rhughes1691

I have a recently bought C6 A4. I don't know any of the history of the car and I'm a complete Vette newbie :-) Everything seems to work fine except...

Once cruising at freeway speeds (65-70), if I press the throttle nothing happens. I can press the pedal almost all the way to the floor and suddenly it kicks down what feels like two gears revs to 5K+ rpm and we take off.

I have discovered by trail and error that if whilst cruising, if I lift my foot off the throttle even for the shortest time, then press the throttle, normal behavior returns and I accelerate swiftly away. Once I'm cruising again the dead spot/lack of power returns. I also discovered that if I use "Competitive Driving Mode" that the problem just goes away completely.

So I'm guessing that TC/AH are cutting in and this is reinforced by a DIC message at freeway speeds that "Traction Control is active" . This only happens at freeway speeds. Around town and any speed under about 60 I have normal behavior with TC/AH in the default mode.

Anyone got any ideas where I should start exploring? As far as I know this is a stock C6 Z51....

Thanks!

Landru

Something's amiss, car should drive/accelerate smoothly.

Before throwing $$ & parts at this (system's many sensors etc) have you checked for any code(s)? If there're any logged it'd hopefully point directly to any issues making troubleshooting a LOT easier w/o guessing.

If you don't have a Tech II & don't want to buy one, most parts stores will loan one to customers. Got to check for codes.

rhughes1691

Thanks for that response, yes, I forgot to mention that there are no codes showing (using Bluetooth OBD adapter and Torque app)

AORoads

It's hard to know where to start, but it would help a lot if you told us more about the car: year (presume it's '05 with A4), miles, where you are located (Canada?), modifications to the car, bought from Chev dealer, dealer or individ., how long you've owned it, etc. Ladru is right above but may not be applicable in Canada re stores loaning it out.

rhughes1691

Yes it's an 05 convertible, 30K on the clock and bought from a dealer, who presumably had it as a trade-in as they sold it "as seen" but with a 173 point mechanical check that was all good. Located in "sunny" WA near Seattle, but definitely south of the border. As far as I know there are no modifications, but with the previous comment on "as seen" goes a "I don't really know". So far I have found nothing other than an aftermarket radio and tinted side windows :-)

KWC6

If you can locate a good corvette tuner he may be able to help you out. It could be engine timing way off or transmission needs adjustments.

v26278

I'm wondering if you have the stock tire sizes. The ratio of front/rear tire circumferences must be very close to the stock sizes, or you can cause TC to activate. If you get the ratio to high, at speed, your computer thinks your rear tires are spinning faster than the fronts and cuts throttle.

rhughes1691

Hmmmm, interesting thought. That will be easy enough to check. I'll have a look when I set of home and report back. The wheels at least look "normal" for an 05 being a five spoke chrome design.

AORoads

Excellent thought by v26 above. And do tell us if you know of a good Corvette shop/technician nearby. Never hurts to have one. So, I'm also guessing you bought it "south of the border" but you live north of it, right? (that's a thumbs up to our great neighbors to the north!)

duckvett

rhughes - please call Tom Wong at Wong's Engineering in Vancouver, WA. Sorry I don't have his phone number, but it should be easy enough to find. Tom is the best I know of.

Also, post your questions in the Pacific Northwest forum, some of the regulars there will have some suggestions on people to talk to closer to Seattle.

rhughes1691

Nope AOroads I'm a US person and so is the car to my knowledge. Not sure where the Canadian thing came from :-)

I'll repost to the PNW forum, but looking to see if I can work it out myself. I do most of my own car stuff, so was hoping that this was some form of known issue that folk had run into before so I could benefit from their experience.

AORoads

Must be my vivid imagination, sorry about that. If you need some help, you probably know of/about Corvettes of Auburn and Dan who runs the shop. Good luck with it.

rhughes1691

I didn't know about Corvettes of Auburn, but now I do, so many thanks for that!

To V26876, my tires are 245/40 18 on the front and 285/30 19 on the back but I can't find the "right" size in the owners manual anywhere. Do these sound about right?

Dano523

Double check the back tires,

They should be 285/35/19 on the rear to go with the 245/40/18 on the front. Of if the rears are 285/30/19, then the front's should be 245/35/18's.

Hence back tires need to be close to 1.04" larger in diameter than the front's, and if you have the 30s' on the back and 40's on the front, then the size difference between the back and front tires size diameters is too far off, the AH is throwing a fit, and why the car is having problems until you turn all the nannies off.

rhughes1691

So grateful, thank you. Hopefully problem solved!

Sounds like new tires in my future, which is a shame as the look nearly new.

RagTop69

I visited Bellevue, WA many moons ago and I can attest that it is indeed in the U.S. of A. Close to the border, but no cigar, eh. I was in my uncles Army at the time, during the difficulties in Southeast Asia, and I assure you the border guards would have arrested me on the spot if I had tried to cross.

silver74vette

Have you tried removing the valve body and cleaning it? It sounds a bit like a valve body sticking. Good idea even if it is not the problem.

rhughes1691

I haven't, but as switching to competitive driving mode makes the issue go away seems unrelated.

v26278

Dano pretty much explained the situation above.

You have the stock front size, but the stock rears are 285/35 19. Your rears are a little undersized, and therefore rotate faster than stockers at any given speed. With stock sizes, the larger rears rotate 0.96 of a revolution for each 1.00 rev of the fronts, and the computer reads that as normal. Your set up is probably exactly 1.00 to 1.00 - you can check the specs on Tire Rack - and the computer thinks the rears are breaking loose. I can't say for certain how far you can stray from the stock relative ratios without activating TC, but at a 4 or 5% variance I wouldn't be surprised if yours are too far off. To put it into perspective, you are racking up about 105 miles on your odometer for every 100 miles you actually drive.

bolivar

All cars have a sticker on the front drivers door or door jam that shows the correct tire sizes and inflation pressure. Been there for years.