Help to diagnose

[steve porch]

I have a 99 Chevy z71 and the truck idles fine it runs and shifts fine until about 35mph or 3rd gear. I've noticed it will drive around 2500 rpm range in 3rd gear I have to give it more gas to get it to shift into 4th. When giving it more gas the truck shakes and shudders until I either let off the gas or give it a lot of gas then it will kick itself out of the shudder and shaking. Once it's in 4th and I'm driving with a steady hold on gas pedal about 2000 rpm it will get rough for a few seconds then drive normal. Then back to rough and back to normal constantly. There is only a slight change in rpm when it does this. Do I have an ignition or tranny problem? There is a slight grinding noise also

 

[Jared Jackson]

from your description, it sounds like a transmission issue since the problem occurs at shift points. The issue with it being rough in 4th on and off sounds like the TC cannot lock up and you are experiencing shudder. Of course, I say this from just reading your description. It may be a good idea to have a trans shop check everything out!

 

[steve porch]

Thank you for the response I will get it looked into and respond when determined thank you

 

[steve porch]

So I have replaced plugs and wires and ignition coil the shaking upon acceleration has gone away. My shifting points have gotten better but still experiencing the constant off and on vibration in 4th gear between 1000-1500 rpm unless i give itimore gas justjust an update. Will be bringing it to aamco transmission to diagnose transmission

 

[east302]

You might look at the rear end as well. Is there any play (up and down, left to right) in the driveshaft at the pinion?


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[Silverado]

I've had automatics do this too. In my 2000 my shift solenoid was going bad and would cause rough or no shift conditions. My friends g35 also did that at 45mph. Id double check your trans grounding points, fluid level in trans, and driveshaft/bell housing/tranny mount alignment.

Do you have a cam by chance?