One morning in October I started up my car and noticed that the exhaust was very loud. Nearly 5 times louder than normal and definitely coming from the passenger side. My first thought, after having owned two 1986 LG4 Trans Ams in the past, was that the exhaust manifold gasket was shot. A couple weeks later I went to Island Dragway for my local f-body meet and it had gotten progressively louder and more pronounced at lower rpms. By the day of the meet, the car was vibrating enough that I could see the mirrors shake. The car passed tech and I would have run it except that some of the guys looked at it and advised against it. They thought it had a terminal knock.

Later that day, I took my car to the local shop and they told me my crank "likely" had a hairline crack in the back of it and was causing my motor to knock. I put the car up for sale and it has been sitting in my driveway not running since. Yesterday, I got a second opinion from a very experienced mechanic who said that this may not be the case. I jumped the car and it started right up. Same issue was present with a noticeable vibration but not as bad as it was at the track. The harmonic balancer visually wobbles and shakes the entire belt set-up. The belt tensioner is constantly shaking due to this too. We checked the bolts and they are all in place so he is thinking it may be one or both of the rubber seals on the balancer. He said he's heard plenty of rod knock and motors with spun bearings but my L98 runs much too smoothly and has a hollow tone to it.

Now for my questions:
Could a bad crankshaft cause this kind of wobble in the balancer? Oil pressure seems ok, although my gauge does not work very often. The car is not burning oil and there is no weird sound coming out of the exhaust. I did not drive it yesterday, but the car was gradually getting louder and losing more power each time I drove it back in October.

Has anyone else had a similar issue?



My plan is to remove my air intake hoses and MAF and then to drain and remove my radiator to get access to the balancer. He said he would stop by with the puller tool and we can take it off to check the balancer and rubber seals. I would be so happy if that's all it is. I will be calling the shop today to see exactly what they did to diagnose a broken crank since it did not appear as if they took anything off the motor. Any input is greatly appreciated and let me know if you need any more details. I will post up what the shop says later today. Thanks in advance.

~Matt

Check to see if your ignition timing is close or not, usually the balancer slips when they go bad.
Make sure you use an installation tool to get the balancer back on, you don't want strip the crank snout threads.

Just called the shop. They said that they noticed the balancer had a slight wobble and put the car up on the lift. The balancer had a lot of play and it seemed like the crank did too. They said that their best guess was a hairline crack at the back of the crank or the front bearing was bad behind the timing cover or possibly the journal there.

I would be checking out the balancer fully before blaming the crank,very common for rubber in balancer to degrade
with age allowing outer ring to slip or even come off in extreme cases

Tomorrow's weather: sunny with a high of 50.

I'll be draining and pulling the radiator tomorrow after work. Not sure when I will get the balancer off yet, but I'll post updates and pics. Possibly even a video of its current running condition.

I don't think you'll need to pull the radiator, maybe just the fan.
You'll need a universal puller to get the dampner off.

And one of these to put it back on, might be able to rent them.

My friend has both tools and said he'd stop over after I had the rad out to pull, inspect and re-install the balancer if necessary. Not sure if that will be tomorrow though.

Before you pull the damper, check your front to rear play on your crank.
You can do this by simply tapping the front of the damper with a soft/rubber mallet to settle the flange against the thrust bearing in the rear, then prying the damper forward with a large screwdriver or other prying tool and measuring the movement.

Ok I'll give that a shot. I'm assuming that the more play, the worse the rubber seal is between the balancer and timing cover?

The pulley is bolted to the area of the balancer that is not affected by a bad rubber ring. Can see that in the picture posted by vettoz. Two of the three bolt holes can be seen in the pic. They are the shiny ones with the threads in them, those retain the pulley.

So if the pulley is wobbling, and there is a sound like a rod knock, odds are there is something a bit more drastic wrong with the engine then a bad balancer.

I would remove the belt and start the engine. Just don't run it too long as there isn't any coolant being circulated.

Then observe the pulley. If it is wobbling, odds are the crank is bent or there is a bad main bearing.

RBob.

The rubber seal has nothing to do with any of this.

The damper has a rubber ring between the damper hub and the outer inertia ring. That was referenced above. When they deteriorate the inertia ring can slip, but that has nothing to do with front-to-rear crankshaft play.
Excess play in the crank is normally caused by a worn out thrust bearing (on rear main).
It is fixable, but often requires a new crankshaft along with new main bearings.

Ok thanks guys. How common is this in a L98 car? Or would it be just as common in all gen 1 SBCs?

I would not worry about how common this is - rubber deteriorates with time and twisting action of the engine tend to weaken and destroy rubber compounds that have been exposed to oil, moisture and heating cycles. IMHO, whenever I rebuild an engine harmonic balancer and flex plate (among other things) are always replaced.

//RF

On Saturday I jumped the car and moved it onto some concrete for the tear down. Removed intake/MAF, removed fans, drained radiator, removed radiator, took off serp belt and unbolted the balancer pulley. Started the motor with no accessories and it still knocked. Sounded a little worse too and definitely from the center of the motor. It didn't sound good, but it wasn't a metal on metal noise and the motor still idles fine. Still no noise from the exhaust either. My oil pressure gauge is shot and the balancer itself looked fine. There did seem to be some oil coming out the back of the balancer where it goes into the timing cover. I'm guessing it's a spun bearing. Thanks for your help and very special thanks to //<86TA>\\ for all the help wrenching.

You might have a cracked flex plate.

Well - feel for your pain, man.

Remove the 4 screws that cover torque converter and see if flex plate wobbles or move it by hand (engine stopped). Cracked flexplates are rather common on Gen 1 SBC pre 87 engines and a little bit less common on single piece RMS SBC's. Indication of a cracked flex plate is pronounced and loud noise during start. Spun rod bearing noise follows engine RPM and get louder with RPM. If the later - complete tear down is required. Flexplate can be replaced without removing either engine or trany, but it is a PITA!
//RF

I'll check, but there are no odd noises at the start, just when it's running. I took a video but I'm still trying to get it from my phone to my computer.