Surge/Stumble Rears It's Ugly Head

[ZBass28]

Now that the cooler weather has hit it appears that I am getting some sort of surge or stumble just off-idle. The car was down for about 5 weeks while waiting for a new rear end assembly, but the engine fired right up and idled perfectly right off the bat.

Prior to this waiting period, the engine was not surging or stumbling in the 1500-2500 rpm range like it does now. Basically if I put the car in gear (6-speed) and let it roll at idle in 1st or 2nd it will develop a surge that rocks the car back and forth. To complicate matters, when leaving from a light in 1st and 2nd gear there is a "sloppy" feel to the engine as it revs from about 1500-2500 and then clears up and feels great.

First off, is the cooler weather partially the culprit here, at least on the coasting surge?

Second, I assume a great deal of my problems are related to AE things ever since my injector change back in early summer. The car has always felt somewhat 'flat' in the lower rpms but then pulls well after let's say 3000rpms. Since the AE tables are a little difficult to understand in terms of operation (at least for me they are), is there anyone that can give a very brief overview of where to apply the changes or what to look for? It would be hugely appreciated.

Tuning for good in-town drivability is gonna drive me nuts

 

[ZBass28]

All these views and not even one reply?

As logged last night....car in 4th gear at about 2000rpms(maintaining that speed, 95kpa). Stab the throttle hard and get a massive hesitation and backfire sound. Looking on the log I can see the car goes to 9.19 A/F at this spot. and then slowly clears itself. The actual O2 correction at that spot is showing that it is adding fuel.

I assume this is an AE issue, but which and where?

 

[Chuck Leeper]

I'll try...

Running the car in gear at idle has a very limited air flow over the O2 sensor.. could be showing rich, then attempts to correct and goes lean.. thus the surge... What's it do if you increase the rpm by adding a bit of throttle??
Sloppy feel could be due to rich condition and that would go back to the VE table and timing. Also look at the AE vs temp.[ I assume the engine is at op temp??]

You don't say what the combo is.. Lots of cam? Low speed perf will be "flat", so to speak, as compared to a stocker...
What gear ?

As for the weather.. could be.. you say only that it's "cooler".. Cooler weather could have lower humidity and thus more O2.. Engine could be leaning out w/ "better" air.
As for the VE tables... you should just log some driving time without WOT. Then make a WOT and see where the cursor goes on the logs.
VE at/near idle should be lower in that area.
The backfire is most likely due to a lean out. I would look at raising the AE vs TPS curve and see where that takes you. It could be the curve is too high at lower rpms.. a cause for the flat feel.[fat] If the curve falls too fast, then you could be getting the lean hole and thus the backfire..
Where is the commanded A/F ?? Does the log while driving show minimal correction? Should be in the 2-5% range max..
where is the O2 sensor mounted?

My $.02 worth.. Sure some gewroo will add to this..
Some idea of what the car has for modifications would help..

 

[ZBass28]

Combo:
350cid LT4, stage2 heads, cam (236/236 .560/.560 114), Vortech 10-12psi of boost, aftercooled, basically a well built stock cube LT4 with forged insides, 6 spd, 3.73 posi. Car ran fairly well and made good power last year with 55# injectors, but they were maxed out so I switched to some 65# this summer. None of these problems existed prior to this change.

If I ease into the throttle no hesitations at all. The VE's for the most part are dialed in to less than 2% corrections over the entire normal driving condition range (and most of the rest of the table up to about 6000rpms and 170kpa), the low rpm/kpa areas being the only giving me troubles.

Cooler weather meaning about 45* / 40% humidity with good barometer as compared to summertime 80*/ 50% humidity and some of these stumbles not occuring at all.

Commanded A/F in the area where I am seeing the hesitation would probably be about 12.8-13.1, somewhere in that range. Log does show minimal correction.

O2 sensor is mounted in the y-pipe just before where the 2 join together, so its mostly reading from the passenger side header.

 

[bobc455]

Zbass28, a couple of questions for you.

- What is your IAC doing when the engine surges? Is it causing the surge, or following it, or not moving?

- Do you have your timing trim set to zero?

- Have you played with your AE enrichment? There is both MAP and TPS AE, and since I have an NA car I can't tell you how to balance between the two, however I would guess that you want mostly TPS enrichment at low speeds, and mostly MAP enrichment at heavy throttle when the boost kicks in... can a turbo tuner help more here?

-Why are you so rich? I would think that 13:1 is a bit rich there, unless you need it for some other reason (most of my A/F map is set to 14.5 - 15:1, except for WOT where I drop to about 12:1).

- Can you describe the problem a bit better? You say a "surge or stumble" - these are kinda different to me.

- Have you checked to see if your TPS is staying steady? I wonder if your foot is moving the gas pedal without you realizing it (happened to me, took a while to get used to that).

- How is your throttle follower set? Do you have it off, or is the IAC setting bouncing around?

Let us know if any of these trigger any thoughts.

-Bob Cunningham
bobc@gnttype.org

 

[ZBass28]

Lots to answer here, I'll surround my replies with '***'

Originally posted by bobc455
Zbass28, a couple of questions for you.

- What is your IAC doing when the engine surges? Is it causing the surge, or following it, or not moving?

** I have not been monitoring the IAC during this, I will try this tomorrow **

- Do you have your timing trim set to zero?

** I know there is a little timing trim added, maybe .75 - 1.0 **

- Have you played with your AE enrichment? There is both MAP and TPS AE, and since I have an NA car I can't tell you how to balance between the two, however I would guess that you want mostly TPS enrichment at low speeds, and mostly MAP enrichment at heavy throttle when the boost kicks in... can a turbo tuner help more here?

** I've done some minimal playing with the AE stuff but my reluctance to try more stems from not fully understanding the numbering of the horizontal axis. Example: 7, 15, etc up to 51 on the AE vs MAP....what do those #'s refer to? If I better understood the tables this would be a breeze. **

-Why are you so rich? I would think that 13:1 is a bit rich there, unless you need it for some other reason (most of my A/F map is set to 14.5 - 15:1, except for WOT where I drop to about 12:1).

** The 13.1 A/F area is the stumble when I am hammering the pedal and getting the hesitation/backfire, normal driving conditions below 100kpa I pretty much have at 14.5 - 14.7, my low rpm surge is in the 14.7 range for sure **

- Can you describe the problem a bit better? You say a "surge or stumble" - these are kinda different to me.

** Problem 1: low-rpm surge in any gear when under let's say 2000 rpms and holding steady around 45-60kpa...the sound kinda reminds me of a motorboat going thru choppy water. The car will jerk back and forth in these spots.

Problem 2: Stumble in basically any gear when I first mash the gas really hard. Feels exactly like a carbed car that hesitates or coughs. Power cuts for a split second then comes on. **

- Have you checked to see if your TPS is staying steady? I wonder if your foot is moving the gas pedal without you realizing it (happened to me, took a while to get used to that).

** TPS is staying good, everything staying as steady as possible **

- How is your throttle follower set? Do you have it off, or is the IAC setting bouncing around?

** Setting is very low at idle, around 10 perhaps. No bouncing when at idle, but haven't checked to see what its doing as I am driving. IAC target at idle is around 22-25 **

I think I would basically be fine if I understood the AE tables just a little better, the horizontal axis always confuses me as to what the values really are.

Thanks for the help so far though, appreciated as always