I just notice that my 91 N/A viibrates like crazy when i rev it to like 6k or 7k rpm...is this normal??? is this the "hesitation" everyones talking about???
(please excuse my horrible knowlegde, im new wit cars )

 

Could be your engine mounts crapping out.

 

no,,, the hesitation is like a hiccup or a burp as you rev the car...

get it there and yank on yer motor and see if it slops around from side to side, you shouldn't realy be able to move it to much but it will move a bit... if it moves a lot one way and not the other.. you probably got a broken motor mount...

Check the spark plug wires for cracks and oil around the boots (this will cause arcing and missing at high rpms and will cause the motor to take a fit)

if you haven't changed your plugs yet... you probably should... when you get the new ones, compare 'em to the old ones so you know what a good electrode is supposed to look like for future reference!!!!

The shaking is caused by vibration in the engine, you motor mounts may be bad, but you should also try to figure out whats causing the engine to miss and therefore shake!

 

I had terrible vibrations above 35mph. Woudl rattle the **** out of all the plastic pieces in the interior. I later changed the tranny out and realised I had 3/4 driveshaft bolts. Replaced all 4 and I had a perfectly smooth ride (for as rx-7s go anyhow). I was amazed at how nothing rattled!

 

is it normal for it to vibrate a little then???

 

it could be bad u-joints on the driveshaft, or would that make the car rattle all the time?

 

A driveshaft problem will be directly related to speed, completely unrelated to engine RPM.

It's not normal for the engine to vibrate at high RPM, but it's reasonably common. Misfiring is the usual cause (ignoring silly things like having the wrong counterweights, etc).

If you don't know when the plugs were last changed, they're overdue by a LONG ways. I'm amazed how much better my car runs when I clean/change the plugs. Plug wires are probably due for a change as well.

If you have the time and skill to do so, it might not be a bad idea to pull the injectors off and send them in for cleaning. I'm sending mine in here at some point... when I get around to it. I know they need cleaning, I just have some highway driving to do here and my beater won't do it.

Don't expect your engine to be smooth as the turbine-smooth Renesis engine, but it shouldn't be vibrating noticeably at high RPM. It should be quite smooth.

-=Russ=-

 

it dusnt rattle, it just....how do i put this..... vibrates??? i dont know... is the car supose to be realli smooth at high rpms???

 

I think you better check your Transmission Mount. Mine was all worn out (the rubber part). Once replaced, the vibrations were gone.

 

Some of the most obvious sources of vibration:
Unbalanced tires
Worn Driveshaft
Worn/Broken transmission mounts
Worm/Broken motor mounts

Check the engine mounts first by trying to rock the engine side to side. If you can get it to move, odds are you have at least one broken one. The passenger side usually goes first.

If you have time, take the car to a tire shop and tell them to rotate and balance the tires. You'd be surprised how often this is overlooked as a source of vibration at higher speeds.

If your driving and you can see the shifter bounce up and down or side to side, odds are that the transmission mount is shot. This is also very common. See Mazdatrix for a replacement.

The driveshaft is a small pain to check, but the u-joints are a known weak point. Jack the car, unbolt the 4 bolts from the rear diff, and pull the shaft. Check both u-joints and make sure they both rotate smoothly. Any roughness in there motion will cause vibrations.