New strange behaivior.

[Gatt9]

Hi guys!

If anyone recalls I had an issue that turned out to be tranny related a few weeks ago that people here helped resolve. I've got a new one that I've just noticed recently.

At low RPM's and speeds greater than 40mph I have an odd vibration. At low RPM's and around 40mph it's simply that, an unusual vibration throughout the car. But at higher speeds, like 60-70mph, the vibration changes. It becomes a rhythemic soft thump, very regular, almost like the feeling created by a badly missing sparkplug. The engine runs fine, new plugs and wires and no sound of missing from the engine. It's just this soft regular "Thump" feeling.

It only occurs at low RPM's, with a very slight amount of gas. Like if I'm on the highway in Overdrive, I'll feel it. But if I keep it in 3rd, where the RPM's are higher, I don't.

I'm wondering if it's driveshaft? I do have the stock driveshaft on the car, and it's pretty rusty.

1998 3.8L Firebird
Whisper Lid
NGK irridium plugs
Taylor Spiro Pro 8mm wires
Stock suspension, driveshaft, 3.02(non-performance package gears, they're 3.02 right?)

Any help is appriciated!

[CDNFB]

Re: New strange behaivior.

No one has any ideas?

I just guessing here but could be the fwd universal joint (the one closest to the tranny). Maybe tranny mount. or a combination of both?

Anyone else?

[CamaroJeff™]

Re: New strange behaivior.

if the vibration only occurs at low rpms its highly unlikely that its caused by anything behind the motor.

only thing i can think of that would cause that (off the top of my head anyways) would be the harmonic balancer. ive had that same issue before and it turned out the rubber in the balancer was deteriorating badly. check for cracks or chunks of the balancer missing, or look at it while the engine is running while varying rpms.

if it were driveshaft, tire, or wheel related the rpms wouldnt matter. it would shake by vehicle speed and not engine speed.

[Gatt9]

Re: New strange behaivior.

Thanks CamaroJeff,

At the risk of sounding stupid, where's the Harmonic Balancer located? First time I've heard of this part. I wouldn't be surprised if it's crapping out, especially if it's partially rubber and exposed, as the car is 10 years old.

[cam98aro]

Re: New strange behaivior.

your gears are 3.23

[cam98aro]

Re: New strange behaivior.

Thanks CamaroJeff,

At the risk of sounding stupid, where's the Harmonic Balancer located? First time I've heard of this part. I wouldn't be surprised if it's crapping out, especially if it's partially rubber and exposed, as the car is 10 years old.

its that huge circle thing that nothing is connected to in the front of your engine

[Gatt9]

Re: New strange behaivior.

Ah, Thank you twice Cam98aro!

I'll take a look at the Harmonic Balancer in the morning using the pic in the other thread as a reference point.

3.23 gears? So, would I see a big difference in converting to 3.41 then? I'm actually doing an Alluminum driveshaft, headers, hi-flow cat, cat-back, and tune in the near future and was trying to figure out what part I was going to look at next. I think short of the torque converter and gears/LSD I may be out of things to do without tearing into the engine?

[cam98aro]

Re: New strange behaivior.

Ah, Thank you twice Cam98aro!

I'll take a look at the Harmonic Balancer in the morning using the pic in the other thread as a reference point.

3.23 gears? So, would I see a big difference in converting to 3.41 then? I'm actually doing an Alluminum driveshaft, headers, hi-flow cat, cat-back, and tune in the near future and was trying to figure out what part I was going to look at next. I think short of the torque converter and gears/LSD I may be out of things to do without tearing into the engine?

just about, you could also upgrade the intake also, get a lid, and the little CAI plate

you would probably notice a difference with 3.41 gears, but it wouldnt be too drastic

[bigbrian442]

Re: New strange behaivior.

Your gears could also be 3.08.

[RadRacer]

Re: New strange behaivior.

Your gears could also be 3.08.

they should be if he's an auto, which he is. 3.42's will be a great improvement for you :)

[greasemonkey]

Re: New strange behaivior.

Auto's are 3.08 and 3.42.
Manuals are 3.23.
Mine is auto and has 3.42s and no posi stock.
Check your RPO code. GU4 is 3.08, GU5 is 3.23, GU6 is 3.42.
G80 is limited slip. (sorry had to clear that up)