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Transmission "jerk"

Rmodeler

Well-known member
Joined
Oct 2, 2005
Messages
57
Location
Goshen, Indiana
Corvette
'82' Silver Blue
82 corvette This may have been discussed before, but couldn't find it. When putting transmission in drive, it jerks, especially if the rpm is up at all. At 5-6 hundred rpm it not quite as bad. It jerks and the back end squats. Shouldn't it go in smooth, without the jerk. I don't notice it going into reverse, just feel the back end raise up. Wife always complains about it(I know, don't let her ride in it.)
 

zachh

Well-known member
Joined
Aug 1, 2005
Messages
221
Location
virginia beach, va
Corvette
1979 black 'n' silver L82 1978 Trans Am Y88
That is normal at higher rpms (around 1000), a small jerk. The jerk is just the pressure getting put on the rear suspension, thats why it sits down. If you put it in gear, hold the brake, and hit the gas, the higher the rpm, the lower it'll get. If there is a "clunk", thats just loosness in the rear end, driveshaft, trans, half shafts, and rear hubs.
Best Wishes
zachh
 

gmjunkie

Well-known member
Joined
Nov 11, 2004
Messages
4,547
Location
Old Fort NC/Vero Beach FL
Corvette
03 Z-51,78 L82 Coupe
Check for a Broken Transmission Mount and Worn or Missing Differential Snubber Mount too!!:upthumbs
 

wishuwerehere82

Well-known member
Joined
Sep 6, 2003
Messages
2,314
Location
Rochester, NY
Corvette
Red '82 Coupe,Sebring Silver '98 Coupe
Mine has always done that even efter a tranny and driveline rebuild. It grabs into gear and squats the rear, even at idle. Definately not a smooth engagement like a normal passenger car. It wants to go!
 

Rmodeler

Well-known member
Joined
Oct 2, 2005
Messages
57
Location
Goshen, Indiana
Corvette
'82' Silver Blue
Thanks to all for the information. That was probably disappointing to spend all that money for the rebuild and still have the rough engagement. If they did that when new, it seems people would have complained about it. I was wondering if a rebuild would correct it, guess I'll save my money. The car has 40K miles on it and I'm the 3rd owner, runs great, except I have trouble with the idle staying high at times, one time you come to a stop and it will be 5-6 hundred, next time you come to a stop and it will want to run 1000-1200. Haven't had time to check it out completely yet.
 

gmjunkie

Well-known member
Joined
Nov 11, 2004
Messages
4,547
Location
Old Fort NC/Vero Beach FL
Corvette
03 Z-51,78 L82 Coupe
Thanks to all for the information. That was probably disappointing to spend all that money for the rebuild and still have the rough engagement. If they did that when new, it seems people would have complained about it. I was wondering if a rebuild would correct it, guess I'll save my money. The car has 40K miles on it and I'm the 3rd owner, runs great, except I have trouble with the idle staying high at times, one time you come to a stop and it will be 5-6 hundred, next time you come to a stop and it will want to run 1000-1200. Haven't had time to check it out completely yet.
Check out the Choke and fast Idle cam,it's sticking!!!:upthumbsMy 78 L82 automatic don't slam when I put it in gear unless it is not warmed up!!! But it will squat when it loads the suspension!!:upthumbs
 

wishuwerehere82

Well-known member
Joined
Sep 6, 2003
Messages
2,314
Location
Rochester, NY
Corvette
Red '82 Coupe,Sebring Silver '98 Coupe
Check out the Choke and fast Idle cam,it's sticking!!!:upthumbsMy 78 L82 automatic don't slam when I put it in gear unless it is not warmed up!!! But it will squat when it loads the suspension!!:upthumbs
Crossfire injection doesn't have a choke, Junk.:L Remember?
The idle speed is controlled by the IAC motors on the throttle bodies. They open and close pintle valves on each TB to vary the flow of air into the intake, below the throttle plates. The throttle position sensor tells the ECM when the throttle is closed, and should be at idle. Then the IAC motors close off the air supply until the tach signal lowers to correct idle speed.
The system is not fast, sometimes taking a few seconds to equilibrate all that data. But a vacuum leak, old TPS, or a worn throttle shaft bushing can cause the idle speed to wander around a little.

Or.... it could be some dirt or gunk in the throttle linkages, causing them to stick and not return to the idle position.

That reminds me to go out and spray some carb cleaner on my linkages and throttle plates.
 

gmjunkie

Well-known member
Joined
Nov 11, 2004
Messages
4,547
Location
Old Fort NC/Vero Beach FL
Corvette
03 Z-51,78 L82 Coupe
Crossfire injection doesn't have a choke, Junk.:L Remember?
The idle speed is controlled by the IAC motors on the throttle bodies. They open and close pintle valves on each TB to vary the flow of air into the intake, below the throttle plates. The throttle position sensor tells the ECM when the throttle is closed, and should be at idle. Then the IAC motors close off the air supply until the tach signal lowers to correct idle speed.
The system is not fast, sometimes taking a few seconds to equilibrate all that data. But a vacuum leak, old TPS, or a worn throttle shaft bushing can cause the idle speed to wander around a little.

Or.... it could be some dirt or gunk in the throttle linkages, causing them to stick and not return to the idle position.
:rotfl Duh!!!! I knew That!!!!!:chuckle:chuckleI don't know what I was think'n!!;shrug:cool
 

minifridge1138

Well-known member
Joined
Feb 2, 2007
Messages
908
Location
USA
Corvette
1982 Black Fastback
I had that same problem on my 1982. Your idle is the problem. Mine used to literally DROP into gear. The car moved forward an inch and the back end dropped 2 inches. I lowered the idle speed and the problem is gone.

Lucky for me, my car has a 4 barrel holley sitting where the CROSSFIRE used to be, so it was easy to adjust.

I'm not sure how to adjust the idle on the CFI, but lower the idle and your problem will go away. Although your 0-60 might be affected, too.

Good luck!
 

wishuwerehere82

Well-known member
Joined
Sep 6, 2003
Messages
2,314
Location
Rochester, NY
Corvette
Red '82 Coupe,Sebring Silver '98 Coupe
The main problem with dropping the idle RPM on the crossfires is that the ECM won't let the RPM drop below 500 RPM. If it does, it will give it a double shot through the injectors, and the speed jumps up. This is the notorious "surging" that the crossfires are noted for. So 650 RPM is the spec. for the idle speed, with a TPS voltage of 0.525V.
 

minifridge1138

Well-known member
Joined
Feb 2, 2007
Messages
908
Location
USA
Corvette
1982 Black Fastback
Well if the CFI won't let you get the idle low enough to eliminate the jerking, then i guess you'll just have to live with it. As I said in my first post, i have a carburetor and it is easy to adjust the idle. I don't know what idle i have though, because my tach does not work. Tuning a carburetor by ear is fun.

Good luck!!!
 

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