Help with off idle stumble

[Loose_Screws]

Need some help with correcting an issue with the engine stumbling/hesitation coming off idle. Trying to gentle pull away from a dead stop, the engine stumbles 2 to 4 times before driving away normally. If I somewhat aggressively try to accelerate from a stop, there's generally no issue. My tuner (WS6Green on here?) has been trying to figure it out as well. The car has a few thousand miles on it, no other issues than the stumble.

5.3 Gen IV / E38 ECM
243 heads with upgraded valve springs
AFM/VVT delete
Mild Texas Speed cam (truck cam I think?)
Mid-length headers (Church Boys)
Trailblazer SS intake and injectors
Gold blade throttle body
Card style (LS7?) MAF
Corvette fuel filter (returnless)

4L60 transmission
Built locally
Slightly higher stall converter

LC-2 wideband

I've posted the tune and a quick log from tonight. At the 1:53s mark, you can see the RPMs jumping while car is in gear and light throttle (holding foot brake).

I can see in the log the engine goes lean with the injector pulsewidth dropping. However, I the exhaust smell is strong (no cats). I'm assuming it's going lean enough the engine stops igniting the fuel?

Lastly, if I need to log different channels...let me know which ones.

Thanks!

[LS ROB]

Log Cylinder Airmass, Dynamic Air, Volumetric Efficiency, and figure out what's up with your DTC's. Looks like you have a few codes popped up.

[edcmat-l1]

VVE.jpg

VVE is completely jacked. Easy fix. Put it in speed density and dial it in.

[Alvin]

Looks like VVT was deleted? If so the cam tables need to be zeroed out and the VVE modifiers for camshaft movement need to be zeroed out.

The VVE and the timing are a mess.

[Loose_Screws]

Log Cylinder Airmass, Dynamic Air, Volumetric Efficiency, and figure out what's up with your DTC's. Looks like you have a few codes popped up.

Just got home from work and grabbed a quick log with the channels you listed. How does the attached log look?

[Loose_Screws]

VVE.jpg

VVE is completely jacked. Easy fix. Put it in speed density and dial it in.

Looks like VVT was deleted? If so the cam tables need to be zeroed out and the VVE modifiers for camshaft movement need to be zeroed out.

The VVE and the timing are a mess.

I copied my tune to a new file and looking through it now, may try a few changes myself. Anything more and I'll need my tuner to take a look at it...I'm just too new to HP Tuners to dive off into it.

[Loose_Screws]

I failed to mention in original post that the car has an E38 ECM.

[LS ROB]

Just got home from work and grabbed a quick log with the channels you listed. How does the attached log look?

My opinion, the tune needs a bit of work. I'd start with your fuel injectors and make sure that all the data is correct. You can get the part number off them and pull a tune from the repository that uses that injector, and copy the info over. Idle timing is set at 25* and ECM is pulling timing to keep idle around 750 likely due to excessive BRAF. My experience is limited, but id be shocked if the VVE is anywhere near correct.

[edcmat-l1]

My tuner (WS6Green on here?) has been trying to figure it out as well.

I copied my tune to a new file and looking through it now, may try a few changes myself. Anything more and I'll need my tuner to take a look at it...I'm just too new to HP Tuners to dive off into it.

No offense but, if your tuner couldn't figure it out before, might be time to find a new one.

[Loose_Screws]

How do you zero out VVT on an E38?

[edcmat-l1]

How do you zero out VVT on an E38?

It's turned off. You need to fix the janky VVE table.

[GHuggins]

I'm not so sure the VE is his actual problem. When I look at the VE table that his is actually using and look at the data then yes his VE table is 2 to 4 g/s off in a lot of places, but is that enough to cause the 20% lean tip in that he's talking about??? Might want to make sure the engine is mechanically sound and you have good fuel pressure and flow. The VE table is very "blocky" and climbs fast off idle. Just not sure it's "the problem".

[Loose_Screws]

I'm not so sure the VE is his actual problem. When I look at the VE table that his is actually using and look at the data then yes his VE table is 2 to 4 g/s off in a lot of places, but is that enough to cause the 20% lean tip in that he's talking about??? Might want to make sure the engine is mechanically sound and you have good fuel pressure and flow. The VE table is very "blocky" and climbs fast off idle. Just not sure it's "the problem".

The engine is mechanically sound. I built it 6 years ago and runs great, drove it cross country and back and then some. Oil pressure, water temp and WB readings are all good.

I did buy a used TBSS intake, rail and injectors back when I was putting the engine together. A couple of weeks ago I pulled the injectors and flushed them, wondering if I possibly had a bad injector causing this stumble issue. Nothing of concern found while flushing the injectors and it made no difference on the off idle (tip in) stumble when I retested.

The TB is a gold plate 87mm (RME-87) throttle body...genuine GM bought new. Anything to do with this TB used with E38 that would cause the stumble? Or the volume of the TBSS intake?

Fuel pressure is confirmed with mech gauge at 58 PSI.

[blindsquirrel]

Anything to do with this TB used with E38 that would cause the stumble?

Wrong question... the E38 was used from 2006 all the way thru 2017 - different service numbers (the internal hardware), different applications/platforms, many many different throttle bodies that do not interchange. TB has to match what the ECM is programmed as.

Or the volume of the TBSS intake?.

Maybe? Did you change it when the intake was changed?

[Loose_Screws]

Wrong question... the E38 was used from 2006 all the way thru 2017 - different service numbers (the internal hardware), different applications/platforms, many many different throttle bodies that do not interchange. TB has to match what the ECM is programmed as.


Maybe? Did you change it when the intake was changed?

I'll assume then that the TB matches what the ECM was programmed as.

I didn't change/modify the intake (or volume) if I understand your question correctly. As far as what intake the engine came with...it didn't have one, but assume it was a "truck" intake.

[gtstorey]

I'll assume then that the TB matches what the ECM was programmed as.

Why?

[Loose_Screws]

I put together a list of the parts used to build this engine. I have also attached a couple of logs and current tune. I am not a regular tuner, and what I have tuned has been SD (Original DIY Megasquirt / Holley Sniper). The guy who tuned my car for me is a friend and not a full time tuner. When we first tuned this car was back in 2018 (I think) and the E38 ECM probably wasn't the best choice on my part.

Before I start trying to make adjustments to the tune myself, I am working back through basics to insure everything is setup correctly.

Is the TB compatible with the OS?
Are the injectors setup correctly?

I understand from previous comments the VVE needs some work. I do see in the logs and on the WB display the engine goes lean (16-18:1) at tip in, then immediately drops to typical AF.

So, basic E38 tuning question...is the VVE table the 'base' table all fueling calculations are based on? I understand (somewhat) the VVE table is a calculated coefficient. Increase/decrease these coefficients to adjust primary fueling?

In the logs, am I logging the correct data needed to setup graphs/histograms for VVE adjustments? Any channels I should add?

NB O2 sensors vs single WB sensor...I read some comments that the NB should be used for both logs and calculating EQ error? And should I try to think/work in AFR or lambda?! So much to try and digest, trying to take it slow and methodical, actually understand the effects from making tune changes.

Also, I know the IATs get stupid...have seen nearly 190F sitting in traffic in hot summer. Chevy II's do not have much room under the hood, but I need to get some cooler air. I see knock retard in the logs, but not convinced the engine is actually knocking as I have never heard it. Wondering if the stainless headers, although wrapped, could be causing a false reading.

• 5.3 Gen IV with truck accessories
• 243 heads
• E38 ECM (VIN 1GC2GTDG1A1175124) from Chevy Express Van
• T42 TCM / 4L65 with VSS and slightly higher stall torque converter
• Custom built complete wiring harness
• RME87 Throttle Body (gold blade) (Delco / bought new)
• 15865791 MAF (LS7 card style) mounted 15" from TB in 4" aluminum intake tube, cone filter (Delco / bought new)
• 12594942 MAP (1.3 bar) (Delco / bought new)
• 12613411 Injectors in TBSS Intake and fuel rail (bought slightly used)
• Gen IV knock sensors on block (Delco / bought new)
• O2 sensors (bought new, believe they are Delco)
• PCV fresh air connected to port on intake tube between MAF and TB
• PCV crankcase air connected to top of intake (full vacuum), 1/16" restriction inline on PCV hose
• In tank fuel pump / external Vette filter+regulator (returnless), confirmed ~58 PSI with manual gauge on fuel rail (no flex sensor)
• TSP Stage 2 Low Lift Truck Camshaft, 212/218, .550/.550 112 LSA, LS6 valve springs (no DOD/AFM or VVT)
• Mid-length headers 1 7/8" primaries into 3" collector (wrapped)
• 2 1/2" dual exhaust with X pipe, no cats
• Innovative LC-2 wideband

[Loose_Screws]

Some pics of the car. Pic from Cruisin' The Coast this past year. And in 2018 we took it on a road trip from Alabama through Jackson Hole, WY to Yellowstone. Also saw Mt. Rushmore and Crazy Horse.

IMG_2037.jpgIMG_4759.JPGIMG_4833.JPG

[GHuggins]

So are we to understand that nothing on the engine has recently been changed and this issue just started out of the blue?

[Loose_Screws]

So are we to understand that nothing on the engine has recently been changed and this issue just started out of the blue?

The stumble issue started after the car sat for 3 years while I made some changes/repairs. The "cut down" harness I originally used when I did the engine swap in 2018 was replaced with a new, custom built harness (LS Harness Connection / Texas). The 87 octane (with ethanol) did not age well over the 3 years, destroyed my in tank pump and filter/regulator, so both were replaced. I also wrapped the stainless headers to help minimize under hood temps. No tune changes, no changes to the engine other than header wrap.

Correction, my friend/tuner did make some changes to the tune trying to solve the off idle stumble. He made the changes, so I can't say what was changed.