The Stumble/Stutter Thread

[ripemeat]

Cobb OTS maps do knock at low loads, if you have some learned knock there, the stutter you feel is the car moving in/out of load cells that have a jump or dip in timing from one to the next. Solution is to get a better tune. Another possibility is fueling. Best bet is to check a learning view to see if either of those two things are a reasonable cause for your particular car. Another option is that the Cobb timing tables (the base and advance) are not smooth, this is the fault of cobb themselves and can very well cause a stutter on it's own, even if you aren't knocking. This is why open source, or a custom pro tune is beneficial for driveability.

[Rattsl]

Now that I am done with classes for this semester, I'm gonna see about going to the Subaru dealer and have a leak check done, and a diagnostics run done. This low rpm biz is bugging the hell out of me.

[BlackWagon GT]

Cobb OTS maps do knock at low loads, if you have some learned knock there, the stutter you feel is the car moving in/out of load cells that have a jump or dip in timing from one to the next. Solution is to get a better tune. Another possibility is fueling. Best bet is to check a learning view to see if either of those two things are a reasonable cause for your particular car. Another option is that the Cobb timing tables (the base and advance) are not smooth, this is the fault of cobb themselves and can very well cause a stutter on it's own, even if you aren't knocking. This is why open source, or a custom pro tune is beneficial for driveability.

 

If mine still does it after being flashed with a proper ROM (see my post on page 5), Cobb is not what is causing my issue.

Edited December 13, 2012 by BlackWagon GT

[ripemeat]

but did you check a learning view? what k&n are you using and do you confirm it's not causing fuel trim issues?

[BlackWagon GT]

but did you check a learning view? what k&n are you using and do you confirm it's not causing fuel trim issues?

Me? I have a stock air filter.

[ripemeat]

oh ok.

[O.C.D.]

TYPE OF STUTTER/STUMBLE

Steady throttle/Increasing throttle/Both: I generally see it at low throttle inputs from about 2,000 and up. This happens in all gears and all the time. Hesitates and stumbles. Always pre-boost.

 

MODS

Intake: Stock

Exhaust: GS Uppipe and COBB DP

Fuel: Shell 91

Turbo: IHI VF40 and Grimmspeed EBCS

Engine Management: Stock

Tuner and revision of tune(if you don't mind): Cryotune Stage 1 Rev: Final - test and now Cryotune Stage 2 w/EBCS Rev - 4

 

MAINTENANCE

Sensors changed: None yet

Plugs changed: 20 miles ago - factory

Coil Packs changed: N/A

 

OTHER SYMPTOMS

Check Engine Light: None

Etc:

 

RESULTS

What has and hasn't worked for you: I have done a tune, belts, air filter, oil and filter, plugs, compression test.

If something worked, how long has it been working for: So far nothing.

Edited April 30, 2013 by O.C.D.

[700watts]

problem solved, replaced stock turbo "inlet hose ?"...engine builder over tightened to clamps which made a hole....

[rlefevre]

I seem to have solved the problem for the most part on my legacy. I was having the problem where it would stutter at certain RPMs when cruising or going up small hills. I found this thread http://legacygt.com/forums/showthread.php/stutter-studder-hesitation-etc-198070.html?t=198070

 

Basically the theory is that a resonance builds up at certain RPMs in the fuel injector line which causes one of the injectors to get starved of fuel and then that cylinder runs extremely lean. I followed the modification suggested in the thread above and replaced one fuel line with a longer line.

 

It seems to have worked well for me. I can still get the car to stutter, but it is much less common now. I have driven about 100 miles since replacing the tubing, and have only had it stutter maybe 3 times. Before the replacement, my car would stutter more than 5 times every time I drove.

 

As a side note, there are some compensation tables that subaru tried to use to "fix" this issue which you are supposed to zero out when you do this mod. I have not got a chance to do this yet, so hopefully my stutter will further improve when I do that.

 

It may not be the fix for everyone, but it cost me about $20 in fuel line and clamps, so it is worth a shot.

[BlackWagon GT]

I seem to have solved the problem for the most part on my legacy. I was having the problem where it would stutter at certain RPMs when cruising or going up small hills. I found this thread http://legacygt.com/forums/showthread.php/stutter-studder-hesitation-etc-198070.html?t=198070

 

Basically the theory is that a resonance builds up at certain RPMs in the fuel injector line which causes one of the injectors to get starved of fuel and then that cylinder runs extremely lean. I followed the modification suggested in the thread above and replaced one fuel line with a longer line.

 

It seems to have worked well for me. I can still get the car to stutter, but it is much less common now. I have driven about 100 miles since replacing the tubing, and have only had it stutter maybe 3 times. Before the replacement, my car would stutter more than 5 times every time I drove.

 

As a side note, there are some compensation tables that subaru tried to use to "fix" this issue which you are supposed to zero out when you do this mod. I have not got a chance to do this yet, so hopefully my stutter will further improve when I do that.

 

It may not be the fix for everyone, but it cost me about $20 in fuel line and clamps, so it is worth a shot.

 

At this point, I'll try anything, but I have my doubts that this will fix my problem. According to the links in that thread, the resonance peaks at about 3,000 RPM. Some back of envelope maths on that give a natural frequency of about 100Hz for an STi's fuel system. In my case, the engine runs the worst at about 2100 RPM, which would be somewhere around 70Hz. I suspect that the LGT's fuel line is longer than an STi's but I don't think it's enough to drop the resonance by some 30%. In fact, I went back and looked at some of my old logs, and my correction 1 is consistently 5-10% negative.

 

I really hope this works for some people, but with how defiant my car has been so far, I admit I'm cynical.

[rlefevre]

At this point, I'll try anything, but I have my doubts that this will fix my problem. According to the links in that thread, the resonance peaks at about 3,000 RPM. Some back of envelope maths on that give a natural frequency of about 100Hz for an STi's fuel system. In my case, the engine runs the worst at about 2100 RPM, which would be somewhere around 70Hz. I suspect that the LGT's fuel line is longer than an STi's but I don't think it's enough to drop the resonance by some 30%. In fact, I went back and looked at some of my old logs, and my correction 1 is consistently 5-10% negative.

 

I really hope this works for some people, but with how defiant my car has been so far, I admit I'm cynical.

 

It may or may not work for you, but it's a fairly cheap thing to try. For me the stutter occurred at multiples of 700 (1400, 2100, 2800) with the worst being probably the 2100.

[ripemeat]

I've ridden in many LGT's, both auto and manual and certainly notice the stumble that's mentioned in this thread. It seems to be limited to the manual tranny crowd though (and verified by the posters in this thread and an overwhelming majority). You can always flash an auto map onto a manual car and see if the problem resolves, just disable the neutral safe switch CEL, might be worthwhile...it could eliminate a mechanical fault as being the culprit.

[rlefevre]

I've ridden in many LGT's, both auto and manual and certainly notice the stumble that's mentioned in this thread. It seems to be limited to the manual tranny crowd though (and verified by the posters in this thread and an overwhelming majority). You can always flash an auto map onto a manual car and see if the problem resolves, just disable the neutral safe switch CEL, might be worthwhile...it could eliminate a mechanical fault as being the culprit.

 

FWIW, my car is automatic and the car did the stutter on stock map as well as cobb stage 1. No other mods.

[Brprs]

Mines auto as well. Never had stage 1, but im going cobb stage 2 once the holidays are over.

[cfaber]

it seems that if the problem is related to harmonics within the fuel line between FPR and rail, an analog in line fuel pressure gauge should indicate this (with wild movement at select RPM range)

[m sprank]

Rubber hose does not carry harmonics. FPR is connected to fuel pipes via rubber hose (on 05-06). No hard connection like on the smog pump cars. In fact, fuel pipes are split up and segmented with rubber hose (on all LGT's). Bolted to the manifold in two places. Bolted to each TGV. Segmented twice with rubber hose. Not sure how this "harmonic" is being carried through the rubber hose which should isolate any harmonics.

 

Smog pump cars have the FPR hard mounted to the fuel pipes. STi are different all together. I understand the harmonics in the newer cars, but not the older cars. Maybe I am missing something. We tend to change fuel lines on many cars regardless due to FMIC installs and motor rebuilds. So, possibly we are "fixing" a stumble.

Edited December 26, 2012 by m sprank

[rlefevre]

Rubber hose does not carry harmonics. FPR is connected to fuel pipes via rubber hose (on 05-06). No hard connection like on the smog pump cars. In fact, fuel pipes are split up and segmented with rubber hose (on all LGT's). Bolted to the manifold in two places. Bolted to each TGV. Segmented twice with rubber hose. Not sure how this "harmonic" is being carried through the rubber hose which should isolate any harmonics.

 

Smog pump cars have the FPR hard mounted to the fuel pipes. STi are different all together. I understand the harmonics in the newer cars, but not the older cars. Maybe I am missing something. We tend to change fuel lines on many cars regardless due to FMIC installs and motor rebuilds. So, possibly we are "fixing" a stumble.

 

You have more experience with these cars than me so I will yield to your experience, but are you sure a standing wave cannot be carried by fuel injector line? At least on my 09 lgt the line in question is fairly short and between two hard lines, so I have a hard time believing that short hose provides any significant dampening.

 

Regardless of the reasoning, my car has almost completely stopped stuttering. Maybe it is just placebo, but my car fell for it.

[m sprank]

Anything is possible. 0n a 09 (LGT) the FPR is hard mounted to the fuel pipes. So, this makes some sense to me.

[rtbrjason]

Rubber hose does not carry harmonics. FPR is connected to fuel pipes via rubber hose (on 05-06). No hard connection like on the smog pump cars. In fact, fuel pipes are split up and segmented with rubber hose (on all LGT's). Bolted to the manifold in two places. Bolted to each TGV. Segmented twice with rubber hose. Not sure how this "harmonic" is being carried through the rubber hose which should isolate any harmonics.

 

Smog pump cars have the FPR hard mounted to the fuel pipes. STi are different all together. I understand the harmonics in the newer cars, but not the older cars. Maybe I am missing something. We tend to change fuel lines on many cars regardless due to FMIC installs and motor rebuilds. So, possibly we are "fixing" a stumble.

 

I don't think harmonics is specifically the issue. Google "water hammer" or "fluid hammer" for a better idea of what people seem to think is happening/are solving for with this mod. (http://en.wikipedia.org/wiki/Water_hammer)

 

I certainly had the stutter when stock and even after "stage 1 and 2" tuning. I was able to fix it with a mix of tuning the engine load comp (MP) table, closed loop fuel comp, and timing. In the case of timing, I believe the biggest change that impacted this was the removal of cruise/non cruise timing. I've had no stumbling at all since getting that all dialed in.

 

I did this mod anyway since many people on various forums were mentioning that the engine load comp table could be zero'd and some of the strange maf scaling issues may go away.

 

I've not really found that to be the case and provided some data in the other thread on this that is going right now. After zeroing the comp table and tweaking the maf a bit, I found I still needed to adjust the engine load table and the overall shape of it was very similar to what I had before zero'ing it and doing this mod. This makes sense to me since any particular range (30g/s for instance) is going to require different fueling if you're doing 40 mph in 4th gear vs say 75mph in 6th. These differences in fueling are minor and certainly well within tolerance for the ecu's ability to adjust, but since we all want our long term fuel trims to be close to 0 it's worth tuning this table. Most people using the stock table should rescale the rpms to cover the full range of closed loop. For me that is about 800 to 4000 rpm.

 

One thing I have noticed is considerably less knock sum increments in the 2700-3200 rpm range where I had constant issues with knock at 1.1-1.3 load while mildly accelerating on the highway. I never believed this was real knock but it seems to be completely gone whereas prior to this mod I could make knock sum increment every time I hit 2700 rpm and 1.3 load.

Edited December 27, 2012 by rtbrjason

[Brprs]

Why does it go away after i reset the ecm?

[m sprank]

Water hammering is reduced or eliminated with vibration eliminators. I am very familiar with water hammering. I am a UA Journeyman Pipe Fitter, lol.

 

I understand the theory. Very hard to prove. Not without some serious tools and a more. If it helps, I am all for it. Like I said, we replace a lot of fuel lines at the shop.

[BlackWagon GT]

I did the fuel line mod, and it made zero difference in the running characteristics. Next step is to flash my ECU.

 

I'm frustrated and running out of ideas.

[rtbrjason]

I did the fuel line mod, and it made zero difference in the running characteristics. Next step is to flash my ECU.

 

I'm frustrated and running out of ideas.

 

Are you pro-tuned? Is it AP or ECUFlash?

[BlackWagon GT]

Are you pro-tuned? Is it AP or ECUFlash?

 

AP, sort of. I don't know what map my AP put in my ECU when I divorced it since I think I had a custom map when I married it, so I want to get a verified stock map/rom back into it before I go any further.

[BlackWagon GT]

I spent yesterday morning datalogging with a friend of a friend, Mike Piecko, who runs Red Mist Automotive in Gilberts, IL. By the numbers, my car is running nearly-perfectly. Fuel trims, knock counts, cylinder roughness, all right where they ought to be according to the ECU. We thought it might have been something with the TGVs or throttle motor duty cycle, but research I did today ruled both of those out.