Acceleration Surge

Before I start, yes, I did search the forum before starting a new thread but I didn't see anything that quite answered the questions I have. I just bought a 2008 Legacy GT Sedan, 5MT a couple weeks ago. 88k miles, one owner, 100% stock. The problem I'm having happens in all 3 Si-drive modes but is most noticable in Sport Sharp. When accelerating at moderate to full throttle through the RPM range, starting when it first comes under boost and continuing beyond 4,000 RPM, I get a surge. What it feels like to me is that the wastegate is opening when it shouldn't and is bleeding off boost pressure intermittently. Once I get into the upper RPM range it seems to even out and boost steadily but it also feels like the power drops off more than it should. I'm plenty mechanically inclined but this is only my second forced induction vehicle, and first turbo. Any ideas or suggestions from the wealth of knowledge here on the forum?

Pulling a data log would help. Maybe a boost selinoid issue ?

May not help but I've found my cars run run smoother after I claen the MAF sensor. the first time I did after a few years made big difference. Now I do them once a year.

 

Which is the same time I clean and lighltly oil my air filters.

^^What Max said. MAF can cause goofy things to happen with surging. Also, dheck your front O2 sensor. If you know someone that has a known good one, change it out and drive the car to see if anything changes. It may be on the verge of going bad and is beginning to do funky things.

 

As ibr said, a data log or LV would help immensely if you have that capability.

Unfortunately I don't have the setup for a data log or LV but I'm kicking off turkey day with new front wheel bearings tomorrow so I'll look into the MAF idea while I'm at it. As far as the O2 sensor if the MAF doesn't do the trick that will be my next stop. Thanks for the responses!

 

Sent from my DROID RAZR HD using Tapatalk

If you replace the front O2, make sure you use a OEM one, My aftermarket only lasted about 2 years.

 

My rear aftermarket O2 is still going strong.

 

Oh also only use OEM front wheel bearings, Moog's will work for the rears, but not the fronts.

 

Some have had good luck with NAPA fronts too.

If the O2 sensor or A/F sensor is going back, it would most likely throw a code when readings are outside the parameters. I would stick with cleaning the MAF sensor first and if that doesn't help look into a laptop and download the free software to pull the data off the ECU.

^I would buy this cable and download learning view.

 

[ame=http://www.amazon.com/BestDealUSA-OBD-II-2-409-1-Cable-VAG-COM/dp/B008HO23OC/ref=sr_1_2?ie=UTF8&qid=1385584400&sr=8-2&keywords=vagcom]Amazon.com: BestDealUSA USB OBD-II-2 KKL 409.1 OBD2 Cable VAG-COM for VW/AUDI: Car Electronics[/ame]

If the O2 sensor or A/F sensor is going back, it would most likely throw a code when readings are outside the parameters. I would stick with cleaning the MAF sensor first and if that doesn't help look into a laptop and download the free software to pull the data off the ECU.

 

Not necessarily. I had a front O2 sensor go bad-car was surging all over the place in STFT from +25 to -25 and there were no codes or CEL. I kept watching from scanner and decided that it was an oxygen issue and changed the front O2 with a known good one and the problem vanished.

 

These cars have stinking issues that sometimes are a crapshoot, but I do know that the front O2 has caused lots of problems without ever setting a code. I know 3 other people that had the same issue I did and it fixed every one of them.

 

Not saying that it's a fix all but definitely worth a look.

Awesome thanks again lots of good advice here!

 

Sent from my DROID RAZR HD using Tapatalk

What does the RPM band look like when it's doing this?

Anyone have part numbers for front 08 OEM or aftermarket oxygen sensor?, ebay is like a minefield full of them!

The dealers in my country want $400+ for one!

bosch 15011 was the part number for the front on my 05.

Hello, so I recently purchased a 2008 LGT and drove it back from Chicago. When I would be getting on the highway I was experiencing the exact same problems. Our local Subaru guru had suggested that we change the plugs and reset the ECU. It felt like it was a learned issue and possibly on bad gas. Needless to say, after logging it was looking pretty rough. Got the plugs changed and reset the ECU and let the car learn for roughly 1.5 hours, and I took a second gear pull ( most noticeable hesitation) and she pulled clean all the way through. I then went and did some logging with a 3rd gear pull and everything is looking great. Just something to think about.

Hello, so I recently purchased a 2008 LGT and drove it back from Chicago. When I would be getting on the highway I was experiencing the exact same problems. Our local Subaru guru had suggested that we change the plugs and reset the ECU. It felt like it was a learned issue and possibly on bad gas. Needless to say, after logging it was looking pretty rough. Got the plugs changed and reset the ECU and let the car learn for roughly 1.5 hours, and I took a second gear pull ( most noticeable hesitation) and she pulled clean all the way through. I then went and did some logging with a 3rd gear pull and everything is looking great. Just something to think about.

 

The easier the fix, the better. Good for you.

My car does the hesitation followed by a surge as well, mostly when I go from moderate to full throttle in 2nd. I've cleaned the MAF sensor and am running an OTS Stg2 tune. I haven't noticed it lately since I haven't been paying much attention, but am sure that it's still there to some degree. I picked up a GS EBCS, so maybe that will resolve the issue?

 

Quick question, and pardon my ignorance if it's a stupid question: But when I'm just lugging around town, I'll shift from 1st to 2nd at ~2.7-3k. When I get into 2nd and try to go harder on acceleration (not necessarily WOT), that's when I experience the most hesitation followed by a surge of acceleration as the car "reacts" to the throttle input and lunges forward. Now.. the question is: should I be feeling a consistent and gradual pull of power? or is the hesitation + surge normal due to the change in throttle pressure?

subscribed, my car seems to be doing something simiar. I thought it was the clutch slipping, but what is being described here sounds like it might be the issue. I have a new motor w/ a dyno tune from infamous performance but it has done it ever since I had it tuned. It is most noticable in second gear from 2k-2.8k RPMs. When Mike from infamous logged the clutch slipping he said the AFRs were spot on. The clutch was replaced and this one is doing the exact same thing. I have a full 3" exhaust, FMIC, EBCS, and BOV that I need to install and am hoping to get it re-tuned by his new tuner. I'll update when I figure it out...

As an update to my original issue, I worked on the car on thanksgiving, replaced wheel bearings and checked the maf as was recommended. It is spotless but the car still had an OEM style paper air filter and it was filthy. I installed a K&N and it has gotten slightly better. Now when the car first comes under boost there is a slight dip but it pulls smoothly after that. The old filter seemed to be just barely undersized for the air box, and after installing the new filter the noise from the turbo and bov is way quieter as well.

 

Sent from my DROID RAZR HD using Tapatalk

Here's something else I just found:

http://legacygt.com/forums/showthread.php/jerky-acceleration-178092.html

A lot of this sounds like a typical LGT. Many LGTs have a noticeable dip in power around 3000 to 4000 rpm, I know mine does. I've spoken with my tuner (Jarrad @ PDX) about it (a long time ago), and it's pretty par for the course according to him, even on tuned LGTs.

 

A lot of it can be tuned out, but apparently it is there for a margin of engine safety. Again, a discussion a long time ago...it's correct to the best of my knowledge.

 

Whatever the reason is, it's normal. Your cars are fine

you might try resetting the ECU and wipe out any bad learning acquired from the old filter situation. Especially if there was air leaks around the filter. Could cause some turbulence in the intake tract that messes with the MAF readings.

OK here's a stupid question... I'm a good mechanic and I know cars but I'm not as familiar with these yet as I'd like to be. I hear a lot of stuff about resetting the ecu, are you guys talking plug in a code reader and reset that way or pull the negative battery cable for x amount of time or is this something you do through a tuner?

 

Sent from my DROID RAZR HD using Tapatalk

You can reset by an OBDII reader or by disconnecting the battery for about 30 secs.

Simple enough. I'll do that tonight or first thing in the morning and see if that makes a difference.

 

Sent from my DROID RAZR HD using Tapatalk

So I finally got around to resetting the ECU, pulled the negative cable and just left it off while I was getting ready for work. I can already feel a difference in the way the car pulls through 2nd and 3rd in sport sharp. There's still a slight flat spot which feels like what everyone on here refers to as normal, but the super jerky on/off feeling is gone, which is good.