LS2 Engine Vibration – in Neutral, RPM Coincident

[Aaron]

This is my first service/tech post in the forums. I’ll try to be thorough in providing information, and I’ve done some reading before posting: noise & vibration troubleshooting FAQ pages and several vibration threads using the search feature. I found a post that seems similar to my issue, but not sure:

http://www.ericthecarguy.com/kunena/8-Service-and-Repair-Questions-Answered-Here/56634-engine-vibration#128910

To begin, some background info on my vehicle:

2005 Holden/Pontiac GTO with LS2 6.0L V8, manual T56 6-speed (RWD), currently 61k miles.

GTOs were made in Australia using General Motors sourced LS2 V8 engines from the US. It was sold from 2001-2006 as the Holden Monaro in Australia, Vauxhall Monaro in UK, Chevy Lumina in Middle East and S Africa, and the Pontiac GTO in the US from 2004-2006.

If you’ve ever heard, ridden in, or driven a 2005-2007 C6 Corvette, the GTO uses the same engine, which is basically a refined Chevy SBC V-8. Although not as fast-revving and “refined” as some of the higher-end European engines, The LS-series uses a cross-plane crack-shaft, which results in smooth, balanced operation. The LS2 is an internally balanced engine, meaning the crank-shaft, flywheel, and crank pulley are balanced independently, not as an assembly.

As the subject says, the issue I have is a vibration – almost a shudder – when revving the engine in neutral, with or without the clutch depressed. It also can be felt driving in gear under heavy throttle. In neutral, it is noticeable at ~1200 and ~1700 RPM. In gear, it is noticeable at between 2000-3000 RPM under load. I’m not certain if the vibration when driving is the same one as when in neutral, but they both seem to be in sync with engine RPM.

When I bought the car at 45k miles, I didn’t notice any vibration when in gear, and I didn’t often rev the engine in neutral, but am pretty sure there was less vibration then than now. Over time I noticed it more and more in gear, and of course in neutral once I checked. I think it started around 55k miles, and now have 61k miles.

There is a bit more to the story, but I’d like to get some opinions on how to best diagnose an issue like this, based on what has been mentioned so far. I fear if all the details are laid out, it could overcomplicate the situation. Earlier I recorded a video showing the problem. Please watch and share your thoughts:

Thanks in advance to your help!

Aaron

Topic

[David]

Maybe engine mounts ? or even a spark plug fouling ?

[Aaron]

Thanks. Engine mounts are a common suggestion I’m hearing. I had the car up on a lift and had the engine and trans mounts inspected by a mechanic. He found no evidence of deterioration. Remember, this car has 61k miles.

The sparkplugs crossed my mind as well, although I’ve felt bad plugs and wires before on different cars. This doesn’t feel the same. Still, I bought a set of fresh NGK TR55GP plugs just in case. I will install and update the result. I’m not very optimistic, as the previous owner said he changed the plugs at 35k miles.

[Andrew Button]

Ignition wouldn’t come and go like that, or at least not for anything normal. I feel its harmonics which of course are RPM dependant ? Did you check the crank balancer ? That is where I would start.

[Cameron]

1. Need to have a pro check the harmonic balancer.

2. If faulty and replacing the HB does not fix the issue I would definitely suspect the transmission mounts. Check the rear mounts again in particular.

[Aaron]

Thanks. I did recently have the car driven and inspected on a lift. The mechanic said the trans and motor mounts are in good shape. He also inspected the crank pulley and checked for crank walk. The crank pulley is perfectly aligned and has minimal play on axis, and shows no signs of separation.

I know C6 Corvettes have a problem with the HB failing at low miles, but my understanding is the GTO uses a different design. I’ve watched videos of the failures on C6s. The crank pulley is visibly misaligned in these cases, often accompanied by belt chirp.

Next step is to remove the serpentine belts and run the engine, see if anything changes. This will eliminate accessories, tensioners, pulleys and belts as sources of vibration. Then I guess take another look at the trans mount and replace the plugs…

[Aaron]

UPDATE: there was one small detail I didn’t mention, and this turned out to be causing most of the issue.

Over the winter, I replaced the driveshaft with a lower-mile, used OEM unit. I did this because the 2-piece shafts on these cars have a thin, rubber carrier insulator prone to failure, and mine was at that point. Before I installed the replacement, I thought it would be good to reinforce the insulator by filling both sides of the insulator with RTV silicone to ‘beef up’ the carrier support and increase the life of the insulator.

GTO driveshaft – carrier is in the middle:

The flaw in my thinking was not realizing the carrier insulator was made soft for a reason (to insulate). The carrier attaches via a bracket near the center of the car, and by stiffening that insulator, it was channeling NVH up through the floor. Even when sitting idle, when not in gear, the driveshaft is still attached to the transmission, which is attached to the engine. The idle vibration was probably there all along, just not noticeable until amplified by the new mod.

As a test, I installed a couple vibration-absorbing rubber washers between the carrier mount bracket and underside of car. This helped some, but the symptoms returned the next morning, then went away driving the car later the same afternoon. What I think happened: warm temperature softened the rubber washers, reducing the NVH until next morning when they cooled down. This repeated for several days until the mornings started getting warmer, then the issue almost went away.

I replaced the OEM 2-piece with a 1-piece aluminum performance shaft, which I avoided initially due to cost. Now the vibration is all but gone gone, and I notice a slight (wheel speed) vibration at highway speed also went away. I still feel the engine vibration a little bit, but at least no one riding in the car notices it now.

Thanks for reading and for posting suggestions.

[Aaron]

Bringing this back. The vibration issue seemed to get better for a while, but never completely went away, and has slowly gotten worse over the past 18k miles. Now I think it was two or more vibrations, and the driveshaft was a only part of it. The idle vibration (in neutral, not rolling) is still there. Also, there is a heavier vibration under load; it seems more prominent in the lower gears. I do not know for sure if this is related to the idle vibration, but doesn’t feel like it.

Last year I took the car to a local mechanic. He listened to the idle vibration and said it was not normal, possibly a miss. He suggested starting with smaller things and working toward internal, mechanical parts. Made sense to me. I bought a single coil pack and swapped it to all cylinders with no change. Checked spark plugs and wires – they all looked in good shape.

In the fall (’16), the vibration under load became more pronounced (passengers commented), being felt strongly through the shifter and in the floor/seat. I noticed the OE transmission mount had collapsed and was making metal-metal contact with the crossmember. I replaced this and the vibration is less but still there, especially lower gears at WOT.

The car now has 79k miles, runs and rides great except for the vibration(s). Normal daily driving is good except for when I really get on it from low speeds. For what it’s worth I sent an oil sample to Blackstone and got a perfect report on the engine’s health. Checked trans and rear end fluid levels are good.

My next thought was changing the injectors, running without the serpentine belt to rule out accessories, and inspecting the crank pulley again more closely. It wasn’t wobbling or giving any indication there’s any problem – LS2 is internally balanced, so is there anything really to fail? I was thinking of buying an SLP pulley and swapping just to rule it out.

In the back of my mind I’m thinking clutch flywheel, but the clutch feel smooth and no slipping that I can tell.

Does anyone have ideas or tricks for ruling out certain areas or faults? I’ve read about using EVA2 vibration analyzer, but don’t know anyone who has one in central OH. Thanks in advance for any help.

[Author]

Replies