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Before I explained the problem, NO its not alignment or balancing problems.

Around 45 MPH my car shakes violently. I'm talking where it rattles everything in the car from the dash to the glovebox. However I can somewhat control it but either letting off the gas or giving it more during that speed to make it stop. If I'm driving and get around 45 MPH and let it shift into whatever gear is goes into at that time, it does it everytime. But anything under or over that speed it never shakes again. I just recently had new tires installed along with balancing, but this problem was happening way before then. Before I take it to the shop to see if maybe they can find something, I figured I would ask on here. I like to do things myself, and If i new the exact problem, I can fix myself. First thought that came to mind, was the cv joints.But it never pops, or shakes at any speed. any suggestions?? thanks

Btw. car is a 99 Civic non-vtec with a auto. 135,000 miles

 
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all i can say is check steering components, motor mounts, ball joints etc, its prolly steering or trans related

 
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I'm thinking transmission related.. I've already changed out the motor mounts a while back, so thats out of the question....

 
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dot know what to tell u

 
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Could be as simple as not installing your tires/torquing your lug nuts properly. Try popping off those tires, reinstalling in a criss-cross pattern starting from the top-left nut (assuming you have a 4 lug rotor, of course), and torquing them down to 70-80 ft./lbs.

Worth a shot. ^^ Btw, 45 MPG isn't a speed, it's a gas mileage, lol. Just thought I'd point that out.

 
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my bad. Can't help to get gas prices off my mind.lol

I'm gonna take it to a tranny shop and let them test drive it.

 
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well believe it or not, many shops dont know how to balance rims. sams club, an expensive tire shop, and tires plus all failed, and sears got them so theres almost no vibration where before there was lots of shake. but if its the whole car, prolly engine or tranny mounts. did you check them all?

 
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hmmm. Total engine mounts i checked where two on the top and two on the bottom. I had to replace the drivers side a couple months back, but the other side mounts are still in good shape. I don't know if "tranny mounts" are the one's your refering to on the bottom. I'll check it back out tomorrow. thanks

 
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well, i checked all motor/tranny mounts again today and very little play. However I played around with the throttle with it in park and found something interesting. After turning on the a/c and letting it idle back down it shakes quite a bit more than without the a/c. It shakes to the point, well if it does the same thing while I'm driving, then i would def feel it. Isn't there a valve or something that keeps the engine idled up to prevent this "bogging". The difference from a/c on and a/c off is not comparable. I really think the motor mounts are fine, however I thought the same thing when I found this problem. What signs of defects do I need to inspect on the mounts. The mount rubber isnt cracked or tore. There is some play but shouldn't be enough to make it shake like it does at 40-45 MPH...thanks

 
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well idle shake is caused by rpm bogging below 700 +/-50 rpms. you can take off the throttle body and clean it, and also take off and clean the IAC idle air valve on the side of the throttle body where the two coolant lines feed into. it usually gets clogged and then doesnt work properly. once you clean and put everything back, reset the ecu and do the idle learn to reset the idle speed.

 
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where the tires mounted with the correct rotation foward?

 
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this might sound stupid, but some tires have INSIDE AND OUTSIDE written on the wall and its possible to mount it backwards if you go somewhere stupid.

 
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wheels/tires are mounted correctly.This problem as been going on before I had the new tires installed. It's not a wheels/tires issue....

Anyways I did the TB cleaning as well as the IAC cleaning, and still does it between 40-45 MPH. I guess I'll swing by the tranny shop one day and let them test drive it. I don't understand what else it can be.

 
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look at your cv axles. and if its when your a/c is on. then it might be a bad a/c compressor. mine fell apart and at idle with the a/c the whole car would shake and even the exhaust would rattle. take it to a good shop and have them check out your a/c pressures.

 
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It does it with both the a/c on or off. I did inspect the cv joints, although I didn't really know what to inspect. Boots aren't torn and seems have no slack in the joint when the vehicle was in park...

 
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ok, can you feel the shaking in your hands/steering wheel, or in your seat? answering this question can decide where to go next

 
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I get a shake in the wheel from 50 to 55mph...I swore it was the tires...4 re-balances and two alignments later=still shakes: I will be reading up on this thread to see what comes from it...If i figure anything else out I'll post it. sorry to be an ahole and not offer any advice; did anyone suggest engine stiffy?...I'll prob. catch crap from that

 
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if you feel it in the steering wheel then its a vibration from unsprung weight. unsprung meaning below the springs. so maybe try rotation tires and see if that changes anything.

 
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It is not the steering or the tires/wheels... I've said this like 3-4 times in the thread. It's something from the engine/tranny. And no the steering wheel never shakes, just the dash,seats, and other parts of the car..