I have a 1996 Firebird Formula with an automatic 4 speed transmission. I had my whole transmission replaced about a year ago, and the mechanic installed the flexplate wrong---I had that fixed awhile ago and it was running fine, but now when I'm stopped, if I step on the gas to hard, I can hear, and feel a loud, heavy thunk sound. And then its sounds like its got a bit of a grinding sound up until it shifts into 2nd, where if i'm on the pedal, it will thunk again. The shifts from 2nd to 3rd and 3rd to 4th, are fine. If I'm going highway speed and i step on it to pass someone its fine. Though when it downshifts at lower speeds, it thunks again.

I'm an ameture mechanic teaching myself everything as i go along, and am actually getting pretty good at this. I haven't opened up the tranny yet to see what the problem was, but I'd like an opinion on a possible diagnosis if anyone can give me one.

Firstly, welcome to the site! There's a great (and smart) group of people on here who can help you with things like this.

For your problem, i had this exact same thing happen to me. The symptoms are pretty much identical. I would put my money on your stock transmission mount being torn. This happens all the time with the stock mounts, it's really just a matter of time. And the "thump" you are feeling throughout the car is probably the bellhousing or the driveshaft slamming into the frame of the car when there is enough load from the engine because of the bad mount. I would check for that and if it turns out that is, in fact, the source of the problem, replace it with a polyurethane tranny mount. They are much more solid and will last the lifetime of the car. Good news is that it's not an expensive fix. Good luck, and welcome again!

I agree, check the tranny mount. The symptons you posted are what you'll typically hear/feel when the stock ones goes. The easiest way to check it is to get the front of your car on the air and then taking the biggest screwdriver you can find, pry up on where the tranny crosses over the k-member to see if the tranny moves up at all. If the tranny mount is broke the tranny will lift up and you should be able to see where the mount is broke.

And I also agree with the poly mount (aftermarket mount). They're stronger and tougher then the stock ones and unless you're putting out a ton of torque an aftermarket one will last a life time. Instructions on how to remove the old mount and install the new one should be included with the new mount.

Do not, I repeat do not go with the cheap auto parts store mount. You'll be replacing it again in 6 months. Trust me

Tranny mount. GM was asleep at the wheel on this one. I wish I got 6 months the first time it happened to me. I had 6 days.

Welcome to the Source! Could you please drop a post in the New Members Forum to tell us a little about yourself?

Matt

polyurethane transmission mount...I am in need of one. I read not to get one at the cheap auto parts store, but is checkers or autozone considered that. From where did you all get yours? Approx. cost? thanks

http://www.ws6project.com/user_stor/catalo...roducts_id=1039

^^^Cheapest i can find^^^ I always order through this company, EXCELLENT customer service and it will be at your house in 2-3 days

Muchas Gracias!

haha I went with duralast so i have a lifetime warranty on the cheap one....so i break it and bring it in and they have to give me a new one free....

How many have you installed so far? Is it just your 1st one? Depending how long they last it would be a good idea. Only that I don't have the time or willing to put forth that effort to installing it every damn time it decided to go out. Better do it right the first time. Was it like $23?

i go throught one about every 6 months....but im lucky...i have a lift in the garage and it takes about 15 minutes to swap

If you have the stock 10 bolt I see a rear end swap in your future!

my dad wont let me just get a new one....so maybe if it breaks... i know that sounds horrible but i want some more power to the wheels and if i can convince him its necessary....haha

If ya stand on at high r.p.m. enough times on dry sticky pavement you'll get your wish!!

Drive your car on over here and I can arrange that for you!! Well because of time constraints I had to purchase the "Anchroage" tranny mount brand. No "duralast" brand available. Got a lifetime warranty and it cost $17.00 at Autozone. Funny how Checkers sold the same brand but offered only 1 year warranty. Interesting? I'll see how this one holds and compare it when I get the one Freshbake advised on the link.

A polyurethane mount WILL last you for the rest of your life, i promise you. If you are going to get a new one, just go that route. I guarantee you won't come back to me and tell me that i was wrong

I know I know but timing was critical. When all else is settled I will get my refund at Autozone and order a real one from ws6store.com and I'll even upload pictures as proof that it's installed. how's that?

I had the same problem...I put a polyeurathane mount in and it made my car vibrate like hell...I thightened it, loosened it, made sure it was in there right, and it still made my car feel like total crap...my whole dash was vibrating and so was my shifter knob...i took it out, took it back to the store, and got an OEM replacement-MUCH more comfortable to me...then again maybe i got a bad mount?

ah crap, that's something I did not want to come across. You see what I'm saying now?

I don't think you got a bad mount, they do vibrate more, but it really shouldn't be that bad. I had a poly mount in my 95 and after I installed it, i could hardly notice a difference. There was only a bit more vibration while idling. My dash never vibrated but all cars react differently. But the reason it vibrates is because it holds that transmission in place and is very stiff, making it extremely durable. That's why when you start looking at all the modded cars, almost all of them switch their bushings and mounts to poly mounts because they hold up MUCH better and can take a serious beating from the power these cars put out.

However, if you REALLY don't want your car to vibrate at all, go with the OEM's, just plan on having to keep replacing them. I guess there's a sacrifice to everything with these cars haha. I have no problem with mine vibrating, i mean it IS a muscle car haha! It gives it character i think.

I dont mind discomfort for performance...not at all...my tranny snaps my neck, my exhaust is loud, and my car fishtails all over the road...thats muscle, and i love it...

the vibration was very abnormal, i assure you...didnt feel right at all when it shifted...it made the cd player skip...

i think they may have given me the wrong one, it fit in fine, but im not sure.

i had to get to work that afternoon, so i just took it back and said "give me an OEM", it was the same guy behind the counter...

oh wow yeah that definitely doesn't sound right. It may very well have been a bad one then!!

Well i thought i would add to this, because yesterday as I was replacing the heater core hoses, i found the source of my rediculous vibrations while idling. Also, the "grinding" feeling under acceleration. Yep, my tranny mount is going bad. The front of it looks fine, but when you look at the sides, it appears to be caving in or something. It's splitting and looks like the tranny is crushing it. Ordered a Poly one and is en route to my home now :-)