Shifting and gassing issues that im curious about in my truck?

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sniperlif3

New member
I drive a 1990 GMC Sierra with a V8 5.7L small block 350, auto transmission and everything should be stock.
Well i will first start out with problem that i get when i first push the gas pedal moderatly from a stop. When i do this, it feels like something in the back of my truck jerks (im thinking its either the transmission, axel, or drivetrain). I have no clue at all whats causing this but let me know anything that you may have an idea about.

The next issue that i have is when i floor it, and it shifts gears, it seems to shift very hard. What would cause this? I did check my transmission fluid and it seems to be fine, but i dont know if my clutch is going, that is, if auto transmissions have clutches

Let me know any and all usefull things that i should check
 
C

Christopher

Member
The problem is carbon build up in the idle air control valve passage in the TB/throttle-body. I know this is a problem because I had the same issues with my 89' caprice which has the same engine as your truck. With the engine off and the key out of the ignition, remove the air breather and look down inside the TB. All the black charcoal is carbon and when it gets in the idle air passage restricts air flow causing the engine to run rich. The part can be removed, cleaned with a TB cleaner that is O2 sensor safe, and reinstalled.
First it is located on the right/passenger side of the rear corner on the TB. It is black in color with a large metal hex, it's about an inch in diameter, about an inch long, and has an electrical plug in on the end. Remove the plug in and unscrew the IAC valve, it is standard thread. I think it is 22mm, but I use my large adjustable wrench. Once it is unscrewd from the TB, remove it out of the passage. There will be a pintle valve on the end that looks like a bullet, be careful not to bump or rotate that pintle valve it could through it out of calibration and the vehicle will have to go through a relearn procedure. Spray out the air passage several times, allow a couple minutes each time for the cleaner to soak in and loosen the built up. Make sure to spray out the air inlet and throttle plate before putting it back together.
This will improve start up, idle, acceleration, and economy.
The hard shifting is do to the detent/kickdown cable being out of adjustment. With the air breather still off go around to the drivers side. Just behind the TB is a bracket that holds both the throttle cable and the detent cable. The detent cable is on the bottom and has a D shape plastic housing just behind the bracket. Inside that D shape is a release tab that is used for adjustment. To properly adjust the detent cable the engine must be off. Manually rotate the linkage with your left hand to wide open throttle and hold it there, while depressing the D tab with your right hand. This will pull up the slack in the cable and the trans. will shift easier. Once the D tab is released, then the linkage can be released. Don't release the linkage before the D tab or you could damage or dislodge the transmission end of the cable.
I had to clean out the idle air passage about once a year and the crazy thing is, I have to clean the same passage in my 97 c1500 vortec engine on a regular basis.
I know this will correct your problem and it only costs about 10 bucks and takes about an hour to do.
Good luck and keep your bow-tie right side up. Christopher
 
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