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Rough idle on a LS3

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Old 03-22-2018, 04:55 PM   #1  
rumen1
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Default Rough idle on a LS3

Hi all, I have just bought a '08 base Corvette. Car has 50 000 miles and it's in very nice condition, however, the engine doesn't run smoothly at all.

Whenever I buy a new car I like to change everything under the hood - liquids, spark plugs, spark wires, pumps, filters, 02 sensors, belts, rollers, water hoses and so on, so I can try to guarantee myself more or less trouble-free ownership of the car.

So I thought that changing the 02 sensors and the spark plugs/wires will solve this problem. However, after doing this, there is absolutely NO change in the way the engine idles. It's like there are little misfires all the time and that obviously translates in vibrations on the whole car.

I have saw many threats in this forum from people that had the same issue on their C6's - some people say it's normal, some say their engines are working as a swiss clock. I also doubt that the car has been working like this in the showroom, so it must be an issue somewhere. However, I haven't seen anyone to say what was causing his problem, so I decided to open this thread and get some opinions.

The main issue is that I live in Bulgaria and here these cars are not very common and unfortunately my mechanic doesn't have a clue where should he look for the resolution of this problem.

OBDII diagnosys doesn't show any errors either. The only issue with the car when I got it was that after a collision with a curb, the front right wheel was little off position, but it was a very light hit, so after some suspension alignment, everything was put back in place.

Any help will be highly appreciated guys.

Last edited by rumen1; 03-22-2018 at 04:55 PM.
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Old 03-22-2018, 08:36 PM   #2  
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Quote:
Originally Posted by rumen1 View Post
Hi all, I have just bought a '08 base Corvette. Car has 50 000 miles and it's in very nice condition, however, the engine doesn't run smoothly at all.

Whenever I buy a new car I like to change everything under the hood - liquids, spark plugs, spark wires, pumps, filters, 02 sensors, belts, rollers, water hoses and so on, so I can try to guarantee myself more or less trouble-free ownership of the car.

So I thought that changing the 02 sensors and the spark plugs/wires will solve this problem. However, after doing this, there is absolutely NO change in the way the engine idles. It's like there are little misfires all the time and that obviously translates in vibrations on the whole car.

I have saw many threats in this forum from people that had the same issue on their C6's - some people say it's normal, some say their engines are working as a swiss clock. I also doubt that the car has been working like this in the showroom, so it must be an issue somewhere. However, I haven't seen anyone to say what was causing his problem, so I decided to open this thread and get some opinions.

The main issue is that I live in Bulgaria and here these cars are not very common and unfortunately my mechanic doesn't have a clue where should he look for the resolution of this problem.

OBDII diagnosys doesn't show any errors either. The only issue with the car when I got it was that after a collision with a curb, the front right wheel was little off position, but it was a very light hit, so after some suspension alignment, everything was put back in place.

Any help will be highly appreciated guys.
the C6 runs a bit rough when cold. Have you gotten it up to full temp, probably around 95C? Does it still run rough?

With only 50K miles, I can only think that a loose spark plug, or a loose or burned plug wire, would be the only thing I could think of.
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Old 03-22-2018, 08:59 PM   #3  
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the C6 runs a bit rough when cold. Have you gotten it up to full temp, probably around 95C? Does it still run rough?

With only 50K miles, I can only think that a loose spark plug, or a loose or burned plug wire, would be the only thing I could think of.
Possibly a dirty MAF, or loose sensor connector maybe? Loose hose?

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Old 03-22-2018, 09:30 PM   #4  
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Possibly a dirty MAF, or loose sensor connector maybe? Loose hose?
could be that, too,Ed.

when you start thinking about cars in various parts of Europe, we have no idea what is out of the norm. Fuel, additives, temps, etc. Could be a lot of things that don't affect us over here. Or, it could be something some of us have experienced.

Can't imagine not having a good indie mechanic to go to, though...
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Old 03-22-2018, 09:42 PM   #5  
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Try to post a video so we can see what your dealing with. These cars are not going to idle smooth like a bmw or similar luxury car if that's what you are used to.

What is the idle speed? How much does it vary?
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Old 03-22-2018, 09:48 PM   #6  
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2cd for posting a video. When I started test driving. They all had what I considered a rough idle. I just thought most of them had been beat on. After researching, I found many comments about the rough idle being normal. I don't even notice it anymore but a big difference between the vette and my daily drivers.
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Old 03-23-2018, 01:18 AM   #7  
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Mine idles a little rough. But, I figure its a Vette with an LS9 so I live with it. I agree with the comment above that it would be easier to diagnose if we could hear it.
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Old 03-23-2018, 03:00 AM   #8  
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Thanks for the comments, I will try to make a video next time I go to the workshop, because the car is still there.

Yes, I have driven it for several days, and it doesn't get too much smoother when the car gets to a good operating temperature.

Car was working like that when it came from California with the US fuel. I have now put premium European fuel, but I honestly doubt the problem is there.

As I said spark plugs and spark wires have just been replaced and the MAF sensor has also been cleaned. Abput a loose sensor connection - if that was the case, wouldn't it trigger a check engine light?

Is it possible, that the valve lifters need adjusting, or with this kind of mileage this can't be the case?

Last edited by rumen1; 03-23-2018 at 03:01 AM.
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Old 03-23-2018, 04:01 AM   #9  
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Originally Posted by rumen1 View Post
Thanks for the comments, I will try to make a video next time I go to the workshop, because the car is still there.

Yes, I have driven it for several days, and it doesn't get too much smoother when the car gets to a good operating temperature.

Car was working like that when it came from California with the US fuel. I have now put premium European fuel, but I honestly doubt the problem is there.

As I said spark plugs and spark wires have just been replaced and the MAF sensor has also been cleaned. Abput a loose sensor connection - if that was the case, wouldn't it trigger a check engine light?

Is it possible, that the valve lifters need adjusting, or with this kind of mileage this can't be the case?
Nope. Bugged me too when I first got the LS3. I've owned a Firehawk (2 of them) with the LS1, then 1999 Firehawk had a smooth idle, the 2002 Firehawk had a more aggressive cam from factory (SLP) and LS6 heads (from factory) and the idle was a bit off.

The LS2 Pontiac Solstice Mallett (#006) was smooth as well.

I've owned two C6 Corvette GS 6-spds, both 2012. Both idled a bit rough, though my current one is a bit more smooth. The cam is aggressive and creates a little tiny bit of "bouncing around" +/- 100 or 200 RPM at idle. Sometimes it sounds like it almost misses a firing of a piston but it doesn't. The LS3 idle is not rhythmic because it was not designed to be LS1 perfect that way.

There's a few ways you can settle this out:

1. Get heavily shielded sparkplug wires like Magnecore (if they're still in business). This may or may not make a difference. YRMV.
2. Run a can of SeaFoam or equivalent (ie. Techron) through a tank of gas, or if you're into it, a little Marvel Mystery Oil. Lots of opinions on all those products so.. the choice is yours there.
3. Make sure you use 91-93 octane gasoline, preferably a Top Tier, non-ethanol if you can find it. Some States don't offer that, like Colorado.
4. Make sure your oil level is correct. If you have a dry-sump (GS 6-spd, Z06, ZR1) there is a special method in the manual. Use synthetic 5W-30 as per the manual. Many use Mobil 1 5W-30. Some owners prefer other brands such as Amsoil, Redline, Royal Purple, Penzoil Platinum, or Castrol GTX. Many opinions here. I recommend Mobil 1 5W-30 standard.
5. Turn up the radio.
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Old 03-23-2018, 05:15 AM   #10  
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Quote:
Originally Posted by rumen1 View Post
Thanks for the comments, I will try to make a video next time I go to the workshop, because the car is still there.

Yes, I have driven it for several days, and it doesn't get too much smoother when the car gets to a good operating temperature.

Car was working like that when it came from California with the US fuel. I have now put premium European fuel, but I honestly doubt the problem is there.

As I said spark plugs and spark wires have just been replaced and the MAF sensor has also been cleaned. Abput a loose sensor connection - if that was the case, wouldn't it trigger a check engine light?

Is it possible, that the valve lifters need adjusting, or with this kind of mileage this can't be the case?
try and keep your alternator cable as far away from number 1 plug wire as possible and use dielectric grease in both ends of spark plug boots when installing your new spark plug wires
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Old 03-23-2018, 07:52 AM   #11  
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Originally Posted by rumen1 View Post
Thanks for the comments, I will try to make a video next time I go to the workshop, because the car is still there.

Yes, I have driven it for several days, and it doesn't get too much smoother when the car gets to a good operating temperature.

Car was working like that when it came from California with the US fuel. I have now put premium European fuel, but I honestly doubt the problem is there.

As I said spark plugs and spark wires have just been replaced and the MAF sensor has also been cleaned. Abput a loose sensor connection - if that was the case, wouldn't it trigger a check engine light?

Is it possible, that the valve lifters need adjusting, or with this kind of mileage this can't be the case?
The lifters are hydraulic, so there are no adjustments.

As has already been indicated, in my experience the LS3 idles a little rough, especially when cold. You have received most of the basic suggestions to look at already. Just make certain all of the plug wires are firmly connected to the coilpaks, as I have seen several posts where misfires were caused by loose wires (this should result in OBDII misfire codes, but it's certainly worth rechecking).

Last edited by BadAV; 03-23-2018 at 07:53 AM.
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Old 03-23-2018, 08:45 AM   #12  
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My 2010 GS idled rough. The alternator cable was sitting on the plug wire. Just like Irok suggested, I fixed that and it now idles fine.
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Old 03-23-2018, 09:02 AM   #13  
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Quote:
Originally Posted by Corvette_Ed View Post
Possibly a dirty MAF, or loose sensor connector maybe? Loose hose?
This !!! Pull it and clean it.
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Old 03-23-2018, 09:47 AM   #14  
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Bulgaria?? Did anyone mention bad gas? That'd be my first guess.
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Old 03-23-2018, 10:19 AM   #15  
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OK guys, thanks, I will tell him to check these things again. The wires I bought are MSD High performance. The MAF has already been cleaned, but that haven't changed anything. Oil has just been changed - Castrol Edge 5W30.

As for the gas - I only use Shell V-Power Super 100. In Europe we have a different octane classification and the lowest octane is 95. The premium fuel is 100 octane. However, fuel for sure it's not the problem.

We'll check the things mentioned here and I'll let you know.

Thanks for the help.

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Old 03-23-2018, 10:37 AM   #16  
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Quote:
Originally Posted by azvetteguy View Post

My 2010 GS idled rough. The alternator cable was sitting on the plug wire. Just like Irok suggested, I fixed that and it now idles fine.
Thanks for the tip 'Irok' ... I went out in the garage and checked the heavy wire that attaches to the back of the alternator on my 2011 GS ... Yup, it was rubbing right up against the plug wire.

Fixed that, but I have not driven the car yet.

Again ... great tip !

.

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Old 03-23-2018, 10:43 AM   #17  
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rumen1 ....

On these cars, the spark plug wires seem to come loose to where they push into there individual coils.

You need to remove the fuel rail covers (easy), and then push the plug wires hard back into each coil.

Worth taking a few minutes to check it out.
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Old 03-23-2018, 11:39 AM   #18  
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Like others said "MAF Sensor" or a common problem is the intake duct comes loose after the air filter.

Last edited by JJLAI724; 03-23-2018 at 11:40 AM.
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Old 03-23-2018, 02:17 PM   #19  
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Please run a compression test and lets make sure all cylinders are within range. This is not difficult and rules out several possible issues.
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Old 03-23-2018, 03:07 PM   #20  
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Quote:
Originally Posted by Turbo6TA View Post
rumen1 ....

On these cars, the spark plug wires seem to come loose to where they push into there individual coils.

You need to remove the fuel rail covers (easy), and then push the plug wires hard back into each coil.

Worth taking a few minutes to check it out.
follow up with the dielectric grease on both ends and you will never have a plug wire come loose again
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