If you have 2000 or close silverado 4x4

[ukrkoz]

owners, have question for you

2000 silverado Z74 4x4 automatic.

when i drive in 2HI mode, and let go off gas pedal, RPMs do not drop down to 500; they stay relevant to vehicle speed and if truck slows down by itself, RPMs go down; i can drive down the hill and RPMs will, actually, climb up, without me touching gas pedal.

is your truck doing the same? i asked dealer, they are - as usual - clueless. oh, we need to run tests, and such. $120 pretax money. and day there.
mpg is about right, 17mpg. truck drives fine. but it's one and only vehicle we ever owned, and we had 2 AWD ones, that does this. I have no idea if it's designed this way, or what. but it's mechanical 4x4, so who knows.

[jdmccright]

This is normal. When you lift off the accelerator, only the fuel supply is shut off to the engine. The engine still remains connected to the wheels via the transmission, specifically the torque converter. It is used as an engine brake when going downhill. Truckers use it all the time to go over mountain passes without frying their brakes on the downhill side.

Having 4wd bears no relevance since the torque converter is forward of the transfer case in the mechanical hierarchy of your powertrain:

Engine ---> Torque converter ---> Transmission ---> Transfer Case ---> Front/rear Drive Shafts ---> Differentials ---> CV joints/Axles ---> Wheels

Google "torque converter" if you want to learn more about this wonderful little invention.

AWD works differently than true 4wd and depending on what you had owned before, the transmission can be programmed to connect/disconnect when the accelerator is not depressed. But for trucks that are designed for towing use, this is de rigeur. As mentioned above, it is a vital feature in load control when descending a hill.

[ukrkoz]

1. csr at Good Chevrolet told me "it should not be that way"; of course, I'd rather go with what you say, as most likely he's just selling me on full diagnostics, potentially resulting in more sales.

2. here's the good thing. mpg is about right for the truck; 17 to 22 freeway; hence, this "locked" engine is not really adversly influenced by those enhanced rpms.

3. reason i asked this that if fuel supply was not being reduced, and engine is idling with fuel supply equivalent to RPMs, then it's an obvious loss in gas.

4. it does slow down going downhill; matter of fact, i kept getting feeling that ir runs in 4HI, not in 2HI. or AUTO.

THANK YOU, JDM. good reading.

out of curiosity, how do wheels manage to influence engine RPMs? i have locking torque converter or something? oh, and i know how torque converter works. just don't get how it works backwords, unless it's locking transmission to engine.

[j cAT]

when the vehicle is in gear the drive wheels are locked in to the engine. Idle will not drop much with no throttle when the vehicle is moving at a good rate.

going down a hill you will feel the engine slowing down the vehicle with no throttle..down shift one gear and the idle will increase with no throttle...this is all normal...the engine in gear is always connected to the drive wheels...

If you did have the engine rpm drop to idle rpm at 50 mph then you have a transmission problem ,,,,which maybe why the dealership stated you did have a problem,,,,,could be THIS is what they thought you were saying....


the transmission does lock the engine to the drive wheels when in gear ...thats what it is there for..

[4x4junkie]

I have a 2000 silverado 2500 and it does exactly the same thing, I agree this is the Normal operation mode for the vehicle.

[jdmccright]

Whether the TC is locked up or not, power input on one side of it affects the other end. If unlocked, normally power comes from the engine, which turns half of the TC (the pump) which from fluid pressure causes the other half (the turbine) to turn, which is connected to the transmission, etc.

The opposite can happen when going downhill. The power or energy to keep the engine rpms up comes from the wheels being turned as you descend the hill (you're trading altitude for energy). In this event, the turbine is acting like the pump and vice versa for that moment, which forces the engine to maintain it's rpms despite having no fuel input.

Depending on how the transmission is programmed, it can keep the TC locked or unlock it to match the conditions it expects. But typically, you don't get significant engine braking when in OD and full TC lockup. I don't know if/when Chevy changed that, but the 4L60E did get a significant upgrade to the 4L65E (1998?)...I welcome help from others to clarify this.

Speaking generally, most newer trucks will allow the TC to lock up when towing since they are built stronger and more efficient and it also keeps fluid temperatures lower. Some even have a selectable "tow/haul" mode that changes the shift points to maximize power and efficiency. Older trucks simply don't recommend running in OD since the strain of a locked up TC while hauling is not recommended and the aforementioned inadequacy of engine braking.

[ukrkoz]

ok, well, as far as others have it same way, i am happy. i can possibly see a general mpg improvement should it not be doing this, but it's ok.

yeah, i can sort of see wheels back-spinning engine. i know that front half shafts do continuosly connect to t-case.

oh, well. another thing i have learned. oh, yes, mine does have haul mode on gear shifter. disengages OD and changes shifting points for trannie. kicks like an old mule if not loaded either.

thank y'all for input.