occasional stutter - Chevy and GMC Duramax Diesel Forum
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post #1 of 20 (permalink) Old 10-21-2019, 12:06 AM Thread Starter
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occasional stutter

I've been trying to narrow this problem down for awhile and wanted to see if maybe someone here had any ideas.

If I take my foot off the gas and let the truck slow down on it's own I notice the rpm's will kinda fall gradually until it gets to a point around 1500 rpm's where it seems like the torque converter let's go and it falls to idle speed (or close to it).

If I give it gas again at this point the rpm's will climb quickly (almost like it's in neutral) before hitting 1200-1500 when I will feel a "stutter". The rpm's drop and the truck begins accelerating. This feels very similar to the torque converter locking up in 5th gear.

It's not 100% repeatable, although I wouldn't say it's rare either.

Accelerating from a dead stop is smooth. I don't feel any slippage under heavy acceleration.

Is this normal?

2009 Chevrolet 2500HD LTZ CCSB White 4x4
4" turbo back straight pipe
EFI live with Idaho Rob tunes (RIP)
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post #2 of 20 (permalink) Old 10-21-2019, 09:41 PM
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When letting off the throttle then as you slow down the converter kicks out then you coast, YES this is normal.
When you quickly accelerate at this point having a slight delay this is also normal.

Diesels are not exactly like a gasser, the turbo needs to spool up before it gitty-up and goes, depending on the rpms and speed it might instantly take off or delay.

Having good intake airflow and good clean airfilter will make a difference.

Dan
2007.5 GMC Sierra 3500 SRW L.B."BLACK" C.C. 4X4 LMM Z71
DIY stock airbox mod, aFe ProGuard 7, OEM drop-in
EFILive ECM ATP - Heavy Tow Tune with Turbo Brake.
EFILive TCM Duramax Tuner
DHD Cold Side High Flow Intake Pipe
DHD PCV Re-Route
AirDog 4G100

2007 Yukon Denail "BLACK" 6.2 gasser
DIY stock AirBox Mod, K&N, Airaid intake, EFILive TCM only tuning
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post #3 of 20 (permalink) Old 10-21-2019, 11:39 PM Thread Starter
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So it's normal when after the torque converter disengages while coasting to a stop to take a bit of RPM for it to hook back up?

I'm not talking about spool up or anything like that. It's more like driving a stick and letting the clutch out late and fast. Let me play out the scenario where it happens the most.

Coasting up to a red light, foot totally off the gas. The speed falls and the RPM's with it, around 1200 RPM the torque converter let's go and the truck falls down to idle of around 720 rpm. The truck is still rolling at this point at 15-20 mph. The light changes to green, so I place my foot back on the gas (note: this is not a quick acceleration, just trying to get back up to 35-40 mph speed limit and getting there leisurely). I rest my foot on the gas and the RPM's climb quickly up to 1200ish. Then it "catches" (like dropping the clutch in a manual) with a noticeable (although not audible) thunk. It feels like a very firm shift. The RPMs fall to the appropriate number for the speed the truck is traveling at and then it drives perfectly normal.

2009 Chevrolet 2500HD LTZ CCSB White 4x4
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post #4 of 20 (permalink) Old 10-22-2019, 07:47 AM
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When youíve let off the throttle driving while itís coasting you usually feel the trans converter kick out and the rpms drop a little.
When you immediately accelerate there can be a slight delay, thatís where there can be the turbo spool up delay depending on speed, rpms, intake airflow with more airflow youíll have quicker spool-up or sometimes just because.
Have you engaged towhaul while driving around not towing to see how it shifts?
How many miles is on your trans?
When was the last time your trans was serviced?
Have you changed the spin-off filter, itís suppose to be changed about every 10k.
How does the fluid look?
Where is the fluid level on your dip stick?

Dan
2007.5 GMC Sierra 3500 SRW L.B."BLACK" C.C. 4X4 LMM Z71
DIY stock airbox mod, aFe ProGuard 7, OEM drop-in
EFILive ECM ATP - Heavy Tow Tune with Turbo Brake.
EFILive TCM Duramax Tuner
DHD Cold Side High Flow Intake Pipe
DHD PCV Re-Route
AirDog 4G100

2007 Yukon Denail "BLACK" 6.2 gasser
DIY stock AirBox Mod, K&N, Airaid intake, EFILive TCM only tuning
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post #5 of 20 (permalink) Old 10-22-2019, 09:34 AM Thread Starter
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The delay is 100% engagement and not boost related. It feels solid like a shift as opposed to the surge that a turbo brings.

When it happens it is under very slight acceleration and where the turbo would only see a couple pounds of boost anyways. Air filter is new in the last 2000 miles. Boost level are where they should be I believe, I run an idaho rob EFI live tune at +90hp and see peak (sustained) boost around 28-30 psi read on an edge insight 2.

I've never experienced the problem while in tow haul mode.

around 190k on motor/trans. Last fluid change was at 160k miles, I put on a new spin on filter every time I change the oil. Fluid looks good, smooth between the fingers without being watery or gritty. No burning smell. I've done both hot and cold checks and the fluid level falls in the correct range on the dipstick for either.

Otherwise the truck performs excellently. There are zero other problems with shifting, accelerating, or boost.

2009 Chevrolet 2500HD LTZ CCSB White 4x4
4" turbo back straight pipe
EFI live with Idaho Rob tunes (RIP)
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post #6 of 20 (permalink) Old 10-22-2019, 12:27 PM
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Quote:
Originally Posted by Epiclobotomy View Post
Otherwise the truck performs excellently. There are zero other problems with shifting, accelerating, or boost.
Then I don’t understand your concerns.
Maybe take it somewhere to have it checked out if this concerns you.

So far it’s taken 3 posts for you to mention that your running a 90 hp tune on a stock trans.
There are no tunes on this planet that are trans safe.
You’ve explained a lot in your first 2 posts but left out the important stuff.

What’s your opinion on where the proper trans fluid level is suppose to be.
What color is the fluid ?

One thing I would check out a little more is your low boost, I’m running ATP heavy towtune 30-40 hp my boost is 31-32.
But that really should not affect shifting with still in the 28-30 range.
My whimpyest tune 25 hp will maintain 29.

Dan
2007.5 GMC Sierra 3500 SRW L.B."BLACK" C.C. 4X4 LMM Z71
DIY stock airbox mod, aFe ProGuard 7, OEM drop-in
EFILive ECM ATP - Heavy Tow Tune with Turbo Brake.
EFILive TCM Duramax Tuner
DHD Cold Side High Flow Intake Pipe
DHD PCV Re-Route
AirDog 4G100

2007 Yukon Denail "BLACK" 6.2 gasser
DIY stock AirBox Mod, K&N, Airaid intake, EFILive TCM only tuning
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post #7 of 20 (permalink) Old 10-22-2019, 12:49 PM
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Have you considered putting your smallest HP tune in and drive for a long while to see what happens ?
Bigger tunes have a effect with the trans to perform like what your expecting and shortens the trans life much quicker than youíd like.

Dan
2007.5 GMC Sierra 3500 SRW L.B."BLACK" C.C. 4X4 LMM Z71
DIY stock airbox mod, aFe ProGuard 7, OEM drop-in
EFILive ECM ATP - Heavy Tow Tune with Turbo Brake.
EFILive TCM Duramax Tuner
DHD Cold Side High Flow Intake Pipe
DHD PCV Re-Route
AirDog 4G100

2007 Yukon Denail "BLACK" 6.2 gasser
DIY stock AirBox Mod, K&N, Airaid intake, EFILive TCM only tuning
D_R_C is offline  
post #8 of 20 (permalink) Old 10-22-2019, 01:24 PM Thread Starter
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I spoke with a local trans shop and while they told me what I was experiencing was not "normal" they did say that because of the frequency of it happening, the fact the trans had not thrown any codes, and because the trans had never limped it would be almost impossible to diagnose. It could be a loose wire, a sticking solenoid, or another half a dozen issues. Their suggestion was to wait until the problem was bad enough I could replicate it for them, or wait until I limped the trans.


Regarding the trans fluid levels, when checked they are in the high areas of the crosshatching on the trans dipstick annotated as "hot" and "cold" with the level appropriate for the temp of the transmission. Fluid checks are done on a level surface, and after shifting the trans through R D and finally P. Trans fluid is pinkish with a little bit of gold, probably close to time for a drain and fill.

Regarding the boost. I'm not surprised my reported levels are a little low, I also don't drive the truck very aggressively (it is an almost 4 ton vehicle after all). I've never done a 4wd launch, heck I've never done a proper 2wd launch under boost. Never even done a burnout or a donut. The boost levels I quoted are under heavy acceleration while passing for example. I'm sure my turbo it a bit worn after almost 200,000 miles and I expect to have to put a new one in sometime in the next 30k-50k. I've tested the system for leaks, and recently replaced the air filter. If you can describe a method for checking boost levels that would not involve me breaking laws excessively on city streets or putting excessive strain on the trans I can try and do it. I checked my edge insight and it's highest record boost value was 36lbs, although that has probably not been reset for over a year though. I reset it and I'll try and take the truck out tonight and see where I end up.

I apologize for not mentioning the tune, you are correct that is a very important bit of information that I left out. I could have sworn that I had added it to my sig when I was more active on these forums. I am well aware that no tune is "trans safe" and I am fully aware that the stock allison 6 speed is a bit of a weak link in the driveline. Since I am aware of that, I do my best to drive accordingly. I purchased tunes from Idaho Rob specifically because I was told his defueling between shifts was easier on the trans than other tunes. I drive my truck the way I imagine old people drive sports cars. Leisurely driving around 90% of the time with the other 10% being 3rd/4th gear roll on's on highway on ramps or occasional "brisk" starts from stop lights (with traction control on).

Regarding the different tunes. My available tunes are +0 HP towing, +40 hp towing, +90 hp economy, +120 hp "race" (never used), and Valet.

2009 Chevrolet 2500HD LTZ CCSB White 4x4
4" turbo back straight pipe
EFI live with Idaho Rob tunes (RIP)

Last edited by Epiclobotomy; 10-22-2019 at 01:29 PM.
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post #9 of 20 (permalink) Old 10-22-2019, 03:03 PM
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Proper fluid level is just above the low hot hash mark.
Search and youíll find.
Iíve found right in the middle of the low - high hot hash mark works best for me trans temp 170-180.
If your level is on the high side itíd be best if you lower to at least the middle of the hot low - high.
A simple spin-on change will remove about 8 ounces.

I doubt the higher fluid level is causing what your experiencing.

I still suggest to run a lower hp tune, me personally Iíd never run above 50 hp.

I run my ATP heavy towtune 24/7-365, I load one tune at a time.
I have 4 different EFILive tuners with multiple tunes from all 4, all are single loading one at a time.
I found one single tune I like, and it stays.
No chance of dumb mistakes switching on the fly.

Dan
2007.5 GMC Sierra 3500 SRW L.B."BLACK" C.C. 4X4 LMM Z71
DIY stock airbox mod, aFe ProGuard 7, OEM drop-in
EFILive ECM ATP - Heavy Tow Tune with Turbo Brake.
EFILive TCM Duramax Tuner
DHD Cold Side High Flow Intake Pipe
DHD PCV Re-Route
AirDog 4G100

2007 Yukon Denail "BLACK" 6.2 gasser
DIY stock AirBox Mod, K&N, Airaid intake, EFILive TCM only tuning
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post #10 of 20 (permalink) Old 10-22-2019, 03:11 PM Thread Starter
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Quote:
Originally Posted by D_R_C View Post
Proper fluid level is just above the low hot hash mark.
Search and youíll find.
Iíve found right in the middle of the low - high hot hash mark works best for me trans temp 170-180.
If your level is on the high side itíd be best if you lower to at least the middle of the hot low - high.
A simple spin-on change will remove about 8 ounces.

I doubt the higher fluid level is causing what your experiencing.

I still suggest to run a lower hp tune, me personally Iíd never run above 50 hp.

I run my ATP heavy towtune 24/7-365, I load one tune at a time.
I have 4 different EFILive tuners with multiple tunes from all 4, all are single loading one at a time.
I found one single tune I like, and it stays.
No chance of dumb mistakes switching on the fly.
thanks for the advice, I'll go ahead and do a drain and fill (since it's due anyways) and toss on a new filter. I'll try and even out the levels when I do the drain and fill.

2009 Chevrolet 2500HD LTZ CCSB White 4x4
4" turbo back straight pipe
EFI live with Idaho Rob tunes (RIP)
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