96 Suburban 454 cool weather high idle very hard shifting turn key off after warm up turn key back on idle and shifting back to normal going on for long time Whats wrong?

Answer 1

Answer 1I'm not familiar with your vehicle, so this answer will be generic based on the symptoms you describe.

On most vehicles, when the engine is cold, the engine control system sets the idle speed up above what is normal for a warm engine. As the engine warms up, the control system [a bunch of sensors combined with an engine control computer] backs off on the idle speed until, when the engine is up to normal operating temperature, the idle speed is back to it's normal [lowest] setting.

Regarding the shifting difficulty, you did not mention what type of transmission you have, but I suspect that it is a manual.

Manual transmissions have small "clutches," [sort of] on the gear shafts which are called SYNCRONIZERS. When a manual transmission is being shifted, the moving gears which you want to mesh are seldom spinning at the same speed, and do not want to mesh.

The job of the syncronizer "clutch" is to drag on both gears, effectively slowing one down and the other slightly up, so they are both spinniing closer to the same speed, allowing them to more easily mesh. Also, until the vehicle has been driven a bit, the lubricant in the transmission is thicker, which also helps to resist movement of the shifting mechanism, contributing to the shift difficulty.

The shut down time you describe allows the warm up engine heat to difuse throughout the engine, warming up the engine coolant temperature sensor enough, to then send an "engine warm" signal to the engine control system, which on restart, allows the engine to idle at a lower speed, thereby facilitating easier shifting.

Hope this helps until an expert engine and transmission mechanic improves this answer.

__________________________________________________________________________________________________________________________________________________________________________________________

The transmission on the 96 suburban is an automatic overdrive. All of the "control systems" you mentioned should do all of the heat up and RPM slowdown "automaticly" and used to do it just perfectly. Now, the RPMs stay high, above 1100 RPMs, until I shut the engine down by turning the key off. When restarted, the RPMs go high, Instantly, and then come back down below 1000RPMs and the hard shifting returns to the normal shifting. When I say "hard Shifting", I'm talking about the feeling of being "rear ended" at about 15 mph. The transimission had to be rebuilt last year because of this problem, which seem to fix the problem, but it was warmer weather...now that it's "cold" again, the problem has returned. Sometimes, not all of the times, I even get the "Check Engine" light. It seems temperature related to me, but there are so many temp sensors located all through the engine and transmission. Any other help is apprciated.