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Discussion Starter · #1 ·
So I haven't had this vehicle very long. It's a 2006 Tribeca with around 120k on it. Got it with a blown head gasket which I had a Subaru tech repair on the side for me. Now I finally got the car back and noticed a significant dipping in the RPMs at certain times while driving (and letting off the accelerator).

Easiest scenario to lay out is this: Driving down the tollway around 70mph if I let off the accelerator the RPMs will drop like the car was put in neutral. As soon as I touch the accelerator it will pick back up right away and read as it should. Doesn't appear to be slipping and don't notice and hard shifts or jutters and there are no indicator lights on...

This happens in multiple gears and seems to happen when cruising with RPMs over 2k. It's just most noticeable at higher speeds.

I wanted to see if anyone has any clue what could be going on. From my reading so far a Transmission fluid change and filter change may be in order. Unless it's a faulty speed sensor... that's not throwing a code?

So tired of this car already.
 

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Sorry, head gasket on these is no fun.

RPM should be related to wheel speed in the situation you are describing since the torque converter should be locked up. I can't imagine a situation when cruising at 70mph in 5th gear and the RPM drops suddenly when letting off the gas without the car significantly changing speed. So this would seems sensor related.

If you truly want to diagnosis this, you'll need to get familiar with romraider and logging. This will give you access to all the sensors and will let you see what each on is reporting during this situation, and you will be able to narrow it down and hopefully identify the false one. There is a good thread over on subaruoutback.org on how to use it.

http://www.subaruoutback.org/forums...-guides/184170-romraider-logging-dummies.html

FreeSSM is slightly easier to use and can give you some quick info, but doesn't give you logging ability.

However, will no ill effects other than piece of mind, I would say try to figure out how to live with it until it shows a clear drivability issue?
 

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Discussion Starter · #3 ·
Thank you for the prompt reply, much appreciated.

I did make a quick video yesterday of the issue. Looking back I need to make a better one with the MPH in screen just to better show the issue. But this shows me driving 65-70 MPH on the tollway. The latter half of the video shows the dropping more significantly. There is slight loss of speed as expected when letting off the pedal but the drop in RPM is much greater that it should be (unless it was a functional design of the car to help MPG which I don't see any mention of anywhere).

https://www.youtube.com/watch?v=r0uXLmc53qc&feature=youtu.be


A friend mentioned it could be trans fluid related due to pressure sensitivity. Maybe the wrong fluid used by previous owner... Not sure if that would be the cause though but another avenue I may have to explore. Not sure if a plug or cable was missed when the car was being repaired for the head gasket either, but that's a thought.

I'll look into that romraider device. I really don't want to put much more money into this vehicle if I can avoid it.

As you mentioned I'm not seeing any ill effects from the issue. But having just over a week of driving time and months of repair time so far I'd prefer to cut my losses now then have the trans blow or something else catastrophic happen.

Thanks again,
Nate




Sorry, head gasket on these is no fun.

RPM should be related to wheel speed in the situation you are describing since the torque converter should be locked up. I can't imagine a situation when cruising at 70mph in 5th gear and the RPM drops suddenly when letting off the gas without the car significantly changing speed. So this would seems sensor related.

If you truly want to diagnosis this, you'll need to get familiar with romraider and logging. This will give you access to all the sensors and will let you see what each on is reporting during this situation, and you will be able to narrow it down and hopefully identify the false one. There is a good thread over on subaruoutback.org on how to use it.

http://www.subaruoutback.org/forums...-guides/184170-romraider-logging-dummies.html

FreeSSM is slightly easier to use and can give you some quick info, but doesn't give you logging ability.

However, will no ill effects other than piece of mind, I would say try to figure out how to live with it until it shows a clear drivability issue?
 

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It would appear your torque converter is not locking up when expected. When it is locked up RPM and speed are tied together. I'm not sure when lockup occurs, but sure by 70 MPH in 5th. Lockup is best for MPG performance.

I suspect that in fact nothing is wrong, and this is normal and happens in any gear when you aren't locked up. But you need to calculate when you are expecting lockup based on gear ratios and speed.

I am not a transmission expert. You might want to ask over on subaruoutback.org to see if anyone has better knowledge.

It certainly can't hurt to change the fluid to subaru ATF-HP fluid (full flush instructions here: http://legacygt.com/forums/showthre...tml?t=197248&highlight=DIY+transmission+fluid )

Here's another good forum with a lot of experience with the same 5EAT transmission: http://legacygt.com/forums/showthread.php/5eat-ask-your-questions-185412.html

Tom
 
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