David Brock

05 6 speed. When I take the car to about 4K rpm in second then shift into third it will stutter or hesitate for a split second then continue accelerating. I had the harmonic balancer replaced with summit racing brand. It seems to have started after that. Under normal Acceleration the car runs great. First gear is always smooth and fast. Only Mods are vararam intake and borla exhaust. Any ideas? Thanks

subfloor@centurytrans

Check to make sure one of your spark plug wires hasn't come loose. I had a very similar problem in my ZR1 and that turned out to be the cause.

David Brock

I checked. They are all secure. Thanks though.

pauly1119

Could be the clutch? How many miles? Hard driving?

David Brock

57K miles. I don't drive it hard at all. When I bought the car the tires were 8 years old so I don't think the previous owner was hard on it either

K-Spaz

There are some things in a tune which will affect transient throttle and cause stumbling but when you're talking about a car that is still stock, I don't really think any age related components would cause that. As much as I want to comment here I just can't think of anything that jumps out at me.

Matthew Hughes

My LS1 Camaro does this on occasion. Upshift to third, and its like the engine tries to stall. May happen once every couple of months or so, but it's unnerving when it does. Only thing I can think of is maybe its an DFCO issue?

Underachieved

What is DFCO?

Matthew Hughes

Deceleration Fuel Cutoff. It's a fuel saving feature on manual cars. When you lift your foot off the gas either to shift, coast or decelerate, the PCM cranks back on the injector pulse width, leaning out the fuel mixture. It often feels like a jake brake on a diesel truck. Maybe it's stumbling on the transition after you shift and start accelerating.....thus the stutter.

K-Spaz

Unlikely. Dfco has no reason to work most of the time and then spontaneously fail.

I can't begin to diagnose what's going on with the Ops car. Or yours for that matter. About all I could do would be to make some guesses of where I would start. I would probably be looking more towards an intake air temperature sensor or something like that. They can cause all kinds of wacky stuff with your ve table interpretation and if you had a intermittent problem with one of them, that would probably make some really weird stumbles.

When you have transient throttle levels, the engine no longer uses the MAF. The MAF only works when there is a relatively constant air flow across it. When you're shifting and making fast changes to the throttle, the engine falls back to the ve table for a brief. Until it can trust the MAF reading again. If you've got a wonky intake air temperature coming from your sensor, then that's going to fuel the car dramatically wrong and cause all kinds of weird stumbles when you're shifting.

Now there are probably five hundred other things that will cause this and I am by no means an expert. But if you simply look at your tables in HP tuners you will see what things influence the fueling and that should limit the amount of diagnosing you have to do. Just look at the things that are used to reference cells in the fueling tables.