I have a 1987 Iroc.

On overdrive, if I keep the accelerator at around 1500-2000 RPM steady, the car will out of nowhere begin to shake badly as if the transmisssion can't engage in the next gear, but I can accelerate out of it if I step on the gas and I guess force it in the next gear.

Sometime this happends, sometimes not.

Just so you know, I have had a transmission mount problem and have already replaced it once before.

What could be the reason?
Thanks in advance.

 

Lockup torque converter problem? What if you hold it in 3rd (Drive) and try the same thing?

Tranny mount usually gives you one good "bang" underneath the car when you nail it wide open from a stop or hit a strong downshift. Doesn't sound like the problem here.

 

Torque Convertor Clutch locking and unlocking rapidly. Have you done a gear swap in the rearend?

 

About the transmission mount, I know what you mean by the hard bang feel/sound, I used to have that problem, but can a transmission mount ever be installed to tight?

I'm not to sure if the gears were ever swapped, RedIrocZ-28, but it was rebuilt about 1 year ago. Is it even possible to switch out gears in an automatic?

It seems you both agree it is a torque converter problem. How much do they usually go for?

Again I appreciate the help guys.

 

I've had this happen to my k5 blazer with a 700r4. does it look and feel like the car wants to walk instead of roll. I mean my whole front end looked like it hAD HYDRAULICS IN IT. it is the converter getting stuck in lockup. I'd look at the tv cable to see if it snapped or out of adjuustment first but its probably too far gone for that now. good luck.

 

My 93 would shake/surge a bit just like the tranny cannot decide if it wants the tourque converter to be in lock or unlock,, I used a trannsmission additive "friction modifier",, she smoothened right out. Also older tranny fluid will also contribute to this shake/shudder/surge.

I also drive in "D" instead of Overdrive if I am going to be going much less than 50 mph.

 

BTW- in answer to your question, no, you can't over-tighten a transmission mount. The bolts only hold it in place, they don't compress the bushing.

 

Had a a couple tired automatics that were actually improved with the new Valvoline Max life fluid. Might try a filter and fluid change with this stuff, drive a couple thousand miles and see if it helps.

 

I only ask if you had a gear swap because when I went from 2.77's -> 3.73's and the speedo gears in the tranny weren't changed accordingly, what would happen is the convertor would get the signal from the computer to lockup at the normally prescribed 35mph (good for gas mileage). However, with 3.73's in there, the convertor would loc up at the SPEEDO's 35mph but in reality it was only 25mph. At that point there is not enough of a load on the motor/drivetrain to keep it happy. My car would buck and jerk something fierce until I burned a new chip that would lockup the convertor at 55mph (real 55mph) which on my speedo is currently about 67mph. I always scare people by getting on the highway when my speedo says 100mph, but I'm aactually only going 77

 

I'll try checking into the gear ratio. The tranny is gonna get serviced this Firday. Thanks guys.

 

Is your speedo off? If its off, good possibility that the gears in your rear end have been changed.

 

Well, My RPM Gauge stays at 1000 after te engine is shut off. I have no idea why it doesn't drop down to 0 like the MPH gauge does, but I'm not sure.

 

Well a friend of mine had a 1994 thunderbird lx 4.6 auto. His transmission did the same thing when he took off and shifted into second. We traced it back to the clutches in the transmission.It finally died though.when you got on it it just shifted hard., but when you just tried to drive it it would literally vibrate the whole freaking car.

 

That's EXACTLLY what's happening in my situation. I did some reading and am pretty sure it's the "torque converter lock-up clutch", not the actual torque converter I originally thought it was.

Was replcing the problem expensive or a big hassle?

 

I have many 3rd gen camaros and firebirds, and so do my friends. These cars have their querks. Right now I am trying to solve a vibration in my IROC much like yours, but mine pulsates instead of a solid vibration. I think it is from my solid motor mounts. That is all I can come up with right now. But, for you I might have some ideas. I know it is a little long winded, but many things can cause many different vibrations. I have seen a lot of them.

One question I have for you is what speed do these vibrations occur?

Please read through this and try these tests. If you have any questions, feel free to write me back. One suggestion is that you contact me before you make any repairs. Maybe we can save you some money. And as we all know, that is a good thing to try when you have a 3rd gen camaro/firebird.

First of all try manaully shifting the gears from 1st, 2nd, 3rd, overdrive as you accelerate. Note everytime you shift if the rpm decreases, this tells you exactly if the tranny is going to the next gear. When the vibration starts try downshifting manually without changing your position on the throttle. Note the change in the vibration. If it is still there, I would check the u-joints and the driveshaft, also the entire rearend (suspension, bolts, fluid, etc.)

If the vibration stopped, then the next thing to do is put the transmission in overdrive and note the rpm to make sure it shifted. When the vibration starts, try taking your left foot and lightly applying the brake, without changing your throttle postion. This disengages the torque converter clutch. If you let off the brake the TCC should re-engage withing a few seconds. You will see the rpm changes about 200 - 300 rpm everytime you do this. If no change in rpm then your TCC could be your problem or your whole torque converter. To make sure you have a TCC problem try accelerating to a higher speed and see if you can get the TCC disengage and engage.

If this still doesn't change anything with your vibration, I would start checking motor mounts, bell housing bolts and tranny mount bolts. I have experienced vibrations from loose bell housing bolts with trannies that have shift kits. The hard shift tends to loosen things. Maybe when your tranny mount was broke the hard hit at every shift loosened the bell housing bolts.