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Hard shifting issue & loss of acceleration 1999 Subaru Legacy
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: http://www.justanswer.com/car/697fz-...#ixzz252mSRPn0
Thanks for your help!