violent shaking when accelerating

i may have first noticed this problem two days ago, but thought it may have simply been unseen road conditions because it would come and go. but when my car started to shake violently suddenly halfway through a 130 mile trip, there is no mistaking now that there is a problem. when i reach highway speeds (55 and above), my 98 eclipse (80K miles) begins to shake violently if i am accelerating or adding sufficient gas to keep the car at a constant speed. if i let off the gas enough that it begins to decelerate slightly, or if i put the clutch in and disengage the engine, all shaking stops, regardless of the speed.

i called someone i knew with mechanic history, and he suggested that it may be spark plugs problem or a fuel injection problem. i just had the spark plugs changed a week ago, but perhaps one was faulty. last night i used a fuel injector cleaner, but didnt notice much difference (i may have not let it mix well enough with the gas, not sure). on a previous 4cyl car i had a fuel injector go bad and start missfiring, but shaking was not one of the symptoms. my check engine light hasnt come on either, which i think it would in either of these cases. there is also no shaking at lower speeds even at high RPMs/accelerating.

i also thought it may be a cv joint problem, but it was my understanding that the shaking pops up between 15 and 45 mph, smooths out after that, and is accompinied by "clicking" noises, especially when turning. i have heard no such noise.

sounds like your axles or maybe as simple as an allighnment

check to see if your engine mounts are bad. wouldnt hurt to put some fuel cleaner in your tank next time you fill the tank..

I'm thinking mounts as well. I have the same problem on the POS-cort. Typically when I'm in a higher gear at lower rpm, lugging the motor a bit. I have to let off, then reapply throttle, or downshift, to get rid of it. Feels like the motor is trying to climb out of the engine bay. Can't say I blame it though...

But are the mounts the cause, or the effect? ;)

Are the tires old? My bro had same problem with his car... he needed new front shocks and tires. Different car, but same problem.

dude, i had the EXACT same problem. i replaced my right halfshaft and it ceased. haven't had a high-speed wobble since. same exact problem youre describing. im completely sure thats what youre talking about.

Guys, axles, alignment, and tires are unlikely--the problem only persists when he's getting on the gas.

It has to be engine related if that's the case. Perhaps an ignition problem that only pops up under extreme load, perhaps motor mounts like others mentioned.

you didnt say if your car was turbo or not but if its turbo the spark plugs could be the problem. lots of people have trouble with platinum plugs in turbo cars. if you have a turbo use ngk plugs. whether your car is turbo or not make sure the plugs are gapped correctly and that they are the right type.

i really disagree, and realize you have more experience than i do, but i had that problem, where if i was on the gas, it would wobble, and if i let off and coast or put the clutch in, it would stop.... there was play in the axle joints, and getting a new one completely solved the problem....

and the winner is... motor mounts.

the motor mount was shot and the transmission mount wasn't too far behind. to cover bases, checked the axels too, but no play in those so for now they are probably good.

thanks for the replies everyone.

I am having the exact same problem with my 3g 01 Eclipse Spyder GT. At first it was a very subtle shake at highway speeds but has gotten progressively worse. On my way to class this morning the shaking was unbelievable. I turned around and took it to my mechanic. I was also thinking the same thing as scotyeclipse regarding a cv joint. I just dropped a new engine in her last july so I have been recently thinking it was a motormount. Well after I get the diagnosis I'll let you know.

I am having the exact same problem with my 3g 01 Eclipse Spyder GT. At first it was a very subtle shake at highway speeds but has gotten progressively worse. On my way to class this morning the shaking was unbelievable. I turned around and took it to my mechanic. I was also thinking the same thing as scotyeclipse regarding a cv joint. I just dropped a new engine in her last july so I have been recently thinking it was a motormount. Well after I get the diagnosis I'll let you know.

Welcome to AF.
Considering that this thread was last replied to in December of 2004, most of the people aren't here anymore. Try not to reply to old threads unless they're yours or you have something really useful to add. The original poster responded back with what the solution was.

Usually I would lock a thread like this, but post back what the diagnosis is I'll keep it open.