Ok guys I've been testing a bunch of things based on the hundreds of threads I've read searching on the forum and I am finally sick of trying to chase down the source of my issues and would like some advice from the wealth of knowledgeable people on here. This will be long because I'll try to cover the majority of what I've done. The car is an 04 MN6 coupe with 95k miles. Bought it bone stock a little over 3 years ago with 54k miles. The only non maintenance things I’ve had to do are a window regulator and side bolster bladder replacement. Mods along the lines of where problems could be are ProFlow+ CAI with K&N filter, Hooker cat back exhaust (not stainless), and C6 shifter.

If you just want the symptoms then skip to the last paragraph, but below is a mostly complete account of issues and the things I’ve done to troubleshoot.

So the first issue was that I had a little bit of vibration while driving, not terrible but noticable. It was time for new front tires so I went ahead and bought 4 new all season tires which were balanced but not road force balanced (i will not be back at that shop again) and the problem didn't really go away but maybe a little better. I believe atleast one of the rear thinspokes is bent a little because they always take quite a bit of weights in various spots to balance. BTW, i can feel the shaking in the whole car including steering wheel, seat, and shifter. I'd say the steering wheel the most but definitely all of the above when below the 2k range.

Another old issue was that I thought my non stainless cat back was starting to rust out a little internally and causing some rattling. I don't think this is really true now but I'll mention it since the rattle is another symptom. One day I also noticed the hanger springs make a little noise if I move them so maybe that was the issue. I tried to prevent them from rubbing but I don't think it worked so they may be the cause of the rattle as well (I will go buy a spring from Lowes to fix that as soon as I dig up the thread that showed what I need).

So just to point out, until today all of my efforts have been aimed at curing the shaking that the car, either stationary or moving. So now we get to the conflicting symptoms.

So if I am driving and push the cluth in the car will usually overshoot (below) idle speed when it winds down but then will settle around 750. Sometimes it will bounce around an additional time or two but not that often. But last week it was having more trouble than usual and was idling rougher than usual. So I cleaned the MAF and throttle body and it is now better but still a little rough when idling (not just the slight cammed car feel that is normal I mean the needle wiggles around a little bit more than I feel it should, nothing really drastic though).
Ok so here is the stuff that really concerns me. There is a little bit of a stutter in the 1k to 2k rpm range. This is whether clutch is in or out. And it seems to coincide with the exhaust rattle. One of the things I had also thought might be wrong is maybe one or more of the cats had broken apart and was rattling where it might get stuck at the over axle bend. This also might have explained the idling issues. I banged on the exhaust system with a rubber mallet and wasn’t able to hear anything that would suggest anything was in there, and the cats didn’t make any strange sounds either when tapped. So I am temporarily ruling that one out.

Back to the stutter. The problem has now progressed to when actually driving too. Power is lacking a little all through the rpms but down low it is pretty bad. This is why I am concerned because my baby is a daily driver and does 60mi round trip a day, half highway half stop and go. I do not want to cause further damage but I can’t find out what is wrong.

Also, there is a sound which I would compare to a wine glass spinning while a sharp piece of metal was in contact with it. The sound can not be heard when the throttle is depressed even a tiny bit or when cruise control is set but the second you take the load off it makes this sound constantly. Like if I rev it a little then let off it makes the sound as the rpm wind down. I think the sound can also be heard if I release the clutch pedal while in neutral. While in the car it sounds like it is coming from just in front of me in the center of the car but today revving a little with the door open I got the impression it may be further back behind me. Side note, the exhaust also sounds a little meaner than previous years.

So today I changed the belts and took a video of the harmonic balancer while I was down there. It has a little bit of movement but from what I’ve read a little bit is normal. However it does seem to only really move when in that 1k to 2k range, so maybe it is related. It is also hard to trouble shoot noises because the serpentine tensioner whines fairly loud, I’ll order a replacement tomorrow. I first wanted to be sure that’s what was causing the noise when I changed the belts.

Another test I have done is to turn the engine off while moving to try to see if the vibrations are mainly from the engine, driveline, or wheels/rotors and the vibration seems to get a little better but there is still an underlying vibration, so I believe both are causing vibrations.

I also think I am ingesting some oil, more so than typical for an LS1. I noticed there was a few drops of oil in the throttle body today (maybe 400 miles since I cleaned it). And I also pulled all of the spark plugs. They all had burnt oil along the circular tips, but the #2 actually had some oil all the way to the top of the threads. They show more wear after 20k miles than the original set did with 75k so I am concerned. I will replace these plugs but not before we find out what the source of the oil is. Next weekend I’ll try to find someone to give me a hand so I can do a compression test since I was surprised that the first plug I pulled had oil all the way up the threads but then none of the others did.

Ok well there have been other things I’ve done to try to test things but this is getting really long and unorganized so I’ll just recap real quick and then feel free to ask me any questions or possible causes and I’ll say what I have done to test for that.

Recap: Problems are hesitation/stutter below 2k rpm, sound when letting off the gas, rattling in the rear, bad vibrations when driving but still minor-medium vibrations when rolling with engine off, losing some oil somewhere with signs of oil on throttle body and spark plug tips.

 

Yikes.....I'll wait for the condensed version......

 

Also forgot to add that I had been getting a low oil warning but the dipstick seemed good enough so I chalked it up to having just moved to very hilly MD compared to flat FL and that occassionally parking downhill caused it to think oil was low. However when I changed the oil the warning never came back so I think it actually was a little low.

 

Read and post your DTCs!

READING YOUR Engine Diagnostic Codes (DTCs)
The Diagnostic Display Mode is entered with the following procedure:
1) Turn on the ignition but don't start the engine.
2) Press the RESET button to turn off any warning messages. (i.e. door open, trunk open etc…)
3) Press and hold OPTIONS
4) While holding OPTIONS, press FUEL four times within a 10-second period.

Initially, on-board diagnostics go into an Automatic Mode which shows diagnostic codes in a pre-set sequence: PCM - TCS - RTD - BCM - IPC - RADIO - HVAC - LDCM - RDCM - SCM - RFA. All codes will be displayed for each. ( i.e. PCM = 4 codes)
If none are present in a given module, you will see No More Codes on the display.
There are two kinds of diagnostic codes, Current and History designated with a letter suffix, C or ;H. A current code indicates a malfunction is present in the module displaying data. A history code indicates a problem existed sometime in the last 40 or 50 ignition cycles. When not accompanied by a current code of the same number, it's potential evidence of a previous problem, now resolved, that was not removed by clearing the codes.
More likely it's an indication of an intermittent malfunction.
Intermittent codes are the most challenging of the diagnostics. An intermittent code may have happened once, may have happened more than once but is inconsistent or may be happening on a regular basis but not at the time the codes are displayed. History codes can also be caused by a current malfunction in a system that is not operating at the time codes are displayed. An example is the rear window defogger which doesn't operate until the Body Control Module detects engine rpm. For history codes set by a module that does not operate with the key on and engine off, a special diagnostic tool called a Scan Tester is necessary to properly diagnose the malfunction.
Once the system has displayed all modules, it goes into the manual mode which allows selection of each module using combinations of Driver Information Center buttons. Manual mode can also be entered during the automatic sequence by pressing any button except E/M. Once the display shows Manual Diagnostics, select a module by pressing the OPTIONS button to go forward or the TRIP button to go back. Once a module is selected, a code is displayed, and if more than one are present;
press GAGES to go forward or FUEL to go back.
To exit the diagnostic mode at any time, press E/M. If you want to erase codes in a given module, press RESET
To reset the codes once in manual mode, press and hold RESET until it displays NO CODES Press OPTIONS to go to the next module. Repeat the steps until you have reset the codes in all the computer modules.
NOTE!! Only reset the codes IF you want to - it is NOT necessary to do this. Clearing a code does not repair a problem. You are simply erasing the evidence of it in the module's memory. If you clear the code/s, and extinguish the Check Engine Light, your emissions status ready will NOT allow you to pass an emissions test until you have completed the required driving cycles.
Once you have the codes, the next question is: What to do with the information?
First, consult the factory service manual. Any serious C5 Do-It-Yourself owner should invest in the Corvette Service Manual of the appropriate model year. The Service Manual is really a requirement if you want to understand and work on your C5.

Here is very good site of DTC definitions:


http://www.gearchatter.com


Make sure to include the H or C suffix!!

 

Yeah sorry I got carried away in my frustration. Fact of the matter is I'm worried because below 2k rpm the car doesn't run that great and I think the problem is oil leaking somewhere internally because the plugs are more oily than would be considered normal. So let's focus on this issue and leave the other vibration stuff and noises out for now.

 

Bill I was hoping you would drop in :-)

I have had ZERO codes when pulling them through the DIC and never have had the check engine or anything come on. I had been having the code P0492 for the driver side AIR valve come on but cured it with the forum solution of carb cleaner and WD40 into the AIR tube.

 

You are obvioiusly in touch with this car and I applaud you for that. However, from your descriptions it seems that you are chasing several issues. You know someone is going to ask you to pull codes but since you didn't mention anything I will assume there are none that you think are significant.
Your vibration issue could be many things and it might be good to think about this from a system approach. If it were me, I would try to isolate the vibration. It seems like the most logical place to start would be in the torque tube/differential for all the reasons you listed above but that is purely my OPINION and I am sure I could be easily mistaken. You have a MN6 so you have many components which tend to complicate the diagnosis like the clutch, throw out bearing, pressure plate, etc. and which really have to be visually inspected and measured to either confirm or eliminate them as problems. If you have the skills to do this then great. If not, I would suggest a professional that you trust. With the mileage you have and depending on how it has been driven you could easily have multiple issues in the drivetrain. The systems that get the most wear would be the suspension, tires, and wheels, the clutch, the torque tube bearings, the differential. I would start with the simplest and move to the more complicated and expensive. After I found the source of the vibration I would then work on the engine issues.
The people on here have a wealth of knowledge but it is very difficult to help from a tech standpoint until a specific issue is identified. I know this doesn't help much but you have to start someplace and perhaps Lucky or some of the others will be able to help you more if you will isolate something and evaluate it to see if it is the source of the problem. Replacing parts on the hope that it is the problem will be a source of frustration and it will be expensive. Good Luck.

 

Pull off manifold PVC hose connection and plug the manifold. start the engine and see if you have excessive blow by coming out of the PCV Hose.

Sounds lie you should do a compression & leak down test

Could have a bad spring or bad ring,

BC

 

Where are these located? If I plug the hose then what do you mean by blow by?


As far as isolating the vibration to a system that's what I've been trying to do and why I am so frustrated now. Prop shaft/torque tube are on the possibiliteis list but doesn't explain the stutter when sitting still and simply revving the engine a little. I also can not determine when the vibrations are worse. At first I'd say they are worse at lower rpm but then again when just cruising with cruise control at 60 mph the vibrations are perfectly bearable even though rpm is around 1500.

Also worth mentioning for the vibration issue I replaced the rotors and pads a few days ago with no change apparant afterword. I have also inspected every suspension component I can get my hands on and all appear fine.

Someone mentioned how the car has been driven - I baby the car, never done a burnout, never been tracked, etc.

 

If all the cylinders are NOT firing properly, you can NOT properly diagnose a vibration issue. Get the engine running properly first.

PLUG THE INTAKE MANIFOLD and OBSERVE THE GASSES COMING OUT OF THE HOSE

 

BC ......you may be right about this but it seems odd that the whole car would shake like the OP says in his first post if it was a bad spring, a vacuum leak or a misfire. No codes either which would seem to point towards drive train to me. But like I said before you have to start someplace.

 

Pull off ONE plug wire and take your car for a drive.. See what you feel.

BC

 

I get your point but wouldn't that throw a code?

 

Ive seen a LS1 with one broken coil on part of the intake valve spring run horrible and NOT throw a DTC. The cylinder still fires but power is a LOT less on that cylinder.

BC

 

Looked it up and now I understand. I'll have the results tomorrow night. Thanks to both of you for your inputs so far.

If that's where the oil is coming from it would definitely explain why there's more at the #2 plug right where that hose comes in.

 

I certainly bow to your superior experience and I am trying to learn here too. So, after the OP does a pressure leak down test and that is good then what? Should he still try to find the hesitation, rough idle, and engine skipping issues before going on to the drivetrain?

 

Just to add a little more, I believe the car runs pretty rich from the factory but I feel like I have a lot of fluid that comes out the tips. Oddly, it doesn't seem to have an odor like it is just water. It's clear, no indication of burning oil or coolant. And coolant in system looks great. I also had the battery tested a few days ago and came back good.

 

I pulled the PCV hose (the lower of the two hoses going into the manifold right behind the throttle body) and there was a lot of suction on the manifold side and nothing at all coming from the hose. I let it idle for a couple minutes and nothing spectacular happened so I shut her down. The engine was already warm before I did this.

A couple of questions about the process of checking the compression. First I pull all of the spark plugs. Then makre sure no fuel will get in there by pulling the fuse to the fuel pump. I know I had read something awhile back about grounding something, is this necessary and if so what is it that I was supposed to ground? I want to say it was something about the coils. And then I simply screw the gage into one spark plug hole at a time, throttle to the floor, and try to crank it a few times correct?

If I can find someone to lend a hand I'm going to check the compression tomorrow after work and then throw some new spark plugs in there to see how it runs.

Thanks again guys.

 

Watching your Thread. Good luck and sweet Avitar!

 

I would definitely pull off the manifold covers and look for a broken spring. Your vibration symptoms are similar to what happened with my 2004 C5 just before a valve spring broke.



Here is what it looked like in the engine.



If that valve drops in the engine, it will destroy it - well worth checking.