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Discussion Starter · #1 ·
Hey guys. I have been experiencing a weird RPM fluctuation with my 2015 FXT. This is my first turbo car so I am not familiar with how turbos work. I bought the car used at 36k miles and I don't know how it was when new.

This fluctuation only happens at cold start when the car sits overnight. When first started up, the boost shows -4 PSI for about 15-20 secs. During this time, RPM jumps up and down from 1500-2000, more like 1500-1700 but you get the idea. Car shakes quite a lot. After the first 20 secs or so, boost shows -10 PSI and everything is stable at this point. Idle is smooth and RPM stays at 1500 until blue light goes out. Car still runs fine. I am still able to hit 16 17 PSI so I don't think the car has any boost leak.

I dropped the car off at a dealership last week due to CVT leak. I mentioned the rough idle issue and the advisor told me that the tech updated the ECU or ECM. Unfortunately, the update does not address this issue. I have read many threads on this forum and WRX forum since both cars share same engine. I only found one other forum member has this same experience but unfortunately, he did not mention anything more about it. So here I am posting up a thread to see if anyone else experiences the same thing or has any ideas. I just need to know is it normal or not. If not, what could be the cause so I can drop off the car at the dealership again. It is still under powertrain warranty.

Thanks guys.
 

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I had the same issues on the 15 FXT, and I had the dealership do two things, at separate times:

1. Top Engine Treatment - This is "supposed" to remove all the soot on the valves from lack of fuel, due to DIT. @$176, was pretty pricey, and did NOT help the cold start. It did, however, reduce the amount of soot under full boost.

2. New battery - Not sure if the battery removal reset the ECU, or if a weak battery gave the electrical system gremlins.

My FXT has 65k miles on it and is performing wonderfully, minus the bluetooth going out.
 

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Discussion Starter · #3 ·
I had the same issues on the 15 FXT, and I had the dealership do two things, at separate times:

1. Top Engine Treatment - This is "supposed" to remove all the soot on the valves from lack of fuel, due to DIT. @$176, was pretty pricey, and did NOT help the cold start. It did, however, reduce the amount of soot under full boost.

2. New battery - Not sure if the battery removal reset the ECU, or if a weak battery gave the electrical system gremlins.

My FXT has 65k miles on it and is performing wonderfully, minus the bluetooth going out.
I am looking into top engine treatment as well as my car is close to 38k miles now and I have no record of what service has been done to it. I've searching around quite a lot to learn more about FXT since I want to keep my car as long as possible.

So I assume the second time you dropped it off (whether new battery or ECU reset) solved the problem with cold start RPM fluctuation? I am also looking into unplugging the negative terminal and leave it overnight to reset ECU just to give it a try. I hesitate because I think that the whole ECU relearn process is complicated and unnecessary. I don't think leaving the car idling for 20 mins is safe due to fuel dilution.
 

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I had a blonde moment.

Yes, the new battery got rid of 95% of the cold start issues. I don't if it was because of A: New battery with good voltage or B: ECU reset.

Supposedly, when the car got flashed with new ECU recall/updates a year earlier, the trims were reset, but I did not notice a difference. A couple months after, I went in for the Top Engine Treatment. The new battery was installed in the beginning of this year. It has been doing great ever since!

I wouldn't worry about fuel dilution from idling if you get on it every now and then. >:) My drive to work is about ~20 miles and I make it a point to floor it while merging into traffic. Even with extended oil changes (12k miles), my UOA from Blackstone Labs come back great.
 

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Discussion Starter · #5 ·
I had a blonde moment.

Yes, the new battery got rid of 95% of the cold start issues. I don't if it was because of A: New battery with good voltage or B: ECU reset.

Supposedly, when the car got flashed with new ECU recall/updates a year earlier, the trims were reset, but I did not notice a difference. A couple months after, I went in for the Top Engine Treatment. The new battery was installed in the beginning of this year. It has been doing great ever since!

I wouldn't worry about fuel dilution from idling if you get on it every now and then. >:) My drive to work is about ~20 miles and I make it a point to floor it while merging into traffic. Even with extended oil changes (12k miles), my UOA from Blackstone Labs come back great.
Thanks Subyguy. I am dropping off the car at the dealership this weekend. It has CVT leak again. The first repair was done 2 weeks ago and I guess the bond or seal fails again. Anyway, I am bringing this issue up with the advisor this time. Hopefully he knows what to do. If not, I will try to reset the ECU.

On a side, does anyone know if it is possible to request to have a service history of the car from the dealer? I would like to know what the previous owner or other dealers have done to the car before I bought it.
 

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2008 Subaru
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Thanks Subyguy. I am dropping off the car at the dealership this weekend. It has CVT leak again. The first repair was done 2 weeks ago and I guess the bond or seal fails again. Anyway, I am bringing this issue up with the advisor this time. Hopefully he knows what to do. If not, I will try to reset the ECU.

On a side, does anyone know if it is possible to request to have a service history of the car from the dealer? I would like to know what the previous owner or other dealers have done to the car before I bought it.
Possibly, if the car was serviced at that particular dealership. If it was serviced at a different dealership you'll be out of luck, as the systems are not typically integrated across the dealership network.
 

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2014 Forester XT
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On a side, does anyone know if it is possible to request to have a service history of the car from the dealer? I would like to know what the previous owner or other dealers have done to the car before I bought it.
Same boat as you. Bought my 14 used. The only thing the dealer may be able to tell you, particularly if it wasn't the same dealer the previous owner used, was any work done under warranty.
 

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I had the same issue with rough idle when I bought my 15 XT with 66k miles with a new battery in August of 2018. In the next few days after purchase, the check engine light came on after a rough cold start idle and it was a misfire code. They installed a new alternator. Next day I had the same rough idle again at cold start and again the check engine light came on for misfire. This time they did some kind of a turbo related update/patch on the ECU and that basically almost entirely took care of the rough idle issue, the idle hasn't be rough at cold starts since then and no lights have come on. I have 92k miles now. Although idling is still fluttering lightly everytime at cold starts I think it's just a normal characteristic of these turbo engines in the XTs and they're supposed to be driven hard from time to time so that gunk crap doesn't build up as much inside. I run 1 spray can of CRC intake valve and turbo cleaner just before every 2nd oil change which helps get rid of that internal build up in my opinion. Hope this is informational, thought I'd share.
 
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