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Bogging, bucking, backfiring under light-medium load


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Hey all, I have a 2005 Legacy GT Limited with 96k miles. I just barely got the car back from getting a new motor and am having issues with it bucking and bogging under light to medium acceleration. It seems fine until about 2,500 rpm and then the turbo spools, car bucks, and when I let off, it backfires. It bogs while doing this, and AFR richens up (according to AP, 11.0 or lower) and won’t hit over 1 lb of boost. It ran just fine for the first 200 miles of break in and then started doing this while I was doing a short freeway drive. It popped a misfire CEL on all 4 cylinders as well as one for the front O2 sensor(P0031). It would hardly drive and I had to be towed home. I replaced the front and rear o2 just to be safe, still had the same problem. So I checked all the vac lines visually and couldn’t see anything. I checked the plugs and they were fouled, so I replaced them with NGK iridium one step colder plugs. No difference. Cleaned the MAF. Still no change. I still need to get it smoke tested or air tested for boost leaks or vac leaks. I was thinking besides that, it could be the fuel pump or maybe my injectors. Any other ideas? Or steps on the best way to clean my injectors out if they’re plugged? I have videos of what the car is doing on my phone, as well as a video of the accessport, I’ll try and see if I can get them uploaded.
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Make sure all the vacuum hoses are routed correctly.

 

Did the car run well before the rebuild ?

 

To upload, you may have to log out of the forum, reboot your PC and then log back in.

 

Log out on the home page in the upper left under your avatar.

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

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Well it leaned out and melted a hole in one of the pistons, due to a major inlet leak I couldn’t find because I was smoke testing it post inlet. But other than that, it ran just fine. Do you have a link to the vac line diagram? I saw one on here a while ago but I couldn’t find it again, only the ‘vacuum leaks and where to check for them’ thread..
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http://people.csail.mit.edu/ilh/vacation/

 

In the back of the service manual is a picture of the vacuum hoses, I think one of the sticky's has a link too.

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

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I've got two datalogs from the Accessport. I don't think timing is the problem because if it skipped timing, it would've bent valves and not run, correct? Or am I misinformed? Anyway, first log is it doing it while driving, second log is the same thing, but at the end I am parked and give it quick rev and the idle drops and there's a couple misfires and the idle returns back to normal. the videos won't upload because they're not a valid file type, and unfortunately, I have no idea how to change them.

datalog9.csv

datalog10.csv

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  • 2 weeks later...
Bump for more assistance.. car isles fine until I rev it and then it dips, misfires a couple times, and returns to normal. Running really rich. I don’t really have money to throw at it so I just can’t go at replacing a bunch of parts.
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One step colder plugs might just exacerbate the problem. Smell a post turbo leak? I had a poor fitting intercooler to throttle body hose once (clamped down but not not seated correctly on the tB)---caused similar issues (not as bad though). It can be tricky to line up actually so might be worth a look!
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I found the problem. I forgot on the old vf40 turbo that the cold side threads for the TMIC connection strip fairly easily. Mine stripped out on my new vf52 and cause the connection to become very loose and let in a bunch of air. Installed a through bolt w nut to fix it. Luckily I had a buddy that was willing to rev my engine for me whilst idling otherwise I would’ve never seen it. Thanks for all the advice guys, I’ll keep that in mind if I ever have a similar problem again.
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