorrectly by my previous mechanic and that would solve the issue. The hard shift came back after a day of in town driving so conductive grease was not the culprit.

Took it back to the dealership and left the car for an ENTIRE week at dealerships request! At the end of the week they still could not tell me what was wrong. The closest they could come to giving me an answer was that it may be electrical or that it is related to the engine wiring harness. Nothing conclusive though in terms of a diagnosis.

When I dropped it off to the dealership for the week, I set the trip counter to zero to see how much the car was road tested and it showed only 8 miles had been driven for road testing purposes for the whole week it was with the dealership mechanic.

The latest attempt to resolve the issue was in February 2013 where we had our mechanic who replaced the throttle position sensor have his diagnostics person run a series of tests. The diagnostics found two bad/ dirty grounds and that the throttle position sensor waveform had lots of electrical "noise" in the pattern. They cleaned all 7 engine and PCM ground points and road tested car and had no issues.

After all of this work was completed we had no issues for almost an entire month and thought the issue was resolved. Then the exact same issue returned! Feels like we are back to square one again and is a consistent annoyance.

The other symptom we have noticed which appears to be related to this issue is while driving on the highway above usually when driving between 60 and 70 mph and higher (2500-3000 RPMs), when letting foot off of accelerator to coast down to a lower rate of speed, when you put your foot back on the gas, rather than a smooth acceleration it jerks into a higher rate of speed and not a smooth acceleration of speed. You can see the RPMs jump rather than gradually go up. This makes it very hard to maintain a constant rate of speed. Then when you pull off the highway, the hard shift as described above happens.

At this point I am not sure what the next step if our mechanic or the Subaru dealership cannot solve? How on earth can I get an expert to pinpoint the issue without throwing money at possible problems?

Any suggestions are greatly appreciated - The only other place I have found a discussion related to this issue is here:

Thanks for your help!


  • saedavesaedave Chicago, ILPosts: 694
    The 1997 model year Legacy had knock sensor problems that made accel. tentative and hard to predict. I can't remember the details very well but it may have been the way the sensor was mounted as well as the sensor itself. You might try checking ....or replacing the knock sensor. My local dealer needed the factory rep to make the diagnosis.
  • xwesxxwesx Fairbanks, AlaskaPosts: 13,159
    That's a nightmare, for sure! I think your mechanic was on the right track, and that your problem really was "fixed" (for a time). In looking at those codes (P0731 and P0122) and how they're related, I strongly suspect that a bad ground is the source of this issue, as it would result in inconsistent voltages, etc., and could manifest all sorts of odd, difficult to replicate problems. The issue now, possibly, is how to ensure that the ground remains good so that the functioning of the necessary sensors is relayed properly to the on-board computer(s).

    Older Subaru vehicles are, in my opinion, somewhat notorious for being "under-grounded". So, I'm not going to tell you the best way to proceed with your money. Instead, I'll suggest that you have a conversation with your mechanic (since his shop sounds competent) about steps you might be able to take to improve the ground circuit reliability on your car before you spend more money on further diagnostics.
    2014 Audi Q7 TDI, 2008 and 2013 Subaru Forester(s), 1969 Chevrolet C20 Pickup, 1969 Ford Econoline 100, 1976 Ford F250 Pickup
  • emkz4430emkz4430 Posts: 3
    edited April 2013
    I just bought a '99 Subaru Legacy and have this SAME EXACT problem and have yet to diagnose it. I can't believe you went through all that and it is still not fixed, I don't even know where to start.
  • filmplanefilmplane Posts: 4
    edited April 2013
    Thanks for all the responses and input so far!

    In response to xwesx and emkz4430 regarding this issue: I am convinced the ground wires are the place to start to further diagnose and hopefully repair this issue. If it is not the ground wires, then the next place will be the engine wiring harness.

    emkz4430 I feel your pain. I can drive all day on the highway but can barely make it to the grocery store without this problem occurring.

    I would highly recommend finding a local Subaru mechanic and first ask them to check your engine ground wires as this seems to be the culprit as xwesx mentions in this thread. Chances are they will find a loose or bad ground(s) causing signal noise which is causing the issue. Related to that after this is done, if the issue still persists, I would have them test your throttle position sensor (tps) and make sure that it is working properly and that it is set to the spec voltage.

    The throttle position sensor is an expensive part so I would recommend starting with the grounds and a basic check of the tps.

    I found a Subaru mechanic in Asheville, NC that is familiar with this issue and his shop will be looking at my car tomorrow. Hopefully I will have some good news and no more issues!
  • emkz4430emkz4430 Posts: 3
    I just took my car to Advance Auto and had them run a diagnostic. It ended up saying it was an issue with the Knock Sensor and to replace it. After removing the old knock sensor sure enough it had a hairline crack in it. A faulty knock sensor can cause this acceleration issue due to the crack. I put a new knock sensor in and sure enough the car has had no problems!! I'm hoping it doesn't come up again as it shouldn't - I drove the car for one hour and no problems whatsoever. Definitely check your knock sensor. It's right under the throttle body connected by one 12mm bolt. All you have to do is unbolt it and unplug the other end - takes 5 min to do if engine is cold. If yours has any crack whatsoever or just looks in bad shape replace it! Good luck! Hope this works for you!
  • emkz4430 that is great news! I hope your hard shifting troubles are over! Can you say more about the diagnostic they ran at Advance Auto? Was your check engine light on when they ran the diagnostic and what was the code that they pulled from diagnostic? I have never had any diagnostic run hat points to the knock sensor being the problem so I am curious if this is worth looking into for my car's hard shifting issue. Any more details you can provide would be great! Thanks!
  • emkz4430emkz4430 Posts: 3
    My check engine light I think is burnt out because it does light for a few seconds when I turn the car on, but with this knock sensor issue I've heard that sometimes the light won't even come on. Anyway I don't know what the code number was but I do remember it said "knock sensor" and to replace it. I've read that this is a common problem with these Subarus and will cause those exact acceleration issues. It is definitely worth checking on your car! Like I said its right underneath the throttle body, connected by one bolt. Simply unscrew the 12mm bolt and unplug the other end. If you google a picture of it it is a plastic ring with an electrical tail. It takes 5 min to do and once you have removed the piece look for any kind of crack on the plastic ring and clean if its dirty. Mine had a tiny crack and was full of grime. The crack alone will mess the whole car up but I don't know about the grime. Since I replaced it yesterday it has had no problems, driving it to school this morning it had no problem. Let me know if you have any questions!
  • filmplanefilmplane Posts: 4
    Ok so I promised an update after taking to mechanic. here's what they found:

    1. One of my cooling fans was failing, bad wire so it wasn't even running.

    2. Positive battery terminal wire completely shot so bad connection there which was replaced.

    3. Mechanic adjusted Throttle Position Sensor above the recommended spec voltage (says from experience he does this because the car responds better to that setting).

    Thought our problems were solved but then Hard shifting and same old symptoms didn't happen again until after almost 300 miles of driving.

    Took back to mechanic and decided to have knock sensor replaced. Knock sensor replaced and so far NO HARD SHIFTING ISSUES and any of the symptoms as described previously in this post.

    BUT ALAS a new symptom has manifested itself. Maybe folks have some idea about this new issue?

    Now when putting gas on the accelerator, there is an uneven shift from 1st to 2nd gear and car seems to be struggling to get acceleration between gears. Very sluggish and even. This is most noticeable going from second to third. It feels like it isn't getting gas, accelerating very slowly up to around 25000 RPMs where it for lack of a better word BURSTS into third gear. The RPMs move very gradually and then ZIP up to 30000 RPMs. This issue is more frustrating than the hard shift issue as it happens ALL the time now during in- town driving.

    Very frustrated but at least now a mechanic can feel and experience this new problem unlike the previous issue where no mechanic could replicate.

    Any thoughts? Thanks in advance!
  • Hello emkz4430 I am having the EXACT same problems that you had with my 1999 Subaru I am just curious how the car has been running and if the knock sensor fixed it?
  • Sorry the msg above was intended for filmplane
  • guest28guest28 Posts: 2
    edited July 2015
    What if this whole time your car was designed to do this. What if the shifting problem that your experiencing now is more about the cpu being programmed to harm the car causing premature wear and functionality issues so the car will require service, the desire to replace the car will be induced, and you'll either spend money in their shop or trade it in on newer. Another question that should be asked is, is there a newer year of model that no longer has this issue, and what was changed about the car to help it run better? They don't advertise design flaws. If nothing changed, go back to my first suspicion.
  • xwesxxwesx Fairbanks, AlaskaPosts: 13,159
    Wow; how very philosophical of you. I am going to go with, "no."
    2014 Audi Q7 TDI, 2008 and 2013 Subaru Forester(s), 1969 Chevrolet C20 Pickup, 1969 Ford Econoline 100, 1976 Ford F250 Pickup
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