taysubaru Posted November 30, 2019 Share Posted November 30, 2019 Hi All, My 05' Legacy GT has been acting up. First, at cold idle, the RPM jumps up and down in an unusual and cyclic way. Then when I start accelerating and cruising, between 1500-2000 RPM, the RPM is "hunting" (or jumping up and down) even though I have a constant throttle position. You can feel the power surging in a cyclic way. Finally, when I give more gas, the car downshifts and powers normally through 3000-5000 RPM with boost no problem. Unfortunately it hasn't kicked a CEL, so the fix isn't obvious. Replaced spark plugs, air filter, and checked the throttle, seems clear. Unplugged the MAF and car ran okay at first, but the problem started again, so I doubt it's the MAF. I wonder if this might be a bad O2 sensor, throttle position sensor, or cam position sensor, but hard to tell without a check engine light...? Before I start ordering new sensors, has anyone experienced a similar issue? Thanks in advance. Taylor Link to comment Share on other sites More sharing options...
Carmania24 Posted November 30, 2019 Share Posted November 30, 2019 Have you checked for possible intake leaks? If not get a can of starter fluid or something similar and spray all around the engine. If you hear it surging, you have a vacuum leak. Link to comment Share on other sites More sharing options...
Nonamedude Posted November 30, 2019 Share Posted November 30, 2019 Another thing to do would be check your learning values with romraider if you have the tools. Or with an access port. Then you could also look into the deeper stuff and see if your afr is doing weird things (bad o2 sensor) or you're having knock and it's pulling timing or something weird. I think with those you can also look to see if the trans sensors are acting weird. Link to comment Share on other sites More sharing options...
taysubaru Posted December 3, 2019 Author Share Posted December 3, 2019 Hi All, Thanks for following up. I unplugged the front O2 sensor (even though no CEL was kicked), and the problem seems to have subsided. This leads me to believe it's O2 sensor related, so I am thinking I will replace both front O2 sensors, and see how that goes. Any insight on replacement part? Looking at either the Denso OEM or the Walker part on rock auto... Cheers! Link to comment Share on other sites More sharing options...
gkinslow Posted December 3, 2019 Share Posted December 3, 2019 OEM only for O2 sensors. Link to comment Share on other sites More sharing options...
taysubaru Posted December 3, 2019 Author Share Posted December 3, 2019 Thanks gkinslow. So the Denso OEM (#2349120) is a good bet? I was checking out the Walker (#25054058) as it's a bit cheaper and my friend has done exhaust replacements with their parts before, but I certainly appreciate your advice! Don't want to mess around with a junk O2 sensor. Link to comment Share on other sites More sharing options...
Max Capacity Posted December 3, 2019 Share Posted December 3, 2019 (edited) https://legacygt.com/forums/showthread.php/idea-web-links-saved-various-parts-219238p5.html That thread will have a post with this Denzo p/n 234-9120 That's the one you want to use. Find the DIY on this forum, It's easy to change once you pull the right front wheel. https://legacygt.com/forums/showthread.php/replace-upstream-oxygen-sensor-cel-p0031-242521.html In a post in that thread you'll see I mentioned putting a screw type hose clamp around the O2 socket. https://www.homedepot.com/p/Lisle-22-mm-Oxygen-Sensor-Socket-LIS12100/306763188?mtc=Shopping-BA-F_HC_A-G-D25T-25_31_GARAGE_AUTOMOTIVE-NA-NA-Feed-PLA-NA-NA-Auto_RLSA&cm_mmc=Shopping-BA-F_HC_A-G-D25T-25_31_GARAGE_AUTOMOTIVE-NA-NA-Feed-PLA-NA-NA-Auto_RLSA-71700000037147753-58700004141485307-92700048672352091&gclid=EAIaIQobChMI_qbSqumZ5gIVVtyGCh06dg9WEAQYBCABEgL78vD_BwE&gclsrc=aw.ds Edited December 3, 2019 by Max Capacity 305,600miles 5/2012 ej257 short block, 8/2011 installed VF52 turbo, @20.8psi, 280whp, 300ftlbs. (SOLD). CHECK your oil, these cars use it. Engine Build - Click Here Link to comment Share on other sites More sharing options...
Nonamedude Posted December 3, 2019 Share Posted December 3, 2019 For the oem vs non oem thing, my civic only gets oem ps and mt fluid cuz they are a weird formulation, nothing else because it's an inexpensive vehicle with no power and a tiny engine. The outback gets a lot more, almost exclusively oem stuff. The increase power output and engine size along with all the ecu adjustments due to the turbo make it more sensitive to parts performing at only 90% where the civic might not notice of the parts only kinda work. These cars seem to have lots of stuff like that. Sensors, axles, various suspension components Sent from my iPhone using Tapatalk Link to comment Share on other sites More sharing options...
gkinslow Posted December 3, 2019 Share Posted December 3, 2019 Crap O2 sensor can cause all kinds of odd issues on these cars without a CEL. There are a few things that you should not substitute on these cars. O2 is one of them. Link to comment Share on other sites More sharing options...
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