Rough Idle & Engine Shaking under load like AC

Posts specific to the 2.5 litre turbo engine

Re: Rough Idle & Engine Shaking under load like AC

Postby Stifull » Sat Dec 19, 2020 8:38 pm

LOL.. Yes.. We have had many subarus for many years. We currently have 5 in our household. We have NEVER taken ANY of them to a dealer except for airbag recalls and true to form they damaged the dash on my Lib GT in the process...Great product...but never take it to them for service or repairs..We only take them to Subievolution when it's something we can't do ourselves. They do an awesome job. I think Russell spoils us because we have soo many .
Current Subarus
07 Liberty GT (mine)
98 WRX Club Spes 2 with 2.5ltr upgrade(mine)
99 Ver 6 Sti also with 2.5ltr upgrade (son)
02 WRX Bugeye Hatch lots of goodies(wife)
09 Non Turbo Impreza(daughter#4)
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98 GC8 WRX Hatch(daughter#1)
02 Bugeye WRX (daughter#2)
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Re: Rough Idle & Engine Shaking under load like AC

Postby ilib11 » Sat Dec 19, 2020 9:48 pm

Hi All,
Thanks for all the hints!
In addition to leak tests, I'll ask to examine spark plugs too.

Re air filter: the air filter has been changed to a new one, installed ok.

Re MAF - First I did I took it out myself and cleaned it with a CRC MAF spray - as I usually do on a regular basis. That did not resolved the issue. O-ring looks ok, and MAF sits tight.

Re upper engine clean - I cannot tell if it was actually done. I saw stories on Whirlpool about subi services which just do not do that to save time/effort and throw out these upper clean chemicals cans. Where would you connect in 2.5T intake to do the upper engine clean?

Re spark plugs torque - may I kindly ask on how the spark plugs torque make a rough idle? On any other cars I had I did not have any problems by just screwing in spark plugs hard enough. These engines were not boxers though.

Thanks and regards, ilib11
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Re: Rough Idle & Engine Shaking under load like AC

Postby bigBADbenny » Sun Dec 20, 2020 3:39 pm

ilib11 wrote:Re upper engine clean - I cannot tell if it was actually done. I saw stories on Whirlpool about subi services which just do not do that to save time/effort and throw out these upper clean chemicals cans. Where would you connect in 2.5T intake to do the upper engine clean?

Re spark plugs torque - may I kindly ask on how the spark plugs torque make a rough idle? On any other cars I had I did not have any problems by just screwing in spark plugs hard enough. These engines were not boxers though.


UEC: see the seafoam link in my tune FAQ (link in my signature), usually the BOV ref hose on a turbo car, wrap the straw in tape to get a good seal on the hose.

Sparkplug TQ is definitely a factor, misfires are calculated from discrepancies in crank vs spark timing plus afr.
I'm not sure exactly what triggers the cel in this instance, could be compression related?
Last edited by bigBADbenny on Thu Dec 24, 2020 12:08 pm, edited 1 time in total.
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Re: Rough Idle & Engine Shaking under load like AC

Postby ilib11 » Sun Dec 20, 2020 5:05 pm

Thank you, adding UEC to the list of actions....

FYI - Here is what I found for misfires - page GD(H4DOTC)-95 in the attached file describes the algorithm - as I understand, a misfire would change immediate crankshaft speed so that is now it is detected.
The file is from 2010 Subaru Legacy and Outback Factory Service Manual. I am pretty sure you have it, or let me know if not and would like to have :)
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2. Diagnostic Trouble Code (DTC) Detecting Criteria.pdf
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Re: Rough Idle & Engine Shaking under load like AC

Postby bigBADbenny » Sun Dec 20, 2020 7:51 pm

If that’s a general market/adm GenV FSM absolutely I’d like the link to add to the list.
I’ve not found one yet online, only usdm :)

Did you invest in your own Romraider setup with Tactrix or vagcom?
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Re: Rough Idle & Engine Shaking under load like AC

Postby ilib11 » Sun Dec 20, 2020 10:49 pm

This particular one was USDM from here: http://jdmfsm.info/Auto/Japan/Subaru/Legacy_Outback/2010/ There are bunch of other stuff both for Subaru, other cars and relevant info on that site.

Yep, I have a laptop with romrider and a vagcom cable. I did have a chance to work on some fun cars & engines ( VW Corrado & Passat, G60 Syncro) in my previous life but do not have a workshop place & toolsets anymore :(
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Re: Rough Idle & Engine Shaking under load like AC

Postby bigBADbenny » Mon Dec 21, 2020 5:00 am

Yep I’ve been using that site a lot, linked in the faq.
Some of the gen4 FSM’s there are “live” as well, that is direct access from a browser, but not the GenV FSM’s alas.
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Re: Rough Idle & Engine Shaking under load like AC

Postby ilib11 » Mon Feb 22, 2021 4:23 pm

Hi All,
just an update on this problem...
Went to a service for a searching for leaks and nothing found. As a workaround, increased idle RPM so it is >=800 under load (Auto Trans at R, A/C is on). At least that masked the problem and the shaking is not that violent (but still exists).

Reading the forums, bumped into the following https://forums.nasioc.com/forums/showpost.php?p=44412059&postcount=31 - which could be another theory to test, even if I do NOT have P0340 code:
"it appears that indeed, a bad alternator can truly cause the P0340 code. apparently a bad diode or voltage regulator sends a noisy signal through the harness and the cam sensor reads this as a fault. So anyone experiencing this code and has the primary symptoms of momentary rough idle especially when the AC is turned on, please check the alternator. it may appear to be sending proper voltage or even pass testing, however internally its faulty ".
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