johnblacksox

I have an 86 SC, 305 V8, automatic, 230+K miles.

Drove about 300 miles on Sunday (warm humid day), when I could feel & hear a vibration which I've never had before.

I could feel the accelerator vibrate when I went up through 0 through 30 MPH. If I took my foot off the gas and coasted, it stopped, as soon as I gave it throttle, it vibrated again. On the highway, I could feel it most at 55 MPH, when I got it up to 60 to 70 MPH, it went away.

Took it to the mechanic today, he checked it out, test drove it, checked the driveshaft, U joint, and could find nothing wrong. It isn't making the vibration/noise today at all.

Any ideas what is causing this? What else can I do to diagnose it?

John

19doug90

first thing to do is pin point it. Is the vibration speed or rpm related? If its rpm related throw it in park/neutral and rev it all the way up and see if you get the vibe...if so its engine related. If it is speed related chances are that its in your drive train because your engine will work differently at different rpms regardless of speed but everything after your tranny just gets faster and faster at speed. The other thing you can do is take the serpentine belt off your car and drive it until you get the vibration. You have to be super careful because your water pump wont be on and you will have no fresh coolant flowing through your engine so watch your engine temp and make sure you dont over heat. This test has to be done as quickly as possible. What it will tell you is if the vibe is accessory related or not. If you dont get the vibe with the belt off its something that runs off the belt. Hope those tests helped a bit. Make sure to figure out if its speed or rpm related.

johnblacksox

Hmm, well, three days later, and my car hasn't gotten the vibration anymore.

I've driven it city & highway, runs fine again.

Could the engine being really hot have caused it somehow? I'm worried now that the engine is starting to go.

19doug90

an engine cant "start to go" things on your engine can break or be breaking and when they go can cause some major damage but an engine wont be about to go (that is unless hypothetically speaking every part was simultaniously about to break). There very well might be something wrong but if your having this much trouble getting started id just bring it to a mechanic just to give it a good check up/tune up for ya and ya should be good to go.