Running a bit rough
 

Haven't been on the Corvette Forum in quite a while ... according to my last login, it was over 3 years ago. Since then, I've bought an '09 Vette to replace my wrecked '03. The new purchase is a GM Certified Pre-Owned car and has a 2-year, bumper-to-bumper warranty.

The one I bought last September had just over 12,000 miles so it had "sat up" a lot ... averaged maybe 4,000 miles/year.

It's been running a bit rough ... "missing" at idle, and another perhaps related symptom ... "surging" when coasting down from highway speeds.

I took it to Dealer A twice ... the first time, he replaces several things, like vacuum hoses, and I think he even replaced the computer module. But the problem continued. I took it back a second time and he could find nothing ... told me that since it's a performance it's built and tuned "on the edge" I couldn't expect it to run really smoothly.

So I took it to Dealer B a couple of weeks ago. He kept it for 3 days and while he was able to duplicate the complaints, he could find nothing mechanically wrong ... pulled the plugs, checked the injector balance, and many other things ... and the only thing he could come up with was moisture in the gas. Said the fuel volatility was "extremely low". So he advised putting in a bottle of AC Delco "Fuel System Treatment Plus" to remove the moisture. I did that, and the car actually ran worse for a while, and still does to some degree.

Going up a hill, it seems to downshift too easily, and you can feel the engine missing a bit. It still idles a little rough. And when coasting from highway speeds, a "surge" can be felt. If I shift into Neutral, the surging goes away so it's not tires. At full throttle, it seems to run just fine.

I'm going to put some Techron in the tank at next fill-up, so it'll have two shots and several hundred miles for the treatments to work. If the problems still exist, I'm going to take it back in to Dealer B and ask them to check again ... things like coil pack, plug wires, and torque converter (for the surging issue).

Anybody else had a similar problem or any ideas? Since it had not been driven much, and with ethanol in gasoline these days, I can see how water in the gas could be a real issue. But at the moment, it doesn't seem to be. I don't know how long it takes to remove moisture using the gas treatments but I would assume a tankful should do the trick. If I still have the problem after another service visit, I'll call GM and see if they can offer a solution.

fuel additives are not needed if you run good fuel except when stored for a while, then use fuel stabil. try cleaning the MASS and throttle body & while you are in there chech your clamps on the intake tube and filter.

I agree with truckplay, clean the MAS. Do NOT touch it with anything but the proper cleaning spray. Also does your Corvette have an aftermarket air intake system? Some of them have air flow turbulence issues that cause similar problems.

No, it's totally stock.

Thanks for the replies.

Yep, definitely check that you have the stock intake AND stock filter. If you have one of those crappy (for street use) oiled filters, then there's your answer. And yes, clean your MAF sensor too. And finally, switch to Chevron (or other tier-1) fuel. By your symptoms, it's a fueling issue IMO. Good luck.

Many people, but not everyone, with the LS3 engine (like our 2009) experience a "stutter" or "shudder" when letting off the gas at highway speeds, could that be one of the things you are experiencing?
"It's not a bug, it's a feature."
Causes no problems, and can't be "fixed", if that is indeed what is happening.

Thanks to the responders to my post, I THINK I have the issues fixed. I took the car back into Dealer B last Thursday. The fuel additives had absolutely no positive effect, and the stumbling and pulsing issues still existed.

The service guy didn't seem all that thrilled to see me ... they'd kept it 3 days the time before and the only think they could come up with was moisture in the gas. I told him that several posters on the Corvette Forum had recommended cleaning the Mass Air Flow Sensor. He didn't want to do that because there was no code. But I asked him to just humor me and at least check the thing. He said he would, but he'd like to keep it several days to "research" the problems. I think what he really wanted to do was to keep it long enough so that I'd never take it back.

But the next afternoon, he called, said they'd checked the MAS and it was defective. They replaced it and it seemed to have mostly cleared up the issues I was having. I went to pick it up and he actually seemed a little sheepish ... apparently because the Forum had solved something that they couldn't.

After several days now the car seems to be running fine. My thanks to the forum for quickly assessing the problem and suggesting a fix. I suppose dealer mechanics nowadays just work by codes and can't really analyze a problem and find a solution.

By the way, everything was done under warranty so it cost me nothing except gas to go to the dealer several times, my time, and the cost of the fuel additive. And the additive probably cleaned out some junk anyway. Thanks again.

Drop in more often:thumbs: