05 LGT 5MT wagon - stutter between 2-3K RPM

I know there's a master list of all sorts of stumble and/or stutter stories, but I'm a little out of ideas here and could use some help.

 

My 05 Legacy GT wagon has 209K on it and has had this stutter since I bought the car a year ago with 202K on it. The car has a little bit of a jump in RPMs by about 50-100 RPMs throughout the 2-2.8K RPM range. If I reset the ECU then the problem typically goes away for quite awhile. The stutter occurs at this RPM regardless of how hard I'm pushing the car.

 

The PO told me that this started to occur when he modified the car. He did:

 

Butterfly valve removal style TGV deletes

A catless downpipe (that I've changed out for a catted Invidia)

A Perrin turbo inlet

Replaced the leaking VF46 with another one

Replaced the OEM uppipe with a catless Perrin uppipe

Had the car tuned by PDXTuning.

 

The turbo has no weird shaft play, the car passed a leakdown test with 4-9% leakage on all cylinders, a pre-dyno smoke test reveals nothing, nothing shows up in logs, zero check engine lights, I've replaced both O2 sensors, the MAF sensor, spark plugs and the problem occurs every time I drive the car. It sometimes seems better for the 1 or 2 minutes after I cold start the car, but it's hard to say. I recently replaced the worn OEM motor mounts with Group N mounts and the stutter is much more annoying now because I can feel it much more. I've blasted Seafoam through the BOV vacuum line, through the fuel, and through the oil. Nothing has solved the problem, apart from resetting the ECU for it to come back later.

 

I've had PRERacing do some diagnostics on the car. They found no exhaust system leaks or leaks in the engine bay. Spark plugs were gapped fine. The tech thought he heard possible valvetrain noise and removed the valve covers and noted the cams were worn, but nothing unusual for the mileage on the original engine. Leakdown and compression test showed zero issues at all. The tech and I drove the car after he spent a day doing diagnosis and we found the car drove almost completely perfectly and only gave us the stutter if you lugged the hell out of the engine at those RPMs. He did note that he removed the battery to check the spark plugs, which pretty much erased the problem during our drive. Their tuner Erich tried a few things with his tuning knowledge and couldn't recreate the problem since the ECU had just been reset. He's since tuned the car and pulled a bit of timing out of the car during those RPMs and the stutter still occurs.

 

I need somebody... and not just anybody...

There's a thread on here about this issue.

 

Take the vacuum hose off #4 intake runner, cap that port. Now T that line into the small vacuum hose off the blow off valve. That will give the fuel pressure system a constant signal.

 

Contact Mike at tuningalliance@gmail.com tell him your issue. He fixed that stumble on my wagon years ago.

 

I'll assume you have a catless up pipe ?

There's a thread on here about this issue.

 

Take the vacuum hose off #4 intake runner, cap that port. Now T that line into the small vacuum hose off the blow off valve. That will give the fuel pressure system a constant signal.

 

Contact Mike at tuningalliance@gmail.com tell him your issue. He fixed that stumble on my wagon years ago.

 

I'll assume you have a catless up pipe ?

 

I've read through the whole master thread and tried a few things. I'm thinking it could be a grounding issue but it seems odd that it would manifest itself after modifying the car. The headlights dim briefly when pushing the clutch in and the alternator is putting out the power it's supposed to when doing a test of it.

 

Sorry to sound dumb, but which vacuum hose would this be? Is it 17 on this diagram?

http://i.imgur.com/62j7Qyc.png

What also is the purpose for this? Testing the fuel system?

 

I don't have an AP or Tactrix cable, but the car has been tuned no less than 5 times since this issue occurred by two different tuners and neither has been able to figure out the problem via tuning.

 

 

I do have a catless uppipe.

17 is a coolant hose to the throttle body.

 

The two bottom engine grounds are easy to replace, with any home made ground wires.

 

Also clean the two terminals on the end of the ground bar by the throttle body.

 

Edited December 24, 2019 by Max Capacity

https://legacygt.com/forums/showpost.php?p=5191064&postcount=518

 

It's a tiny hose going from the fuel breather line to the intake manifold. My understanding is this location is not a very clean reference for vacuum and some sort of idk like resonance causes an unclean fueling signal. (I know those words don't make sense but that's the tl;Dr interpretation)

 

 

That's the guy. It's under the bracket for the intercooler. If you take the effort to trace it, it goes to the fuel vapor line all the way back to the firewall or fuel pressure regulator depending on year.

 

 

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Yes, that's the port you want to plug, I have done that on both my cars. Both T into the small hose off the blow off valve. Reset the ECU after doing this.

 

 

I had some weird hesitation on my 05 OBXT that turned out to be bad OCV valves. I replaced them with brand new ones from the dealer and my hesitation went away,.

Thanks for all of your replies guys. Given the time of year, I have had absolutely no time to work on the car and have just been dealing with it and may need to hand it off to a proper mechanic to work on it for the time being.

 

The theory with the OCVs is interesting. I’ve done a Seafoam treatment of the car before and I noticed that, with Seafoam in my oil, the car ran maybe 95% of the way to perfect. Could potentially have a clogged valve. Looks easy enough to do. I will definitely look into the fuel line issue as well. Thanks!

Wow, replaced both OCVs and the car runs like new. Can not believe how smooth it drives now. Thank you guys so much! Literally haven’t wanted to drive my car for fun because of this issue. I feel confident about going up to Seattle in a couple weeks now. Literally cannot thank y’all enough.

No problem! Glad to help.

I had some weird hesitation on my 05 OBXT that turned out to be bad OCV valves. I replaced them with brand new ones from the dealer and my hesitation went away,.

 

I had a similar issue arise (albeit intermittently) with my 05 OBXT and OCV valves also resolved it for me. New, OEM OCVs. And I got them pretty cheap online from Perry Subaru (https://www.subarusource.com/).

Yes OEM for OCV unless you want to buy another set in short period of time. Or so I've heard.

One question: should I reset the ECU afterwards? Already did, but curious.

Per almost every unabomber faq on nasioc, it never hurts to reset the ecu after doing stuff.

 

 

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I had a similar issue arise (albeit intermittently) with my 05 OBXT and OCV valves also resolved it for me. New, OEM OCVs. And I got them pretty cheap online from Perry Subaru (https://www.subarusource.com/).

 

 

Oh, these, ?

 

Valve Ay Oil Control

Part Number: 10921AA020

Browse more in Engine - Valve Train Components

 

Description: Incl.Oil Control Valve More

 

$126.18

$79.50

Add to Cart

 

$126.18

$79.50

Oh, these, ?

 

Valve Ay Oil Control

Part Number: 10921AA020

Browse more in Engine - Valve Train Components

 

Description: Incl.Oil Control Valve More

 

$126.18

$79.50

Add to Cart

 

$126.18

$79.50

That’s the exact part number that I purchased two of from Wilsonville Subaru. I’ve read that those should be replaced if your turbo ever blows, but I’m not certain mine has ever “blown” previously, just shown extreme signs of age without outright failure. Regardless, something was wrong with both of mine because I replaced the driver’s side, drove for a few minutes, noticed it was better but figured I should go buy the other side too. Bought the other one they had in stock, installed it in two minutes, called it good. Car drives better than it ever has while I’ve owned it.

Well guys, I spoke too soon, I guess? Stutter is back in full force again. Weird, as the problem was completely resolved for a few days. I noticed the stutter popping up very slightly every so often so I reset the ECU thinking maybe that was supposed to be done. Now the stutter occurs exactly as it did before. What exactly is going on?

Quick update: switched the OCVs around, passenger to driver side and vice-versa, and the car behaves much more normally again. Something in the tune? Didn't reset the ECU or anything.

You shouldn't tune the ocv to behave differently per side so I don't think so. Possibly some crud in your connection that got cleaned out and is all good now? Possibly a bad out the box ocv and it takes a few days to start being funny? Keep an eye on it but unless one of them was bad, switching sides shouldn't effect it.

 

 

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How old is the oil? Did the OCV's look nice and clean?

You may want to run a can of Gunk engine flush through the oil system before your next oil change.

 

Follow the directions on the can.

How old is the oil? Did the OCV's look nice and clean?

 

Oil had about 4K miles on it (synthetic) when I did the new OCVs. Oil in the car now has about 300 miles on it. The OCVs looked clean, I guess? I’m not sure as they both looked about the same as the old ones, however a lot of really clean oil poured out of the car as if it were trapped in there when I took the old ones out.

You may want to run a can of Gunk engine flush through the oil system before your next oil change.

 

Follow the directions on the can.

 

I’ll do that, thanks.