Flare or surge going in or out of Park and Neutral

[sam05]

Having an interesting problem with the Denali. When I sit at a light in D, the RPMs look ok but the truck is trying to go. It seems to settle to a stop Ok at first, but then the longer I sit, the harder it seems to try to move forward. Also, when coming to a stop it has a surge or flare when it downshifts from 2 to 1 as I slow down. It also does the surge/flare when I go in and out of Park or Neutral.

Shifting during normal or even "spirited" acceleration is normal. No flares between gears except 2 down to 1 when slowing.

Of course my "trusted mechanic" told me it's all fine! They say it runs normally. They said my transmission has the correct amount of fluid and it's in good condition (because I had it changed about 8 months and 3K miles ago). Mileage on truck is 202K but only about 50k on remanufactured transmission. Prior owner did tow a large travel trailer and there is a tow brake installed but it is completely disabled.

So far have tried: clean MAF and Throttle Body, Exchanged 1 quart of Dex 6 fluid for 1 quart of lucas transmission fix. Neither made a significant difference.

I'm not opposed to doing a full fluid exchange or a (gasp) flush if that's what's recommended.

 

[Scottydoggs]

said flair in rpms is likely because the park and neutral vs in gear rpms are different. so they tend to raise or lower in or out of gear.

 

[sam05]

The flair is different from how this vehicle ran up to a couple weeks ago. Also, my 2WD 5.3 05 Yukon XL never did this. Something is different/failing or in need of maintenance. When slowing down the change from 2nd to 1st is a pretty noticeable bump...but not quite a jerk. Maybe this is nothing concerning but it's definitely different than any vehicle I have owned or driven and I'm just afraid that ignoring it is a bad idea.

 

[Scottydoggs]

might have a vac leak causing it.

i was just out in mine, does a 200 rpm bump in and out of gear. nothing i can feel tho.

 

[sam05]

Ok. Don't know why but a vacuum leak hadn't occurred to me. I'll go a hunting as soon as I'm done with work and dinner!

 

[Rocket Man]

When you say the fluid was changed 3k miles ago, was the pan removed? If so, it’s possible RTV was used on the pan gasket which can cause the servo passages to get clogged up which causes symptoms similar to what you’re describing. The servos don’t operate properly and hard shifts and erratic shifts especially the 1-2 and 2-1. Never use RTV on a trans pan especially a 4l60.

 

[sam05]

Thanks Mark! (I'm a big fan of the Beast and watch your posts.) I am inclined to think something is blocked off like you say. I found out yesterday that the truck doesn't want to back up the driveway. Had to floor it to get it to move up at all in reverse.

I didn't change the fluid. I took it to my normal shop, but I've had several problems with them and they may get demoted to tires/oil/AC only. They may have used RTV.

What is the proper remediation for this problem? The Dreaded transmission flush? Is there something (fluid) that I can run through the transmission for a few miles to "clean it out" or dissolve that and then do a fluid change?

 

[Rocket Man]

My trans builder cleaned the passages out as they were rebuilding the trans. I was the offender using the RTV when I did the 3000 mile fluid and filter change that their warranty required. It ended up breaking the output shaft of the trans due to the HP and the hard shifting. But the pics they showed me looked like it was fairly easy to clean. There might be some pieces that go through though if the trans isn’t completely disassembled so I would think the filter would catch those. I don’t know of anything that dissolves RTV. Hopefully they didn’t use it on you’re but if they did I believe the only way to help would be pull the pan and clean the passages. They go up into the trans from the pan rail area. It’s where the fluid from the servos drain into the pan. If they don’t drain they don’t release right. I have a LubeLocker gasket ready for the next fluid change.