Allison reverse behavior. Also "Piston Slap"

SierraSmurf · July 13, 2005

Is it normal for the Allison tranny to surge/lurch when reversing and off the gas (idling)? Usually when backing out of the driveway, I just put it in reverse and let off the brake without applying the gas. Used to behave smoothly but lately (I've got just over 4000 miles now) it jerks back and forth pretty bad. Does it need more RPM for proper torque converter lockup now that it's "broken-in"? Should I take it in for service, or is this just a characteristic of the Ally?

Next part of the question: My engine rattles like a diesel (it's an 8.1) and I had previously attributed this to the "Piston Slap" issue, only it does it when the engine is warm as well. Now I'm thinking that it may be simply a vibration issue. It is much more noticeable (louder) under the truck than under the hood. I've tried starting it with the hood popped and can't seem to trace the noise. Same when crawling underneath. It is audible in the cab while driving and especially with the windows down and a barrier next to the truck. I've driven lots of Chevys, my whole family owns them, but I'm the only one with the 8.1 and none of the 6.0s are this loud. If it helps, it has done this since new. Of course, the service department says that this is "normal". Can you have piston slap that severe with a warm engine, or is this something else. Maybe an engine mount issue? Thanks in advance for any thoughts and sorry for the monster post!

Edit: 2005 GMC Sierra 2500 HD, Ext cab, long bed, 8.1 L, Allison

SierraSmurf · July 14, 2005

Anyone? The tranny will also do it in first gear while idling, though not nearly as often. Is the torque converter just locking and unlocking due to low rpms? Is there something I can do to lock it up at lower speeds, or do I just need to give it some gas all the time? Can't find anything on a search which leads me to believe that something may not be quite normal about this...

GM-Tech · July 15, 2005

The converter doesn't lock at all in the lower gears. I'm not a tranny tech, but I'd say it probably doesn't come on until you're in at least 4th gear. If it did it at very low speeds, it would stall the motor or at least be very annoying. The "piston slap" issue does not affect the 8.1.

Other than that, I can't be any help. Without experiencing your issues, I can't tell you what is going on. And if you call it an "Ally" again, I think I'm going to scream! That's almost as bad as calling the Colorado a "Colly" like they do over on one of the Colorado forums.

knez · July 15, 2005

Sierrasmurf my 8.1 does the same thing in reverse occasionally there was a post about it in the 2500hd forum if i remember correctly and no one had a answer for it the either.g-m tech told you pistonslap was not a issue, well it is with mine start it up sounds like a diesel for a minute then goes to a faint knockish/ticking noise but to me soundslike its coming from the block buy will go away after about 5-10 minutes.

SierraSmurf · July 16, 2005

Sheesh! I only called it an "Ally" once... And yeah, I know piston slap occurs in the 8.1. I've read tons of posts on other forums which back this up. I've also gone back to the dealer to listen to other new engines which do the same thing. Simply because it's not a official problem for GM yet doesn't mean it doesn't exist. Just like the heat shield banging during cooldown doesn't affect the 8.1... I've always owned Chevy trucks, but this is the first I've purchased new, and the only one I've had problems with! Go figure. Anyway, all in good fun.

GM-Tech · July 16, 2005

Let me clarify... "THE" piston issue doesn't affect the 8.1. It may have some cold piston noise and so do other of our engines, but it ain't for "THE" same reason.