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Vibration problem with 1984 Trans Am. Need advice

 
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teckelhut



Joined: 06 Nov 2015
Posts: 4
Location: Central Oregon

1984 Firebird

PostPosted: Fri Nov 06, 2015 7:59 pm    Post subject: Vibration problem with 1984 Trans Am. Need advice Reply with quote

I have a very strange problem that I have ran past many many mechanics familiar with 3rd Gens. None can explain it, nor can my mechanic get to the bottom of it...yet. Hoping that someone can help me here before my mechanic proceeds forward with his plans for it next week.

The car gets a strange vibration with a harmonic sound at exactly 2500 RPM (is not present above or below that). We have replaced the following thinking they might be the culprits but nothing has helped...

New motor and tranny mounts
Rear end completely rebuilt
New torque converter
New aluminum drive shaft
Harmonic balancer
Flex Plate

Then the mechanic decided to try an experiment. He disconnected everything from the torque converter back and ran the engine. It still gives off the same strange harmonic vibration and sound at exactly 2500 RPM. He says that because of this, it can only be an engine problem. He wants to pull the engine next and have it sent off to be taken apart and balanced.

I do not have an problem with having that done as the engine is custom 350 with an LT 4 camshaft made by another mechanic and it may very well have issues. I have put close to $8000 into this car and will continue to put money into it until its right. The car is a looker for sure. Interior and exterior are showroom. This car is my baby.

Just some added info.

We have done allot of other work to the car as it sat for 6 years in a barn before I purchased it. We have also replaced the radiator, the distributor, the intake manifold and the carburetor. Put brand new Cooper Cobras on. Lots and lots of other things. 99% of everything we put into this car has came from Summit Racing.

Also need to add. The vibration used to be present starting at 2500 and going away at 3000 until we replaced the drive shaft. Now it does it at exactly 2500 only.

Anyone have suggestions about this strange problem?

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Larry
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Joined: 01 Jun 2006
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PostPosted: Sun Dec 06, 2015 9:33 pm    Post subject: Vibration Problem Reply with quote

I apologize for taking so long to respond... guess I missed this question.

It is confusing when you mention the vibration with driveline hooked up, but still there with it all removed. I don't see how the driveshaft change would have changed it if it is still there when all is disconnected.

Questions: have you disconnected all items at the front of the engine? Is there any sort of noise at all? I'm assuming this is the engine that was in the car before this rebuilding- you have a history of no vibration before?

A camshaft "made" by someone? Reputable?

Obviously the "loosness" isn't great or it would show up below 2500 rpm. And when you go beyond, it spins itself true (doesn't fix anything though).

Have you thought of something like a bad rear main bearing? Perhaps a little undersize but not enough to knock? Could be others too, but the rear would make the most problems.

Did you check the flex plate carefully? I've seen them with cracks.

Let us know where this all leads.

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teckelhut



Joined: 06 Nov 2015
Posts: 4
Location: Central Oregon

1984 Firebird

PostPosted: Sun Dec 06, 2015 10:33 pm    Post subject: Re: Vibration Problem Reply with quote

Larry wrote:
I apologize for taking so long to respond... guess I missed this question.

It is confusing when you mention the vibration with driveline hooked up, but still there with it all removed. I don't see how the driveshaft change would have changed it if it is still there when all is disconnected.

Questions: have you disconnected all items at the front of the engine? Is there any sort of noise at all? I'm assuming this is the engine that was in the car before this rebuilding- you have a history of no vibration before?

A camshaft "made" by someone? Reputable?

Obviously the "loosness" isn't great or it would show up below 2500 rpm. And when you go beyond, it spins itself true (doesn't fix anything though).

Have you thought of something like a bad rear main bearing? Perhaps a little undersize but not enough to knock? Could be others too, but the rear would make the most problems.

Did you check the flex plate carefully? I've seen them with cracks.

Let us know where this all leads.



Thank you for your advice, but we have already determined the problem and are almost finished with it.

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Larry
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PostPosted: Mon Dec 07, 2015 10:20 am    Post subject: Vibration Problem Reply with quote

That's good news! Can you share the fix with us so others can be informed if faced with the same issue? It is sure a curious one!
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teckelhut



Joined: 06 Nov 2015
Posts: 4
Location: Central Oregon

1984 Firebird

PostPosted: Mon Dec 07, 2015 11:16 am    Post subject: Re: Vibration Problem Reply with quote

Larry wrote:
That's good news! Can you share the fix with us so others can be informed if faced with the same issue? It is sure a curious one!
''

My mechanic called the original builder of the engine (who had won awards for being so good at what he did. That is why I bought the car knowing that the original owner had used him to build the engine) and asked him if it had ever been balanced when it was build. We were shocked when he said no he had not. So we pulled the engine and sent it to the top machinist in this area, who I personally know and trust.

When Laren got into the engine he was appalled. Not only had the builder NOT balanced it but he put it together with used parts. The parts had scored the cylinders so bad that the block had to be rebored and the engine completely rebuilt. I saw everything. It was a mess. NOTHING the original builder of the engine had told us was true. It was all a pack of lies. The only reason we believed him in the beginning was because of his stellar reputation. I am pissed off that he lied to me, but even more pissed off that he lied to my mechanic.

I should have my boy back up and running in a couple weeks. I really miss him.

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teckelhut



Joined: 06 Nov 2015
Posts: 4
Location: Central Oregon

1984 Firebird

PostPosted: Mon Dec 07, 2015 11:20 am    Post subject: Re: Vibration Problem Reply with quote

Larry wrote:


Have you thought of something like a bad rear main bearing? Perhaps a little undersize but not enough to knock? Could be others too, but the rear would make the most problems.


I had the rear end completely rebuilt.

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