My vette is an 87' with 65k miles. Every time I take it up to about 40mph and the car shifts and the rpms are low, there is this rumbling sound / feeling as I accelerate; almost like I am going over rumble strips. I tried putting this "fish oil" additive into the rear, but it did not help. Any advice would be great. I really dont want to take out and rebuild my rear

 

Repeat your test but keep the tranny in 3rd, don't let it go into OD. If you have a miss (like 1 cylinder not firing under light load), once your car is in OD and the torque converter locks up, it will produce a sound/feeling like you describe. Being in a lower gear and no lockup eliminates that, if rumbling is gone, look to the motor. If still there, then it's rear end or u-joints. Next thing you have to do is determine if it is there all the time, only light throttle, coasting etc.

From your description, it may not be the rear as either bearings or gear issues at 40mph will be higher in pitch, closer to the hum an AC motor can make not 'rumble strips'

 

sorry, I should have been more specific. The car is an a4 by the way. It is normally around 40mph or so (or whenever its in 3rd/4th, low rpms) and while throttle is applied. It does not happen when I am coasting whatsoever. If it was the ujoints, wouldnt the sound be there all the time?

 

a4???

 

automatic 4 speed.

 

I assumed it was when you said "the car shifts".. Unless it's posesed it had to be auto

Re u-joints, no unfortunatly just like the rear end, vibration will often only be under load (either engine braking or engine power), wheel bearings under all conditions but will worsen or improve on cornering one way or the other. Rear ends can be just under power or just under engine braking (this would be gears normally) or both (which normally would be bearings)

The difference is the frequency these parts will produce noise at..
A tire at 50mph might at best turn 100rpm, the drive shafts to it, the same. Very low deep base vibration (think unbalanced tire frequency). Your drive shaft, about 3 times faster than that, closer to a deep buzz feeling. Gear teeth mesh issues, around 100 times that, closer to a range between a small dog growl and a whine, bearings in the wheel or diff, little higher pitch again from that.

Your rumble strip description is a good reference, but our rumble strips up here produce a very low rumble, yours may not!

 

Hmm, that gets me thinking, because it really is a very low deep base vibration as you said. When I had my new rims put on, my friends shop told me that the tires were balanced. Perhaps they weren't...or at least not correctly. Thats differently something to look into before I start looking at the u joints / drive shaft / rear, no?

 

Is it safe to assume the correct lug nuts were used on these "new rims", loose lug nuts or the wrong style for a rim will make a very low rumble as the rim shifts from one bolt to the next (slides slight)..

If you pull a lug nut and see an almost mirror shine under it, that might be your issue..

 

Also move tires front to rear to make sure it isn't a defective tire/belt issue. See if rumble moves to front with tires.

 

Awesome, thanks for all of the ideas. I'll start with balancing the tires tomorrow and get back to you. Thanks again.

 

Alright, I have not gotten the chance to balance the tires just yet. However, I do not think that is the problem anymore.

The car does NOT make this rumble / vibration if it does not go into overdrive. I put the car in the 2nd D (im assuming this is drive without the overdrive) and the car does not make this rumble / vibration. It only occurs when the car has shifted into overdrive and the rpms are low.

Now, where do I start?

 

Make sure you're running on all 8... Once in OD and the converter is locked, any chugging or vibration the engine makes will be telegraphed through the whole drive line. Without the TC locked up, it acts as a buffer or absorber of those vibrations. I don't believe an 87 will throw a code for 1 dead cylinder either. A dead cylinder may only produce a very slight roughness to the idle too. You can often pull a plug wire on a V8 and barely see the difference.

Check your plugs, see if one is very much different than the others. Wet, maybe black, it probably isn't getting fire, whiter than the rest, probably isn't getting fuel.

It's also possible the OD in the tranny has issues such as a slipping clutch pack, but you'd see that as a flare (engine revs when throttle is applied)

 

Try driving in OD , and once the vibration starts , with your left foot very lightly "ride the brakes" (you only need enough foot pressure to turn on the brake lights) but not letting off of the gas peddel, this will disengage the torque converter clutch.

 

good call!

 

I will try this either tonight or tomorrow and let you guys know

 

Alright, i tried riding the brakes as you said and i think it worked. what does this mean? Also, just thought i would mention it again, the rumble / vibration does not occur in D2 and goes away when I "punch it" in overdrive.

 

I was about to jump in and say that 40 mph is too slow for tire balance issues. But it looks like you've already narrowed it down to the converter.

 

So you need to picture the drive line. In every single gear without the TC locked up, you have this big soft damper in there taking any vibrations from the motor and eliminating them from the rest of the driveline. As soon as the TC locks up, your just like a standard, you have a 100% zero slip connection from motor to road, if the motor bucks, you feel it and hear the drive line rattle as it is pulsed back and forth. Any chugging or kicking from the motor is transmitted, without the TC locked, almost all motor misses and chugs are absorbed.

Now picture 7 smooth fires per rev and a pause where the 8th should be, constant chug timed each revolution, this hammering effect will be directly connected to the road and the car will broadcast it, as soon as the TC unlocks (what you did touching the brakes) you'll hardly know there is an issue.

Not to say there isn't some other problem, but I have had this before on Camaro and a little Cavelier. It's also the same reason our Chrysler 300C doesn't lock the TC fully at cruise, it is one of those V8 come 4 bangers when cruising and Chrysler found that with a full lock on the TC the vibration of having only 1/2 the pistons firing was too much.

Long winded but hopefully you get the idea, you need to make sure it isn't a simple miss fire before you start into a bunch of drive line stuff.

 

Got it. My school's auto shop has a scan tool. Can scan tools typically tell if each cylinder is firing? I will definately be scanning my car if thats the case and making sure I am firing on all 8. I'd actually be jubilant if I had a misfire compared to a bad diff or transmission I will scan it and get back to you. Thanks again man, cant tell you how much I appreciate the help.

 

It won't through the cars ECM no, but if you mean one of those big old diagnostic machines by Sun, yes it probably will, however your miss may only be at speed too... (could always put the scanner in a trailer

The old Sun machines will put each cylinder up on a scope and if the guy reading it is good, they will see something on the trace for one of the cylinders unlike the others..

Good luck!

by the way, other things will increase with this too, but if the vibration increases the more throttle you use (up until the TC unlocks) and is non existant while coasting, this lends to a miss too