02 Yukon 5.3L Shifting Issue

CharlesOwnby · May 25, 2019

I’ve got a issue that just started last night. I was driving home, pulled from a red light and engine revved pretty hard before shifting. That repeated all the way home. Every time I slowed down to 1st gear and had to start again it would repeat. When it did shift I didn’t feel any other gears change, just the one change. Part of this trip was on the interstate and at 65 MPH the RPM was about 2500. Trying to figure out if I have a speed sensor problem of some type issue not telling it when to shift or if I’ve got an actual tranny problem. I have very little mechanical knowledge and it’s my first GM vehicle I’ve ever owned for about 3 mos now. Also I did figure out when the RPM gets to right at where it should shift, I can let off and tap the gas and it’ll shift much sooner and smoother than if I just stayed in the gas. Also noticed the oil pressure increased when pressing the gas from about 40 to 60. Don’t know if this is normal but I’d never noticed it before. Maybe only noticed it due to being particularly observant cause I was having issue.

Also, separate questions, I got a check engine light a few weeks ago saying something about both main banks. Shop said it’s most likely vacuum leak somewhere. Previous owner is very knowledgeable and said to change O2 sensors and probably was not a vacuum leak. Any advice on this as well?

Thanks.

Edited May 25, 2019 by CharlesOwnby

Doug_Scott · May 26, 2019

Something you need to understand about automatic transmissions is that 1st gear has 2 ways to engage it. When shifter is in Drive, 1st gear will not provide engine braking, it will actually freewheel when it is engaged by the transmission when slowing down. It may appear or give you the impression it is slipping if you slow with your foot off the gas, and slow to about 5mph, then step on gas. The engine will feel like it has to catch up to the transmission. This is something that for some reason once you have noticed it doing this, you will learn how to make it happen, and it will do it every time. If you put the shift lever into 1st gear at 5mph you will feel the engine braking. The engine braking is accomplished by apply a band in the transmission. When in drive, the transmission uses an over-running or one way clutch to get 1st gear. This clutch will only prevent the drum from turning when driven by the engine, and when slowing, that drum will just free wheel when driven by the output shaft. In manual low or manual 1st gear, the drum is held by a band causing the output shaft to be connected to the engine side of the drum, giving you engine braking.

That was a lot longer than I intended. As for the other issue, I would check for the vacuum leak. It is a pretty common issue. Would be helpful to post the exact codes.

CharlesOwnby · May 26, 2019

I take off, it starts to rev high as I speed up and then if shifts to what I believe is 3rd. I can run as high as 55-60 before it starts to reach the end of the gear. Gotta be 3rd or 4th. That’s it, nothing in between. Just from reading it sounds like it’s in limp mode. When it first started the change engine oil light came on, not the service engine soon. I stopped, checked the oil, it’s fine on level. Then I also bought a engine code reader today and it’s not pulling any codes. If there was something wrong to cause the tranny to go into limp mode it would seem that there would be some code right?

Doug_Scott · May 26, 2019

The change oil light means the oil life monitor has reached its limit and you should change the oil. Note that the oil life monitor has no idea if the oil has been changed or not, you need to reset the monitor after each oil change. The monitor uses time, distance, how the vehicle is driven etc to decide when a change is necessary.

If you are flooring the gas pedal and the first shift you feel happens at 55 to 60 mph it can still be in first gear. It depends on which gears you have in the rear end. If your truck has the 4L80E transmission, and you have 3.08 rear axle, at 5000 rpm you would be doing 60mph. These trucks can have some long legs. The only issue this creates is it is dead slow from a stop. To give you an idea of how those gears behave, my truck has 4.10 gears, same diameter tires, and my transmission is a 4L60E, at the same 5000rpm my truck is only going 37 mph. There is a difference in the 1st gear between the two transmissions as well, the 4L80E has a 2.48 1st gear, while the 4L60E has a 3.06 1st gear.

I have been hearing the term "limp home mode" since the beginning of EFI. I have never experienced it. I have however experienced the "walk home mode". When I was told about limp home mode I was told the ECM would default back to "safe" settings, and you would be hard pressed to feel the difference in normal driving.

If you have the owners manual, give it a read. It is sometimes quite useful to spend an hour in the driveway behind the wheel with the owners manual familiarizing yourself with all the features you truck has.