The Stumble/Stutter Thread
 

Lets see if we can get a handle on this issue and get a repository going of what mods people have, what they have tried to do to fix the stutter, what tests they have run with either good results or no results.

Let's define stumble/stutter as any form of hesitation at part throttle at either steady throttle input or increasing throttle input.

TYPE OF STUTTER/STUMBLE
Steady throttle/Increasing throttle/Both:

MODS
Intake:
Exhaust:
Fuel:
Turbo:
Engine Management:
Tuner and revision of tune(if you don't mind):

MAINTENANCE
Sensors changed:
Plugs changed:
Coil Packs changed:

OTHER SYMPTOMS
Check Engine Light:
Etc:

RESULTS
What has and hasn't worked for you:
If something worked, how long has it been working for:

I'll start

TYPE OF STUTTER/STUMBLE
Steady throttle/Increasing throttle/Both: Increasing throttle (often), steady throttle (very rarely)

MODS
Intake: Perrin Turbo Inlet, FMS TMIC
Exhaust: OBX catback, CNT Catted DP
Fuel: Stock
Turbo: BNR 16g
Engine Management: Accessport
Tuner and revision of tune(if you don't mind): Infamous1, Rev 1 (on a retune, changed mods)

MAINTENANCE
Sensors changed: Cleaned MAF, no change.
Plugs changed: none
Coil Packs changed: none

OTHER SYMPTOMS
Check Engine Light: Just got a PO302 the other day and reset ECU and it's gone and didn't come back
Etc.
Had boost leak a while ago and fixed it. It did help a little.

RESULTS
What has and hasn't worked for you: New tune and/or resetting ECU mostly "fixes" the stutter.
If something worked, how long has it been working for: Only lasts for a few weeks and I have to reset again.

Clearing a CEL is not a fix. If there is ever a CEL regarding a driveability concern like a misfire or lean/rich condition it NEEDS to be fixed prior to diagnosing a stumble/stutter. In most cases the CEL is the cause. Also, to set a check engine light, a number of different parameters need to be met. For example, sometimes a problem like a misfire may need to occur more than once in a single drive cycle to turn the light on. Just an FYI from a subie tech! Ill be glad to help out with this topic as well!

We have seen many different versions of the "stumble". We have come across many different fixes.

For some it is a slight vacuum leak. Post turbo.

For some it is a dirty MAF.

For some it is a dirty or faulty TPS. In older cable TB cars it was the idle air solenoid.

For some it is a front (A/F) O2 sensor that is reading off.

For some it is a leaking BPV (leaking to atmosphere).

It can be a total mystery when combined with 5EAT's too.

You guys made me remember some things I have checked with varying success. Right now it seems like what caused my misfire code it a tune or sensor issue. The longer I run any one tune it seems like the stutter gets worse. Until the day I got the misfire code, I had a very rough idle. When I reset the ECU the idle and drivability got better. So either the tune is funny(which I doubt because even at Rev 1, infamous is pretty good) or a sensor is giving funny readings and messing with my fuel trims, etc. I do, for sure, have some bad endlinks. I have heard people say that they got flash knock readings from the click/thump they make. Those will get fixed ASAP. So maybe the computer is pulling too much timing in some places in closed loop and causing a misfire. But I don't know how much timing is able to be removed by tuner ecu. So maybe this isn't a problem.

False readings not flash readings*

I have personally tested a set of Perrin endlinks that caused false knock. Installed them in three different LGT's. The knock went with them to all three cars and went away when I removed them. Switched to Kartboy.

Stumble is not in the tune. It is the ECU reacting to something. Trick is finding it. Sometimes like a needle in a haystack. Sometimes it is never found.

Gotcha. Thanks Mike. I'll start digging for the needle.

Good luck to you. Did the car ever drive without a "stumble". If not, it might never go away. For sh#$ and giggles clean the ground under the TMIC. Next to the starter. That is the cars main ground and most important to ECU communication. It could actually be a slow communicating sensor causing it. You would be amazed at what can effect CANBUS communication and the steps engineers took to attempt to avoid interference.

TYPE OF STUTTER/STUMBLE
Steady throttle/Increasing throttle/Both: 2000-2250 RPM, light throttle. Most pronounced after a cold start, but often still noticeable when warm. Without even thinking about it, I just got into the habit of driving at 2400+ to avoid it, and I've been doing that for so long that I'm not sure when the stumble started. I think it may have been there since the car was stock.

MODS
Intake: KSTech 73mm CAI
Exhaust: Invidia catted DP, Perrin Mid/Y, stock cans
Fuel: AVO 245, then later Aeromotive 340
Turbo: ATP 3076
Engine Management: Stock ECU
Tuner and revision of tune(if you don't mind): Me, about 175 'official' revisions plus a couple dozen one-offs here and there. Only a few of those flashes were related to the stumble, mostly the revisions have been due to changing mods, adding power, tuning tip-in, changing boost levels, etc, etc... lots of little tweaks here and there.

MAINTENANCE
Sensors changed: I've got two MAF sensors, didn't notice a difference between them.
Plugs changed: Last summer, didn't notice a difference.
Coil Packs changed: Never tried that.

OTHER SYMPTOMS
Check Engine Light: I've had a few, but never anything related.
Etc: Car runs great otherwise.

RESULTS
What has and hasn't worked for you:
I've tried forcing the car to stay in open loop, to see if the was related to the O2 sensor or closed loop - made no difference.
Tried adding and removing ~5 degrees of ignition timing in the affected area - no difference.
Tried adding and removing ~10 degrees of AVCS in the affected area - no difference.
Changed fuel lines, changed fuel pressure regulator to an Aeromotive 1000 - no difference.

If something worked, how long has it been working for:
Nothing has really worked. It's pretty mild after the engine has warmed up, but it doesn't go away completely.

Going to try speed-density as soon as I have the time and energy. I made a speed-density ROM for 2005 LGTs but haven't had time to really tune it, so the ROM is still in MAF mode. If the problem is MAF-related, that should fix it. :)

TYPE OF STUTTER/STUMBLE
Steady throttle/Increasing throttle/Both:
Stumple at idle. first start up and after warm up. Intermittent problem.

MODS
Intake: AVO air filter
Exhaust: Borla cat back, PDE catless up pipe. Flow catless shorty down pipe
Fuel: 93 octane
Turbo: VF40
Engine Management: Cobb AP
Tuner and revision of tune(if you don't mind):
TDC protune stage 2 map (old)
Cobb stage 2 OTS map (old)
Infamous stage 2 OTS map (current)

MAINTENANCE
Sensors changed: PCV valve
Plugs changed: Yes.
Coil Packs changed: No

OTHER SYMPTOMS
Check Engine Light: No
Etc:

RESULTS
What has and hasn't worked for you: I had the V.1 AP with TDC protune stage 2 map. It did a lot of stumbling at idle. I've cleaned the MAF sensor and upgraded to V.2 AP stage 2 OTS map and the stumbling went away. I'm not sure if it's a result of a bad tune or dirty MAF sensor, or both.
Revision: 1
Installed Grimmspeed EBCS and Infamous Stage 2 tune. The stumbling at idle in drive mode is still occurring intermittently.
If something worked, how long has it been working for:
Only have less than 1,000 miles since the upgrades. Will revise if conditions change.
Revision: 1
Stumbling came back, now intermittently does it at idle while in drive mode. :iam:

TYPE OF STUTTER/STUMBLE
Steady throttle/Increasing throttle/Both:

MODS
Intake: AVO
Exhaust: Autospeed
Fuel: 93 and E85
Turbo: AVO380
Engine Management: Cobb AP
Tuner and revision of tune(if you don't mind): Calvin Dotson Cobb Plano Protuned

MAINTENANCE
Sensors changed: MAF no difference
Plugs changed: 25K on plugs
Coil Packs changed:

OTHER SYMPTOMS
Check Engine Light: NO
Etc:

RESULTS
What has and hasn't worked for you: Nothings worked
If something worked, how long has it been working for:

My car is doing this as well. I noticed that if I ever so slightly push on the pedal and watch the Throttle position on the AP it actually goes down for a split second before it increases.

For example at idle with no input it is sitting at say 6. As soon as I just barely press on the pedal it goes to like a 3-4 before going up and the AFR goes a bit rich and then it hunts for a second.

Myself and KCwagon have the same exact issue. Once it goes rich like that as the throttle position number goes down then it is out of whack for a bit until it settles in.
Both of us were tuned at Cobb Plano, but I would think this portion of the tune was not touched.

Sometimes I wonder if its an issue at the throttle body or the sensor on the pedel causing this, but its weird that both of us are having the same issue.

If I hold the throttle at the point when the TP number goes down a bit the car really starts idling rough and will almost die.

Its like it does not like this partial throttle and who knows what the number going down does.

Anyone else have this or can test it?

I just went on the AP monitor section and monitored throttle position while pushing the pedal ever so slightly with my hand.

This might be due to the transition between the idle DBW mode and regular driving DBW mode. The ECU learns to idle at whatever throttle opening angle is needed, but when you touch the pedal it switches to the DBW table. With Subaru's factory tune, the throttle plate angle table has 0% in no-throttle / 750 RPM corner. What you're seeing might be due to the ECU trying to use that value. It would be easy enough to put 3%-5% in that corner to test this theory.

Hey NSFW, I also notice it is worse on really HOT days.

Last couple of days its been in the high 90's here already. This morning though it was in the 70s and the problem wasn't near as bad.

Its also not bad at all hot or not with the AC turned off.

Thing is the idle on mine is set to 850 both AC on or Off.

Mine has something to do with a blip on the throttle or a slight opening of the throttle.

Its really starting to irritate me.

Here is some info from the service manual

1. Engine stalls during idling.
1) Manifold absolute pressure sensor
2) Mass air flow and intake air temperature sensor 3) Ignition parts (*1)
4) Engine coolant temperature sensor (*2)
5) Crankshaft position sensor (*3)
6) Camshaft position sensor (*3)
7) Fuel injection parts (*4)

2. Rough idling
1) Manifold absolute pressure sensor
2) Mass air flow and intake air temperature sensor 3) Engine coolant temperature sensor (*2)
4) Ignition parts (*1)
5) Air intake system (*5)
6) Fuel injection parts (*4)
7) Electronic throttle control
8) Crankshaft position sensor (*3)
9) Camshaft position sensor (*3)
10) Oxygen sensor
11) Fuel pump and fuel pump relay

3. Engine does not return to idle.
1) Engine coolant temperature sensor
2) Electronic throttle control
3) Manifold absolute pressure sensor
4) Mass air flow and intake air temperature sensor 5) Accelerator pedal position sensor
6) Oil temperature sensor

4. Poor acceleration
1) Manifold absolute pressure sensor
2) Mass air flow and intake air temperature sensor 3) Electronic throttle control
4) Fuel injection parts (*4)
5) Fuel pump and fuel pump relay
6) Engine coolant temperature sensor (*2)
7) Crankshaft position sensor (*3)
8) Camshaft position sensor (*3)
9) A/C switch and A/C cut relay
10) Engine torque control signal circuit
11) Ignition parts (*1)
12) Accelerator pedal position sensor
13) Oil temperature sensor

5. Engine stalls or engine sags or hesitates at acceleration.
1) Manifold absolute pressure sensor
2) Mass air flow and intake air temperature sensor 3) Engine coolant temperature sensor (*2)
4) Crankshaft position sensor (*3)
5) Camshaft position sensor (*3)
6) Purge control solenoid valve
7) Fuel injection parts (*4)
8) Electronic throttle control
9) Fuel pump and fuel pump relay

"Before drive by wire technology was introduced, if a throttle stuck open a driver could generally put a toe under the accelerator and lift up. Occasionally after servicing or repair, the wire or cable between the accelerator and throttle would not be correctly reinstalled causing sudden acceleration. However, with the ETC, the movement is all done by electronic controls moving an electric motor. But just moving the throttle by sending a signal to the motor is an open loop condition and leads to poor control. Most if not all current ETC systems have a closed loop system whereby the ECU tells the throttle to open a certain amount according to an algorithm based on the geometry of the throttle. Then, if due to dirt build up in the throttle bore or a damaged TPS a signal is sent from the TPS to the ECU, the ECU can make appropriate adjustments to compensate, though it might result in surging, hesitation or uneven idle."