The_Ghost

I know a lot of guys are tuners here, so thought I would post it here too. I have an '08 Solstice GXP LNF turbo with 82k on it. It's dyno tuned, has had the WP at about 58k done, had a HPFP done last year. Replaced my thermo this spring. Now that this is out of the way....

I need help with a NASTY vibraion at 1k rpm....

A bit of background....

I took my car in May to a tire store to have my rim leaks fixed. When I arrived, I told the guy writing it up that there were lifting instructions in the car as I have undercar LEDs on my rails, so they must follow these instructions. No problem...I've had other work done here with my other cars and they've ALWAYS done really good work and have been careful. But this time I made a mistake and left the store because I had other errands to do that day. When I returned to pick it up, the writer was adamant that I look under the car to make sure my floor was okay. No problem I thought, so I did... Looked fine. As I drove home I noticed a new noise that I didn't have before. A high pitch tinging, sound. When I got home I seen that my Solo exhaust mesh hinge joint was failing....or so I thought. As I pulled it in the garage, I noticed a vibration the car never had before. My car, even though dyno tuned to the ragged edge, has always been rather smooth... I thought maybe they took my car out on a test drive and pulled more then 7k RPM because I have no rev limiter and my car gets to 7k in 1st and 2nd in the blink of an eye. Long story short, I took the car to Midas to have the front portion of the exhaust redone (Solo sent me a new part) and I was shown that the exhaust was pushed against the driveshaft tunnel. Hmmmm.....could this be my vibration? Nope. Vibration was still there after I picked it up.

I have done research on my issue and found that other LNFs experienced the same sort of vibration when they had a balancer chain guide failure, but theirs seems to be a bit larger range of RPMs then mine. Mine seems to be very nasty around the 950-1050 mark, every other LNF seems to be 1000-1500 or even 2000 RPMs that has this issue. So last weekend after my radio got alien lettering, I decided that it was time to replace my 10 year old battery. After replacing it, I started the car and the vibration was gone!! Could it have been my battery causing this issue? I drove it the next day and put about 100 miles on it with no vibration. Hmmmm....did a battery fix it?!?!

Nope. I was going to drive it to work on Friday so I thought I would start it and go put gas in it....guess what? The vibration was back, and back with a vengence! It's so bad that it literally made all the plastic joints in the interior squeak as it shook. So I left it idle for a few minutes, then decided to rev it to 2k and hold it there for a few seconds (20 or so I think). After I did this, the vibration was gone!!! It did not rear it's ugly head again during that idle session. So I started it again today and the vibration is back. It is cyclical while it's there... Today I could not get it to go away. I'm at a loss. I need some advice/help and hoping maybe the right person here knows what might be wrong. I really don't want to tear into the timing chain and replace all of that...and I'm NOT paying a dealer to do it...not at $2000 that I was quoted. I'll buy a brand new long block with turbo for less then that and do that myself! So here's what I know:

Vibration is between ~950 to ~1050rpm
Vibration comes and goes, but it's there most of the time at cold start. Seems to go away after the car gets warm but has been there after warmed up.
It was recommended that I replace the Throttle Position Sensor as they can "lose their parameters" and must "re-learn" when they start to fail, and will create a rough RPM range until they do so. The re-learn phase happens above 2k RPM, which would explain why the vibration goes away after it warms up.

I know that it's motor related. It happens when the car is not moving. And seems to be more of a vibration then a miss. So I was on the path of possible fly wheel weight, but, that couldn't be because it comes and goes. And it is horrific this last time. You can hear every piece of plastic squeaking inside the car right now. But once it gets warmed up, it's gone....but could come back. A balancer chain guide failure would have the car doing this all the time and would also be throughout all RPMs...I would think. My other possibility is this. When I did my thermo this spring, I only pulled out about 1.5 gallons of fluid....but the car somehow took almost 2 gallons when I refilled it. My MAF only has about 10k on it, and the readings are rather steady, so I doubt it's that. Camshaft solenoids were replaced last summer with my HPFP

So here's where my thoughts are:

Throttle Position Sensor
"Lazy" Fuel Injector
Alternator not putting out enough of a charge at that RPM range at times
Possible coil going out, but not bad enough to set a CEL
HPFP not putting out enough fuel - my fuel pressure stays the same at 1k as it does at idle of 720. Then it starts to increase.

2 things that have made me shy away from the flywheel area. It happens in neutral, or while I have the clutch in while in gear.

Furthermore, it seems to be less of an issue once the car is warmed up. It can come back, but happens a lot less and the vibration isn't as severe once the engine is warm if it comes back

So those are my thoughts.

So, before I tear up my motor...any suggestions?

The_Ghost

One other item of interest. My AFR has always gone to 29.4 when coasting in gear. I data logged and it now only goes to 27.58...and is commanded to that number as well. Not certain if this would have anything to do with the vibration.

Solaris99

Have you checked the engine and trans mounts? I know there are symptoms that may not be explained by loose/failed mounts, but it's worth checking them. At least for the sake of knowing you've eliminated those possibilities.

jdbaugh1

iTrader: (1)

First you have to determine if it is a misfire situation or a vibration from a rotating component. I would recommend not driving it besides to troubleshoot as a vibration that drastic could really be tearing the hell out of something. As far as your AFR it should only look like that if you are decelerating.

Is your CEL illuminated? I would think if it was misfiring that bad you would eventually throw a misfire code. A misfire should make the exhaust note kind of sound like a Subaru.

If it isn't a misfire you have an out of balance or out of sync rotating component. First thing I would do is inspect all of the belt driven components as that is the easiest thing to do. Check your harmonic balancer and make sure it is in good shape. We know it isn't on the transmission side of things because you said it does it still with the clutch depressed. If you determine that it isn't a misfire and it isn't a belt driven/driving component then your problem is either internal or something out of what with your flywheel/pressure plate. What I don't get, if it actually is an internal vibration, is how it could come an go. If something is out of balance it should vibrate all the time. If your balance shafts jumped time it should vibrate all the time unless that chain is somehow periodically jumping time making the intensity of the problem modulate. Your balance shafts are driven by the same chain as your water pump which is a separate chain behind your your timing chain.

If it turns out to be your balance shaft chain replacing it and the timing chain isn't real hard depending on how accessible the front side of your engine is which I am not familiar with in your car. On the Cobalt you have to remove all the accessories/motor mount on the front side of the engine, the crank pulley, the valve cover, and the timing chain cover.

ECaulk

iTrader: (6)

4 cylinders vibrate like a sob when there is a misfire at idle. Start with simple things, get new ac delco plugs if you haven't installed new plugs in over 10k miles.

Do you know what your AFR is during WOT?

Also at that mileage you're intake runners could be pretty coked up and causing some of the issues.

The_Ghost

It does get better as the car warms up....and prior to yesterday even almost went away after warmup. I have no CEL. I have no misfire codes. I'll do the plugs this weekend. It's been a bit since (about 30k) since they were done. The car still pulls my 24psi of boost at WOT....but it doesn't feel as fast. I might be used to my ScatPack Charger now though. I have driven the car now for about 2k miles with this vibration and no CEL, no issues. Which takes me back to my initial comment about the test drive and over-revving my motor and the balancer skipping a link. Guess I should have said that I'm well versed in the LNF...I'm stumped on this as I was on my HPFP. I came here for advice on that as many people on our Solstice/Sky forums don't have more then 30k on their cars and haven't experienced the issues I have with my 82k on my car.

I will say this...this last winter I would idle the car for 10 min/wk to keep the battery charged. So I"m wondering if maybe I don't have carbon build up on my valves...as this is a common issue on our cars around the the 40k mark.... I have 82k, so I'm on borrowed time. I do appreciate the help and advice!!

AFR at WOT is between 13 and 13.5. Idle is now at 13.58....which is very rich compared to what it was a year ago. It was around 14.57 last year at idle.

ThePanduuh

iTrader: (3)

This is because your car, much like many cars, has DFCO. Downshift Fuel CutOff. It saves gas and emissions by not injecting fuel when above a certain RPM and no load (like when decelerating by downshifting). This is totally normal.

In my experience, my LNF vibrates between those RPMs when cold too. I've always assumed it's just a DI thing. It's not THAT bad (not like you're slipping the clutch on a hill and you don't give it gas vibrations) but it's different than when the car is warm that's for sure. I have an aeroforce interceptor gauge and I scan a few misfires (less than 10) on cold start and idling after. FWIW idle should be 14.7 (stoicheometric/stoich/1.00 Lambda). This is where it is stock. 13-13.5 AFR @ WOT is a bit higher than normal as my LNF (GMS1) goes into 12's. If I remember correctly the commanded AFR of stock LNF is 12.7? Not 100% sure but it should see high 12's. I doubt any of that is your problem though. I'd be looking for vacuum/boost leaks, and look into your engine & trans mounts. I'm not sure what mount options you RWD folks have but at 80k depending on your geographical location they could be shot (mine were way shot at 120k Northeast US, I replaced both front and rear trans mounts, FWD though). Also, 30k on plugs is a lot. I got maybe 15k out of my last set. Looked at the plugs when I pulled them out and geez they were toast. Lots of soot. Cleaning the valves will help a little but I'd start with the easy (plugs, vac leaks) then work to hard (cleaning valves). If you do choose to do the cleaning, I posted a video on youtube of it. TLDW: toothbrush + cleaner > walnut blasting with a low pressure low volume compressor. And obviously a compression test wouldn't hurt, I need to do one on my car soon I'm getting some blow by I shouldn't be.

The_Ghost

Correct. I know all about DFCO. However, last year it was commanding 29.4 and getting it. Now it's commanding 27.58 and getting it. I have logged and checked this on multiple days and it is always 27.58 now for some odd reason. Not certain if this could have anything to do with the vibration...just thought I would throw it out there.

The_Ghost

Well, went to datalog a bit tonight. Started the car, had my nasty ear shattering vibration happening. Let it idle while I sat up my logger for about 15 seconds. Started logger. Blipped the throttle to 2K and the vibration was gone!! Let it idle for 10 seconds or so....blipped it again to 2k, back comes the nasty vibration. I'm at whits end with this!!! Any help is appreciated.

Things I've done:

Replaced battery
Replaced spark plugs
Cleaned MAF
Made sure that MAP sensors leads were good.

Things I'm thinking
Bad Coil
Bad Injector
carboned up valve (although I don't think this is it because of the weird commanded AFR)
HPFP not functioning properly

2 things that bother me....the commanded AFR of 13.58 at idle when it was always 14.7 and DFCO commanded AFR of 27.58 when it was 29.4. The other item is that I still have no CEL.

ThePanduuh

iTrader: (3)

The_Ghost

The HPFP does not have a filter on the Solstice. It's got less then 2k on the new HPFP from last fall. My rail pressure is 'normal'. It goes to 1800 at WOT. And is around 600 during idle. My injectors have never been replaced, but I ran enough Techron through it to choke a horse a year ago!!

Now that you mention it, I may have an injector that is stuck open. What is the voltage range for the aft O2 sensor? Mine goes from 0.89 to 1.05.

ThePanduuh

iTrader: (3)

I wish I had video of my stuck open injector. Picture a diesel truck except white smoke coming out of the tailpipe. It was running super rich on cyl 4 lean on the other 3 (closest to HPFP is cyl 4). It was DEFINITELY noticeable that something was wrong. and AFR commanded at that point is useless because the ACTUAL AFR was 9.5! If you have a stuck injector you'll see the actual AFR be way under what it should be commanding.

And HPFP has an internal, non serviceable filter. But if yours was replaced already I'd rule that out. Is the HPFP harness chewed through by any chance?

The_Ghost

Yeah, I don't have any smoke. My car still runs like a champ when I get on it. My commanded AFRP isn't that far off....at least not yet. My harness is fine. I have checked over everything, unplugging and re-plugging any plugs I thought it might be....

The_Ghost

Maybe someone can answer my question here. My downstream O2 sensor is only registering between 0.89mV (or Volts, can't remember) to 1.05. Shouldn't the range be normally from like -1.0 to 3.0?