do a burnout, engine will be under load, and it wont be moving

j/k

 

Remove driveshaft and problem might be solved.

Seriously, take it off and check the U-joints for notchiness. Also have the DS checked for balance.

Do a rear end service while you are at it and see if all the teeth in the gears are there.

 

I guess I might be the only one but it sounds like he snapped a motor mount to me. Either that or a tranny mount. Either of these can break and still look intact. Put in drive hold the brake and give a quick romp on the gas and you will hear a bang.

Just my .02 but if it is none of these things make sure to tell us what it was.

Brett

 

I would look at the driveshaft first. It doesn't cost anything to check it. If it only does it under load at higher RPMs that is your best bet.

 

UPDATE
Well i got my car back from my mechanic (will tell about that after) and ran the tests that were sugested. Drove the car around for a while until the problem started. Got it in my drive way and put it in neutral. Revved the engine up and around 3500 rpms i started getting the vibration. It was a little different unstead of it pulsing it was a solid vibration but that should just be attributed to the fact that my drive train was not turning over so i have basically pin pointed the problem to my engine.

The problem is a bit worse now. When the engine is at idle it rocks back and forth pretty good. I dont think there is anything wrong with the engine mounts i just think its just because i have a 350. When you get on the gas the engine stops shaking and goes pretty normal so i think all is well there.

As far as the problem with the engine im thinking one of the problems is i might have a leak on the intake side of my engine. When you get on the gas it feels like there is a slight sputtering and the car doesnt feel like it has all of its power. I had pulled off the plenum and egr and i think there is a chance the leak would be coming from the plenum due to some issues putting the plenum back on.

However that i know has nothing to do with my vibration problem because i had the vibration problem before i took anything off the car it just wasnt as bad.

My mechanic couldnt figure out what was wrong and said to take it to a gm dealership repair place because they have some scope? that he doesnt and some other stuff so they could better evaluate the problem. Any tips for things to check for the reason of my problem? What could cause a large vibration coming from the engine at over 3500 rpms?

(the problem i had when i put on the plenum is me and my friend are idiots and tightened it from front to back not from the middle out. Had some other problems but nothing that could be attributed to a vacuum leak.)

 

I would say, if it is in the engine and was there before,check the Harmonic balancer. They do get old and slip out of wack sometimes. I have seen them be able to be turned by hand,and I even had one on a old Thunderbird come off and hang on the crank shaft (ting,ting,ting)drove me nuts til I got home and cut it off with a torch.

T.

 

im kind of a newb. What does a harmonic balancer do/look like/where is it. My mechanic who is very sketchy he likes to make things up although he did say he checked it and said it was fine. Usually if he finds something thats broken hes very quick to tell me so he can fix it for me although i usually just fix it my self. Anyways dont think thats the problem. Its a severe vibration as i described in my last post so i wont do it again. Thinking maybe it could be a rod bearing? really not sure just throwing things out there. Its going to a different mechanics tommorow to see if he can figure out what it is.

 

To answer your questions,first it does just what it's name implies,balances and kills the harmonic vibations of the crank shaft. It is located on the crank shaft behind the crank pulley and is the thing you look at when setting the timeing (that is where the timeing mark is)it is basicly a flange with a rubber dough-nut around it and then another metal ring around that(the part with the timeing mark). They can slip and through your timeing off or even seperate. The only way that you yourself at home could check it would be to pull the valve cover and #1 spark plug,bring the piston(#1) up to TDC on compression (both valves closed)and see where the timeing mark is. It should be be somewhere between 0-10*,however this still doesn't mean it is ok,just means it hasn't slipped on the crank. Also check the rubber for big major cracks and missing chunks. This should get you started in the right direction.

T.


PS. Have you had transmission work done lately?

 

no i just bought the car....dont think its anything to do with the tranny because in neutral i still get the vibe. I should have some answers tommorow its getting looked at first thing by a second mechanic.

 

I take it you have a manual trans,the reason I asked is most of the time when work is done to the trans(ie clutch,throwout bearing,etc.) the pilotbushing is overlooked,even on an automatic this should be checked and replaced,this can be associated with vibration also.

T.

 

that actually sounds like something very possible. It is an automatic tranny (the stock 700R4 it has a shift kit not sure what else guy i bought it from was pretty car illiterate funny thing being he works on a gm assembly line). Anyways like i said earlier the vibration is a solid vibration when in park/neutral and when in drive it turns into a vibration that sounds like something turning over. What you said sounds like a very good idea to check. Like i said in my last post its going to be looked at by a mechanic first thing in the morning and well see what he says if not ill definitly give that a check.