The Stumble/Stutter Thread

Im having this issue pop up again recently. My gut is relling me its there might be a spring that is loose in the drive by wire mechanism. I saw the stutter/stumble whatever on pump with dw850s and now again with e85 and id1300s. Both times my tuner said it wasnt the tune. For me there is always something a little off when I am barely on the gas which is essentially my foot position during cruising when the tach is 2000-2500.

Ive also become accomstomed to feeling the different vibration frequencys the engine makes when its running lean vs rich. Lean feels more vibrations/insta torque and less when running rich. Wondering if at light throttle in that rev range the car running to rich.

Logs show nonirregularities according to the shop though.

This issue has been diagnosed and a solution found.

It involves replacing the Fuel Pressure Regulator and some fuel lines. Reference 08+ wrx issues.

Adding another case to this list, hoping someone can point me in the right direction.

08 Legacy 2.5i Special Edition, 160kmi.

 

Symptoms: very hard to start, stutters and stalls when coming to a stop in traffic (will not stall if I shift into N and give it some gas to keep the RPMs up). CEL stays off even when it stalls out, but recently had a misfire code in Cyl 3. When idling, if I barely touch the accelerator, the RPMs drop to about 400 and that is when the shaking starts. Other than that it idles fine.

 

What I've Tried: ran a can of seafoam through the last tank of gas to hopefully clean up any injector issues; changed spark plugs (it was due anyway). No help with either.

 

Concerns:

- Could this be the fuel pump going out?

- Coil pack

- Bad injectors

- What else?

 

Maintenance: new plugs at 80k, 160k; timing belt at 100k, all other belts at 120k, regular oil changes, no mods.

 

Any help here would be hugely appreciated!

Post in the 2.5i section so we (other 2.5i owners) can chime in. Your problem isnt associated to this problem as your is completely different.

Just in case it helps - I had almost exactly the same symptoms as turk56, and for me, it was a coil pack.. I do think both 2.5i and 2.5gt could have similar symptoms in this case..

True but that year 2.5i isnt coil-on-plug could be a bad injector, bad AFR, bad plug, jumped timing, MAF issue etc..

Sorry lads , I believe ice just come across this problem over the last month.

My 98 legacy wagon just started doing this, jerking when trying to boost, but without boosting it drives smooth. Would this be the same problem of stuttering/stumbling?

Sorry lads , I believe ice just come across this problem over the last month.

My 98 legacy wagon just started doing this, jerking when trying to boost, but without boosting it drives smooth. Would this be the same problem of stuttering/stumbling?

Woah my bad dude, it was kinda hard for me to understand what you all were talking about as I'm not really familiar mechanical wise.

Plus my mechanic wasn't all too helpful either with trying to diagnose a problem

Anyway I just cleaned my MAF sensor . Now it accelerates fine with no jerking and no more check engine light, ive missed it's boost for a while, so happy to have it going fine now

TYPE OF STUTTER/STUMBLE

Steady throttle/Increasing throttle/Both: sometimes both, but consistently increasing throttle between 2100-2250rpm

MODS

Intake:KNN Typhoon, mishimoto turbo inlet, Perrin TMIC

Exhaust:Invidia catless up-pipe, cat-back magnaflow

Fuel:STi FPR (vacuum line running off BPV)

Turbo:Stock turbo

Engine Management:Cobb AP.3

Tuner and revision of tune(if you don't mind):Erik at Torqued Performance, not sure of revision number

MAINTENANCE

Sensors changed: none

Plugs changed: factory standard plugs with less than 5k miles

Coil Packs changed:none

OTHER SYMPTOMS

Check Engine Light:none

Etc:have a shitty Pilot bearing, and now TOB on my stage 2+ performance clutch (no idea why the P.O. installed this ridiculous clutch)

RESULTS

What has and hasn't worked for you:what has worked is resetting my ECU, then after a couple hundred miles it's back, the FPR mod and vacuum location swap helped a lot by decreasing the overall feeling, but I think I need to get a new fuel pump soon and try that out

I moved my FPR's pressure signal from the intake runner to a T in the BPV hose today, and it made a big difference. I'm not sure it solved the problem 100% but it's pretty close. My car is lowered on 18s with slightly stretched tires (225 on 8") so it's kinda hard to tell whether the remaining roughness is just the road or still the motor.

 

I've tried a bunch of other things in the past (load compensation table, 20" T off of the fuel line, fuel line dampers...) but nothing else fixed the intermittent roughness while cruising in the 2000-2500 RPM range.

 

I'm going to try the same thing on my OBXT next, as it has a similar problem. Being 5EAT, the roughness is somewhat damped out by the transmission, so it took me a while to be sure I wasn't just imagining it. But on the other hand, it's a got a smooth ride so I'm expecting it to be very smooth after this. My next couple weeks are going to be really hectic though, so it might be a while before I make the change.

After some more time, I've realized that the problem is much better when the engine is warm - almost completely fixed, I might not notice it if I wasn't looking for it. But the engine still runs rough when it is cold. That might have a lot to do with my 272 cams though. I still need to get off my butt and redo the FPR reference line for my OBXT to compare.

Adding to this because it's been driving me crazy (and draining my bank account) over the last year. This occurred at some level before, but because more noticeable post motor rebuild.

Type of shutter/jerk: constant speed or light acceleration under moderate load while in closed loop. Runs relatively well in open loop.

Associated symptoms:

Rapid AFR correction #1 variation (~7% swings).

Fuel pressure variation noticed at higher load/engine speed.

Poor idle stability. Idles ether too low or high.

Mods during motor rebuilt a year ago:

New short block

Refreshed heads

New plugs

TGV deletes

Tomei ELH

BNR Turbo

Replaced all seals and hosed (including throttle body and turbo inlet).

Front O2 sensor (changed a month before first motor blew) - Denso

Rear O2 sensor (changed a month after getting it back) - Denso

Mods trying to stop jerking:

Changed injectors to DW740's

Changed fuel pump to AEM, new pre and post filters, and o-rings

Changed MAF sensor

Changed Fuel pressure regulator reference

Changed fuel pressure regulator and pulse damper system to Radium based setup

Changed to ID1050X's with Radium fuel rails, fuel rail pulse dampers, and rubber lines

Results:

None of these helped the moderate load issues. They did smooth out on boost performance, and the ID1050x's have better idle quality.

Previous Test:

Compression and leakdown test - passed

Boost leak test - boost leaking out exhaust?

Fuel pressure - high at startup, stable at low load, unstable at high load/rpm (not tested post ID1050x as sensor failed)

Primary Hypothesis :

Last night I noticed that the front O2 is lazy. Doing a 2nd gear pull from costing to the starting RPM the LC1 in the downpipe came alive ~0.33 seconds before the front O2 did. As there is a lot of piping and a turbo between these so it should be opposite. This APPEARS to be causing the ECU to over correct as it keeps adding short term correction until the O2 sensor catches up. By this point it's already running lean/rich enough to cause a change in torque, leading to the jerking feeling. The sensor has less then 10K miles on it but may have been somehow damaged during the rebuild.

Primary Theorized fix:

New O2 sensor on order.

Compounding hypothesis:

Changes to the system have shifted the exhaust manifold pressure to intake manafold pressure corolation enough to require a changing the front O2 sensor MAP compnesation table. I need to check EGPB vs a stock (ish) car to see if this is needed.

I've been tinkering with AVCS for the last few days and I've all but eliminated the stumble. It's gone during cruise on level ground and there's just a hint of it in top gear going uphill.

Basically, look at the load/RPM region where you get stumble, and add AVCS advance to that area (about 5 degrees), and blend it smoothly with the neighboring cells, and then test-drive... If it helped, or add a few more degrees (or extend your changes into wider load or RPM ranges) and test again.

Also try going up and down mild hills to see what load you're running if/when the stumble happens. Try higher and lower RPM as well.

For me the stumble was worst around 0.75 load and 2500 RPM, so I started adding more AVCS advance in that area, up to 30 degrees, and gradually expanding the region that uses 30 degrees, and eventually came up with the table shown below. I tested it earlier tonight, and so far it seems to work really well.

The shape of this table is obviously a lot simpler than stock, and it might be too simple... I've not been trying to measure power output or anything else, so I imagine this will change as I do more tuning. I got about 5 iterations into the process I described above before deciding that more advance was helping so much that I was ready to scrap the old table and start over with something that has a much simpler shape to it. That may or may not have been wise, but I like the results so far.

Two disclaimers:

I have aftermarket cams (BC 272) and exhaust AVCS (08 STI heads) so 30 degrees might not be ideal for you. Heck, it's probably not even ideal for me, I'm still testing stuff. If more works better for you, use more. Or if you reach a point where adding advance makes the car run rougher, just take a few degrees back out.

Don't copy the bottom few rows of this table - I am also experimenting with AVCS at higher RPM and I make no claim about this being useful. The stock AVCS table ramps down to zero at about 4500 RPM, and yours probably should too. (And no, I don't rev to 9000. I just wanted fewer cells to edit, so I made the last row unreachable.)

You might notice that the far-left column in that table has 20 degrees of advance where the rest of the columns have 30. That's because the stock table used less advance at low load... but I just changed that column to match the others and I like the results.

I think I'm also getting fewer knock-sum increments in cruise and during shifts now, but that's only based on 5-10 minutes of paying attention to knock-sum. I need to watch it more to see if that's for real or just chance.

Yes, that's working well for me. Give it a shot and report back.

I think it makes for a smoother transition between engine braking and cruising, and when getting back on the throttle after a shift.

I barely messed with AVCS tuning over the years, partly because valve timing seemed too mysterious and partly because I was satisfied with how my car was running... Ironically what finally got me to study up on valve timing was buying a 2002 Corvette (Z06, whee!) which does not have variable valve timing. But that makes it even more important to pick the right camshaft, so I have been reading everything I can find, and I started experimenting with my Subaru.

I'm still not very clear on how timing should vary with load, so I have more experimenting to do there. Making every column the same has worked out pretty well for cruise and low boost. I haven't paid attention to how it is affecting high boost yet but of course I'm getting curious.

The main insights that led me to try the table I'm using now:

* Of all of the valve events (intake open, intake close, exhaust open, exhaust close), the intake valve close (IVC) is the most important.

* GM truck cams have the intake valve close at about 33 degrees after bottom dead center

* Aftermarket Corvette cams mostly have an IVC around 45 degrees ABDC.

("Close" is measured at 0.050 lift, which is kind of a lie, but that's the convention.)

I don't have specs for stock cams, but for the BC 272s that I'm using, 30 degrees of intake advance puts the IVC at 31 degrees. 2009 LGTs used 40 degrees advance for a small part of the AVCS table, and had about 20 degrees less duration (10 less on the open side, 10 less on the closing side), so I'm guessing that my IVC timing with 30 degrees of advance probably matches the stock cam at 40 advance.

(That's assuming that my BC272s have the same the intake lobe centerline as stock cams. BC says the 272s have a 130 degree intake lobe center, but I don't know what the stock centerline is. If anyone does know, please share. I'm curious about the stock exhaust centerline too.)

And then I take out 20 degrees of advance at high RPM for an IVC of 51, because the GM aftermarket sees fit to use 45 in cams that are optimized for high RPM... and I bet they would go further if they didn't have to worry about the effect on low RPM. That's sheer speculation that really needs to be tested, though... The car seems plenty strong this way but I haven't done any performance measurements yet. The stock AVCS table drops to zero before 5000 so I'm really curious which approach makes more power.

TYPE OF STUTTER/STUMBLE

Steady throttle/Increasing throttle/Both:Light throttle from 1500-4000 rpm, idle

MODS

Intake:K&N drop in filter

Exhaust:3" catless downpipe, catless uppipe

Fuel:stock

Turbo:stock

Engine Management:stock

Tuner and revision of tune(if you don't mind):Cryotune Performance E-Tune, rev 4 (I think)

MAINTENANCE

Sensors changed:cleaned maf, checked wastegate solenoid function, front O2 changed last year

Plugs changed:Plugs and gap checked, normal

Coil Packs changed:stock

OTHER SYMPTOMS

Check Engine Light:none

Etc:car feels slowish to build boost, but this mostly went away from fixing a pre turbo exhaust leak

RESULTS

What has and hasn't worked for you:JB welded the stock TMIC after one of the end tanks was leaking and got a new BPV gasket. Stumbles and idle got slightly better. Fixed pre turbo exhaust leaks via new gaskets and car is more quiet and seems to idle smoother, however light throttle jerkiness is still there. I have boost leak tested, checked TGV gaskets, TB hose seals and they are holding. The car seems to hold boost. The MAF scaling is correct from my logs and the car doesn't have any obvious points that stick out from logs. IAM is at a constant 1, knock count is ~1 per 20 minute drive, no FLKC or ignition is being pulled.

If something worked, how long has it been working for: The exhaust leaks seemed to fix the boost issues, however the car still has occasional studders / misfires at idle and is jerky under low load, light throttle.

I really want to figure this stuff out. I'm going to try to clean the TB with TB cleaner, then put new plugs in (I have a new set). If that doesn't work, check injectors, coil packs, etc. Why can't it just drive smoothly? It could be a tuning issue but the car ran great right after the tune.

Edit 1 - update: I installed a boost guage. Car hits target boost fine and holds boost well. Spools fast. However, only pulling 15-16 inhg of vacuum at idle. My thoughts are post maf pre turbo boost leak. I'll check the inlet later.

I don't have specs for stock cams, but for the BC 272s that I'm using, 30 degrees of intake advance puts the IVC at 31 degrees. 2009 LGTs used 40 degrees advance for a small part of the AVCS table, and had about 20 degrees less duration (10 less on the open side, 10 less on the closing side), so I'm guessing that my IVC timing with 30 degrees of advance probably matches the stock cam at 40 advance.

As best I can tell, these are the specs for the stock cams:

Duration: 240 degrees @ .006", 204 int / 208 exh @ .050"

Centerlines: Intake 125-85, Exhaust 115

My intake cam with 30 degrees advance has an IVC of 31 degrees

The stock cam with 15 degrees advance has an IVC of 32 degrees

So, based on my car getting smoother with 30 degrees of AVCS advance in the 1500-3200 RPM range (roughly 0 to 1.25 load), then with stock cams I'd start by using 15 degrees of AVCS advance in that region. The stock AVCS table for my 05 had a mix of 0 and 15 in that area.

And more advance might be better, I only stopped at 30 because I started to worry about piston-to-valve clearance, which is a non-issue with stock cams. 2009 LGTs have 40 degrees of advance in some of the same cells (around 0.5-0.9 load).

I've attached pics of the stock AVCS table for my '05, and something that I think it worth trying if any DIY tuners out there want to experiment. The highlighted cells in the stock table are where I had the most stumble. If the proposed table actually helps, try setting those highlighted cells to 25 and see if that helps more.

I barely messed with AVCS tuning over the years, partly because valve timing seemed too mysterious and partly because I was satisfied with how my car was running... Ironically what finally got me to study up on valve timing was buying a 2002 Corvette (Z06, whee!) which does not have variable valve timing. But that makes it even more important to pick the right camshaft, so I have been reading everything I can find, and I started experimenting with my Subaru.

Heh funny to hear about someone else getting a Vette and that actually being factor behind modifying/tuning the Subaru more. I picked up a '10 GS a couple months ago and have since spent more time tinkering with the LGT in that time that I have in the past 2 years. I'm just hoping to make the transition between cars a bit easier.

Sorry for the off topic, carry on!

I went and looked at my AVCS map and it’s already at 25* in the area I get stumble (.8-1.2g/rev 2000-3000 RPM). Maybe that is too much advance?

I doubt that it's too much, because my wife's 09 LGT has 40 degrees from .5 to .8 in that RPM range. But it couldn't hurt to experiment with 20 and 30 to see if it gets better or worse.

It might be interesting to try varying the timing too. For comparison, my wife's car's factory tune (stumble free) has about 30 degrees at .8 r/rev and 16 degrees at 1.2 in that RPM range. My car (pretty good but still experimenting) has about 35 and 15. All of those figures include the base + advance timing values.