A couple weeks ago I started to notice my gas mileage went down. It used to be anywhere from 21-23 MPG, but it suddenly dropped to 17-18 mpg. Around this time I also noticed a vibration with the car. It isn't really noticable until about 50 MPH, and it gets worse as the car speeds up. It isn't terrible by any means, but I can tell it is there. I'm wondering where to look:

I'm thinking it's gotta be one of these areas:
1. Busted tranny mount
2. Unbalanced driveshaft - However, I've had the same driveshaft for a while now, what would cause it to come unbalanced all of the sudden?
3. Unbalanced tires - Same situation with the driveshaft

What else could cause it to start vibrating like it is?

Any help would be appreciated.

Eck

P.S. I've searched, it's looking like a tranny mount is possible, I just haven't had a chance to get under the car.

 

A broken tranny mount clunks when you accelerate aggressively; try a spirited launch and listen/feel for a clunk. U-joints could be failing, again you usually here a clunk in the drivetrain when you shift from park to drive or reverse. A tire could have thrown a wheel weight and caused the vibration. It could also be worn suspension components. Jack it up and take a good look around.

 

If all the things above check out ok and you've eliminated ALL the easy stuff, you may have a bad torque converter. I assume you still have a stock TC. When you're out on the highway next time, see if the vibration goes away when the converter unlocks by doing the brake tap test:

Keep a steady speed of 50 mph in OD and with the other foot lightly tap the brakes to unlock it while maintaing your speed. RPMs should go up a few hundred and the vibes should go away, or at least decrease some. If it does the TC is bad.

What happens is the clutch lining is glazed in certain spots and causes an imbalance when locked. Or the converter may not be fully locking even if it appears to be. It's slipping and can explain why your gas mileage went down the toilet.

If it all sounds like BS, ask me how I know...lol. Mine got to the point where doing the brake tap test didn't do anything anymore and it was just getting worse. Then I replaced the TC and no more vibes.

On the driveshaft, check to see if it didn't lose a balance weight.

 

It clunks every now and then, but not all the time. It didn't used to do this before... Today I was doing about 65 on the highway, then I put the throttle about 2/3, making it downshift into 3rd gear. Then I pushed it all the way, making it downshift into 2nd gear and it clunked like a MOTHER. Tiz gotta be a tranny mount... I'll check it out today.

 

loss of mpg + vibration = missfire...plugs and wires

 

Uhh... no, it's not THAT kind of vibration. This one is relative to speed. I already went through plugs and wires. Thanks though.

 

Swapped it out this morning. The old one was split in half. First thing I noticed was putting it in reverse to back it off of the ramps - it didn't clunk like a mother. Good deal.