Hey...I've noticed this ever since I had the car but seems like it may be getting more pronounced lately...maybe I'm just paying more attention to it.

When shifting from PARK into RVS or DRV...I feel a big lurch/heave in the direction of the gear.

What is causing this?

 

I also have this same problem with my 1991 z28 when i put it into gear .
Could it be the rear brakes not working for some reason......?

 

I have had this problem since I got my car in 2006 (it's an 86).. I got my tranny rebuilt, rear end rebuilt, driveshaft and joints new, rear brakes new...

To describe it, it sorta bangs into drive or reverse and lurches forward or backward, right? I have heard that this is from the lack of an improvement they made in the 1987 700R4's called the accumulator spring (correct me if I'm wrong) that cushions something when it gets put into gear.

Short story: I got pissed when I rebuilt the tranny and the builder told me that there was no difference from an 86 700r4 and a 87 for example.. After it was built he did say there was that improvement and that he didn't think I would care about that because it has nothing to do with the 'performance' of it. The tranny shifts so well now so I didn't bother to change it again to fix that lurching problem, but either way you can't add it to an older 700R4, you have to have a newer one.. But the weird thing is that you have a 91 Z28 so I would think it wouldn't do that like my older 86? Odd..

 

idle speed to high...do not trust the tacho! Get a timing gun w/ rpm indicator and go from there

 

If it matches what I see in Tunerpro when I datalog, does that mean that it's accurate (matching tach with what the ECM sees)? thnx!

 

yes

 

Hmm. My idle sits comfortably around 750 or so, sometimes a bit lower at 650, after all my tuning I got it this low and happy.. But it still does it when I start the car, but oddly, it doesn't do it when the car is cold or after a minute of starting, seems odd though, because it does it when it's warmed up.

 

its the power ready and rearing to pounce

 

Check those u joints

 

The ujoints should be good.. What are the signs of them going south? Looseness?

 

I just keep tellin' this to people who come in the car and ask

 

My 87 LB9/700R4 does this as well, but my LHO/700R4 92 RS does not. I have got use to it, but you do notice it for sure.

 

How about the tranny mounts? I remember my '86 clunked and lurched when they were shot.

 

I had rubber, it was decent, upgraded to ES poly mounts, (had some drivetrain vibration on highway), changed to prothane mounts and there's no vibration but still this lurching problem.

 

Drivetrain slop, be it in the splines, u-joints, or lash in the r&p can cause the sensation, so can brakes that don't hold as well as they should. I avoid autos, but if all those things are good, process of elim says its in the trans.

When a u-joint is on the way out the cross will have play in the cups, rotational, side to side, front to back, up and down, whatever, if it has perceptable play its not long for this world. Don't confuse play in the pinion or trans out for play in the u-joint. Looseness in my mind means the joint moves within the yoke, in which case both should be changed.

 

Throttle cable adjustment. Before I adjusted mine it had no problem... after I adjusted it now its lurching hard when I shift from park to drive. Try doing a real adjustment. Like I'm doing tomorrow.


Posted from Thirdgen.org App for Android

 

I'll check ujoints out.

For the throttle cable adjustment, what is the simplest way to do that?

 

Push the "D" button and while holding it down, push the cable towards the windshield....Release the "D" button (it may be square instead of D shaped, it is on my car) Then simply rotate the throttle all the way to full throttle. The cable will ratchet "perfectly" into place.

I'd also say it's time to take the throttle body and IAC out and clean them with carb cleaner. Mine was amazingly dirty inside for how low of mileage it was. (I did this about 4 years ago). Once you've reassembled it, follow the instructions for setting the IAC, min idle, TPS settings here:
https://www.thirdgen.org/tpimod2

If the min idle is too high, it will bang into gear. If it's set right, it won't.

The other issue is a broken rubber transmission mount as mentioned by another poster.

 

I will try adjusting the cable too, and i'll clean the throttle body as well. I think mine is leaking air slightly so I might buy a new 52mm ported one (one of those EBAY type re-inforced ones). I have a new IAC and TPS and I know the TPS is adjusted perfectly at 0.54 (or 0.52, whatever it was supposed to be at when I did it), but the IAC I'll have to look at to be sure.

For the throttle cable: When you say "release the "D" button, do I release the cable at the time same? Or have I already let go of the cable at this point? And when you say rotate the throttle to full throttle, do I just turn the throttle on the side of the throttle body and hold it at full throttle until the cable stops moving?

Sorry for the Q's, just wanna be sure so I don't burn up the tranny!

Thanks Reid,
-Marc

 

I had the same thing with my car it was annoying as heck. In my accumulator piston instead of the springs to soften shifts there was 3 washers and a nut to make the shifts "harder". Instead it probably played a key role in ruining the trans along with some of **** I seen done to the trans.

 

The D button is basically a way to lock the cable in place. You have to press the button in order to get the cable to move towards the windshield.....After you've let go of the button, the cable will stay there.

You are correct about simply reaching over to the throttle on the side of the throttle body. Turning that to full throttle will ratchet the cable forward to the "exact" spot it needs to be in.

Many people foolishly try to manually adjust the cable in order to get a firmer shift. But it's one of those deals where it's either set 100% perfect or you'll end up burning up the transmission.


This is why I like the Pro Built Automatics "modified" shift kit. Most kits use 1 washer for ambulances, towing etc, 2 washers for performance, or 3 washers for drag race only cars.......The Pro Built shift kit uses 0 washers! Yet it's a fantastic shift kit. I've had mine for 6 years now. Daily driver. But you never get tired of it.

 

Wow.. I wish I knew exactly what went into mine.. Thanks for the info I'll be doing this over the weekend, hope it helps! Thanks Reid!