HammerDown

Ok guys, tried over in C4 Tech, now I'm going to see if any of you engine gurus can shed some light on this.

First, some history. The car is a '94 LT1 with approx. 46k miles. Bought it in 2001 with only 18k miles on the clock. Car ran like a champ (although a little rich, due to bad plugs and the owner taking the vacuum line off the FPR). There was NO vibration at any rpm, all the way up to the rev limiter.

Fast forward to last year. Took the car to the dragstrip a few times. Didn't break anything, but thought the times were a little slow so tried a plug and wire change, due to the age rather than mileage. Shortly after that, I began to notice a vibration over 3500 rpm (present when the car is in neutral, clutch in or out). The vibration is high frequency, and can be felt through the whole body of the car. It vibrates up through the bottom of the seats, and buzzes the shift lever. Kinda like one of those massaging chairs at the mall.

Tried another plug change, no difference. At this time I was getting a pretty loud clacking sound coming from the flywheel clutch area, and thought my dual mass flywheel might be bad. So, took it to a couple of shops and they thought the same thing, although said they wouldn't know until they took the tranny off.

So I tried a few other things first. Went back to OEM Delco platinum plugs and wires (on the suggestion of two shops). No change. Removed the balancer (left the hub on the crank) and inspected for a thrown weight, or deteriorated rubber ring, or slipping outer mass. Looked brand new, and with the outer mass marked, never slipped in the several months I drove it. So I said looks like the flywheel, and had the flywheel clutch and pressure plate replaced. I went ahead and did a Fidanza single mass flywheel conversion at this point. We (the shop and I) made sure EVERYTHING was match balanced to the original equipment. It was found that the loud clacking noise I heard before was caused by the clutch fork rattling around on the ball stud. The dual mass flywheel looked brand new, as did the clutch and pressure plate.

While the car was in the shop, the mechanics hooked it up to an ignition scope and a high dollar Snap-On scan tool. The car was nice and smooth over the whole rpm range. The knock sensors were not picking up a thing (they tested them just to make sure they were working by lightly tapping the block with a hammer, which set them off immediately).

This past weekend my brother and I went back and verified the balancer hub position on the crank by finding TDC on piston 1 and checking the hub arrow. It looked pretty dang close to 12 o'clock to us. So now we are thinking about putting pieces of modeling clay around the inner surface of the balancer's outer ring to see if that affected the vibration. Either that or putting small temporary weights in the outer holes. I'll be trying this in the next few days.

At this point I don't know what else to do, short of tearing the engine apart (or selling it and buying an LT4). The mechanic at the shop said he thought if it was an internal problem, something bad enough to vibrate the whole car, the knock sensors should be picking it up. The engine mounts were also inspected. All I am sure of at this point is that the vibration is in the engine starting at the balancer and ending at the flywheel/pressure plate. The reason I believe this is because the car still vibrates in neutral, sitting still, with the clutch disengaged.

Oh yeah, eliminated the accessory pulleys by running the car without the serpentine belt.

I'm to the point where I'm thinking about just getting rid of the car. So any suggestions would be greatly appreciated.

Sorry for the long post.

Lohkay

Humm.. I didn't see you mention a leak test. If one of ur cylinder is leaking (bad valve ajustment maybe?) and not firing, it would cause that vibration. If thats fine, check if u get a spark at each plug.

HammerDown

The ignition scope would have ruled out a spark miss.

We didn't do a leak down test. I'd thought about maybe an injector fluttering at the higher rpms, however the scanner showed all injectors being nearly identical as far as duty cycle goes. Although that doesn't actually test the spray pattern. If it was easy to swap out a single injector, I'd just go from cylinder to cylinder with a known good injector to see if that made any difference. Also, the mechanic at the shop said you could look at each cylinder's temperature to see whether there was a misfire happening. Haven't tried that yet.

I might try running some injector cleaner through the car to see if that has any effect.

MarkBychowski

For what it's worth, I have a very similar problem on my LT4. It started when I did a single-mass conversion. Of course the first thing everyone thought was that the flywheel was not matched properly to the original DM. So I yanked everything and took the flywheels, and clutch assemblies to the shop to make sure they were exactly matched. Sure enough, they were.
Myself, the shop, and everyone I've talked to now agrees that the engine was slightly out-of-balance from the factory. The consensus is that with the original heavy DM flywheel, the vibration was masked enough to not be noticable.
So basically, I'll be rebuilding the engine in the next couple of years, but I'm driving it as-is for now.

HammerDown

I expected to get a little more vibration from the single mass aluminum flywheel, but the vibe started with the dual mass, so something significant changed in the stock engine before I ever started replacing stuff.

I got my hands on a couple of the hammer-in weights for the balancer. I'm going to try tapping those in at different locations around the balancer and see if that doesn't affect the vibration.

comp

i'd do a leak test to take out those questions

KyRP2NITe Corvette

So what exactly did you do to eliminate the accessory pulleys??? Sounds unlikely but you may have done something that is causing this by changing the accessory drives on the fron t of the motor... just a thought

HammerDown

Ran it without the belt. Basically the only pulley spinning is the balancer.

HammerDown

How exactly is a leak test done? Just screw some sort of pressure guage into the sparkplug hole, turn the engine over until you're on the compression stroke for that cylinder, then watch to see whether or not it holds?

HammerDown

Just put a full tank of Chevron Super in it and a bottle of the STP super concentrated injector cleaner. We'll see how that goes...

Anybody used the Sea Foam stuff??

comp

and just tried Techron on a carboned up 305

HammerDown

Ok guys, here's an update. So far, haven't noticed any difference with the Techron and STP fuel injector cleaner, although I've only burned up about a quarter of a tank. Last night on my drive home I decided to try something. I ran the rpms up into the range over which the vibration occurs, mainly to see if there was any change in the frequency or power of the vibration at different rpms. The vibes begin at about the 3200 rpm point, and as I slowly brought the speed up to 3500, they increased in force. I proceeded onwards to 4000 rpm, at which point things seemed to smooth out. Not Lexus smooth, but nowhere near the levels at 3500 rpm. So I proceeded on to 4500 rpm. After 4200, the vibes got so intense I thought the shifter was going to self-destruct. Onward to 5000 and still pretty violent. So I pegged the tach at 4000-4200 rpms and drove the whole way home like that, about 10 minutes.

The entire time it was at 4 grand, the behavior never changed. It stayed relatively free of harsh vibration. This was under changing load conditions (into the wind, slight grade, steeper grades, etc.) as well.

Now, my question is this: if an injector is acting up, either clogged, occasionally failing, or just having a weird spray pattern, would this still result in the behavior I observed last night? I would think something like a clogged injector would cause the vibes to get worse as rpms increased (once the vibes began), and they wouldn't be as "predictable" as what I observed. This kinda makes me feel like it is a balancing problem after all. The only reservation I have about that diagnosis though is that something major would have to happen to bring on such a violent vibration. Like I said before, as far as we can tell, nothing has moved or come off (externally) that would affect the balance of the motor.

This is a real mystery...

Z06_BluByU

At this point I would take it to the dealer and let it become there headache...

HammerDown

I've already spent $2k (having a shop do the work) to get rid of this problem. If I can't fix it myself, I'm just going to get rid of the car. It's not worth it to me to keep dumping money into it.

At best, I may keep the car and drop another engine in it, but I think I'd rather just sell it and save up for a C6.

93Quasar

I don't think your test rules out a balancing issue. Vibrations have sinusoidal harmonics. At certain rpm's, the vibration effect will be zero, and at other rpms, at the peak of the sin curve, it maxes out. Back when I had flywheel balancing issues, it was easy to visualize this as the speed of the balancing machine was increased and the fixture the flywheel was in would rock back and forth. Balancing was done on the flywheel at one of the rpms where the harmonic frequency was at a peak for worst case situation. Low 3000 rpm onset of vibrations is very typical of balancing.

HammerDown

Exactly!! That's what I'm saying the test seems to indicate.

My question then is, does this sound like something characteristic of a fluttering or intermittent injector??

I'm thinking not, since the vibration has definite rpm points at which it is more severe, and these rpm points remained constant over the length of my drive.

93Quasar

Oh, I missed your statement of that in your previous post.

I'm a little confused on the sequence of events here. You noticed a vibration after you changed spark plugs, thought you had narrowed it down to a bad flywheel, replaced the flywheel with an aftermarket, had the engine running smooth in the shop, and the vibration is back? I'm not understanding why it was running smooth in the shop, but had a vibration before and after.

HammerDown

No, unfortunately the engine has not run smooth since the vibration first appeared. The car DID run smooth when I first bought it, and continued to be smooth for about a year after that. Then I took it to the strip a couple of times, not really thrashing it or anything, but noticed the vibration shortly after. Coincidentally, I changed plugs and wires right after those trips to the track (due to what I felt was poor performance of the car) and naturally I associated the beginning of the vibration with that major change. Unfortunately, two plug and wire changes later, plus the analysis with the ignition scope, and that doesn't appear to be the cause. Then came the focus on the balancer and flywheel, and due to the racket coming from the bellhousing (which later turned out to be the clutch fork), we concluded the problem was a bad dual mass. We figured either I messed it up at the track, or the previous owner drove it a quarter mile at a time (it was very low mileage) and it was just used up. That was wrong, too. While we were in there, I decided to do the aluminum flywheel conversion. The only difference that made was to make the vibration more harsh, due to the reduction of inertial mass which was previously helping to smooth out some of the vibes.

Lately we've been looking hard at the balancer on the front, and just recently, considering the injectors.

Bottom line, something significant changed in or on the engine to cause this. Now, since it appears the balancer is in it's original position on the crank (and the car didn't vibrate at first), along with the fact that we tried another known good balancer which resulted in no change whatsoever, the only thing left to look at, apart from valve springs or the motor internals, is the injectors.

Simmo

This is one of those problems that could drive a man mad. Sounds like a balance problem but cant imagein how it suddenly occured while balancer and flywhell are ok.

UKPaul

I had a similar problem years ago & it almost drove me insane (some would argue that it did!). It was a different engine, but it could be the same principle? Normally for things like this I suggest the blindingly obvious as it's so easy to miss them if it's your problem, but it looks like you've checked them, so I'll throw in a couple of really weird suggestions. It's the drag strip bit that got my attention What I did was to race something a lot faster than me & I gave my engine a hard time. A very hard time. After that race it vibrated from 3000rpm upwards, with a smoother "area" at around 5000rpm. At 7000rpm it was so uncomfortable that you couldn't physically run it at that speed. What had happened was that while thrashing it I'd overheated one of the main bearings. It was a roller bearing & the outer spool started revolving in the crankcase. Once everything cooled back down, the crankcase contracted & gripped the outer spool, so everything appeared OK. Because the spool had spun in the case it had removed a small amount of metal, so the loading on the bearing (from the crankcase) wasn't as great as it should have been. This allowed it to run very slightly slack. It wasn't until I'd spent vast sums of cash & time on replacing & checking everyting & getting nowhere, that I gave up & stripped the engine. I found that at TDC & BTDC with the pistons removed from the rods, there was a very, very slight amount of up/down movement on the crankshaft. A miniscule amount of play, what we call a gnats **** of play. And that tiny amount of play caused massive amounts of vibration. So it could be worth dropping the pan & checking the bearings?
Another problem I've seen in the past was with a bike a mate bought. It was very well cared for, low mileage & looked a good buy. All was well until one day when he gave it some beans & it started acting up. Long story short: it had never been ridden at high engine speeds. All its life had been spent cruising around at low rpms. Consequently, it ran sweet until my mate came along & wrung its neck. What happened was that at high rpms the alloy conrods stretch as they pass TDC (piston wants to keep going up but gets pulled down by the rod, stretching the rod very,very slightly). Normally this wouldn't be a problem, but as the engine had never run at high rpms the wear ridge at the top of the bores was at the point where the rod was unstretched (hope this makes sense). At high rpms the rods stretched just a little bit and let the top piston rings try to pass over the wear ridge. This smashed a ring. Don't know what sort of rods you've got (steel or alloy), or even if the rpms a SBC runs at would cause rod streching, but it's certainly worth checking compression (& leakdown) as suggested. I know this sounds a bit far fetched, but we laughed at the guy that told us about the possibility - until we found a top ring smashed & visual evidence that it had run past the wear ridge in the bore. Then he lauged at us