C7 Z06 engine loping
Pro
Joined: May 2015
Posts: 555
Likes: 49
From: Slinger, WI
Pro
Joined: May 2015
Posts: 555
Likes: 49
From: Slinger, WI
Took the field reps out for a few runs a they totally agreed something was definitely wrong. Ended up hooking up a 3 dimensional vibration device (Picometer) and found the chugging was very cyclical, was even worse when decelerating without any brake. Conclusion is a ring gear in the rear end that has variable backlash runout. So the backlash on one tooth is more than on another so when it comes around it has a space where it is not engaged and then it kind of free wheels till it hits the next tooth causing a noticeable bump. Makes sense to me as it is always the same no matter what gear, and the frequency increases with speed. Also, when he disengaged the torque converter so it was not 100% locked up the shuddering was very less noticeable. Seems the fluid in the torque converter would absorb it. I noticed the acceleration was much less with the torque converter disengaged.
I asked if this was a common problem and he said they had seen it a lot in trucks but he never would commit to it being a corvette problem. They are going to replace the entire rear end which is on back order.
I asked if this was a common problem and he said they had seen it a lot in trucks but he never would commit to it being a corvette problem. They are going to replace the entire rear end which is on back order.
Last edited by K1600GT; Sep 30, 2016 at 07:30 AM. Reason: Add info
Race Director
Joined: Jul 2007
Posts: 10,961
Likes: 2,643
From: Texas Hill Country
Mine was doing this, I found it to be an airflow issue. I really don't think a new rear end is going to do anything.
I seem to have fixed it by backing off the screw for the by pass valve about 1/8 - 1/4 of a turn and doing a "Cleaned Throttle" reset which the dealer could do for you or you can use an HPT Scanner to do it for free.
I always would happen with 3 or 4 psi of boost at part throttle, felt exactly like surging I could induce with some old turbo cars when the BOV wasn't opening enough to relieve pressure from the intake. So for now, I think it is fixed but I could stand to drive it more...
I seem to have fixed it by backing off the screw for the by pass valve about 1/8 - 1/4 of a turn and doing a "Cleaned Throttle" reset which the dealer could do for you or you can use an HPT Scanner to do it for free.
I always would happen with 3 or 4 psi of boost at part throttle, felt exactly like surging I could induce with some old turbo cars when the BOV wasn't opening enough to relieve pressure from the intake. So for now, I think it is fixed but I could stand to drive it more...
Race Director
Joined: Jul 2007
Posts: 10,961
Likes: 2,643
From: Texas Hill Country
Right in front of the engine. It's important to do a proper throttle reset also though. I am not saying this is the fix, but there is a high correlation on my car so far.
Pro
Joined: May 2015
Posts: 555
Likes: 49
From: Slinger, WI
i'm gonna need a picture. Which HP tuners do you guys use? There seems to be a few of them. Some are a little pricey too.
Race Director
Joined: Jul 2007
Posts: 10,961
Likes: 2,643
From: Texas Hill Country
There is one HPT but there are two cable options, one Standard, one Pro. The Pro allows analog inputs like widebands, EGT, and other various gauges, etc. to be logged along with the PIDs built into the computer. You can use the AEM 30-0333 with the standard cable but everything else needs the Pro.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
5 Best & 5 Worst Corvette Daily Drivers
Joe Kucinski
The Headlights of Every Corvette Generation Explained
Joe Kucinski
5 Best & 5 Most Overrated Corvette Track Packages of All Time!
Joe Kucinski
Every 2027 Corvette Engine Explained
Joe Kucinski
Designer Imagines A Corvette That Looks More Like a Corvette Than the Corvette
Verdad Gallardo
10 Ugly Corvettes That We Still Kinda Love
Joe Kucinski
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
Race Director
Joined: Jul 2007
Posts: 10,961
Likes: 2,643
From: Texas Hill Country
Le Mans Master
Joined: May 2000
Posts: 8,729
Likes: 1,508
From: Redmond WA
So our bypass valve has a preload spring adjustment I take it? It really should open around 0. Sounds like it's bleeding pressure off at 3-4 psi which causes you to feel surge. Higgs is suggesting increasing (or decreasing) the preload which should stop the bypass valve from cycling under light boost.
Frankly, I'm surprised this isn't an error (CEL) condition for the bypass valve to open in the presence of boost, but the ECU may very well get no feedback from the valve at all, so isn't aware it's happening (other than watching the MAP I guess).
Frankly, I'm surprised this isn't an error (CEL) condition for the bypass valve to open in the presence of boost, but the ECU may very well get no feedback from the valve at all, so isn't aware it's happening (other than watching the MAP I guess).
Last edited by davepl; Oct 21, 2016 at 05:47 PM.