4l80e slipping from a stop?

[Logan Miller]

I recently picked up an 04 Silverado 2500HD (6.0 gas / 4l80e / 4x4) and it will run shift drive great as if nothing was wrong, but then atleast once a day I'll come to a stop light and when it's time to go it'll just free rev like it's in neutral and not go anywhere. Out of instinct I let off the gas and on the way back down it always seems to grab first and away we go as if nothing happened. I couldn't really find any other posts that state a solution. Truck also has a p0300 code but doesn't feel like it's missing any. I've heard that that code will cause the transmisson to do some weird things. Trans fluid level is normal, trans fluid looks fine but smells rotten. Any advice would be greatly appreciated.

[Jsdirt]

Previous owner may have bandaided a failing transmission with a gallon of "mechanic in a can" - that could be the source of the smell. That stuff REEKS. Very strong smelling. 

 

Some other things to look for, especially since there's a random misfire code, is a failing MAF. That will smoke a transmission. They will normally throw a code for the MAF, though, so this one may be a long shot.

 

Sometimes you can get 0300 codes from a crank relearn not being performed, or crap-quality crank sensors.

 

Another thing to keep in mind, with the half billion miles of wiring required to run transmissions from '96-up, you could be experiencing an intermittent electrical issue, too. On tired trannys, sometimes ATF will make its way into the connector to the vehicle harness and cause issues. Intermittents are THE WORST to try and nail down. 

 

And of course, at that vehicle age, there's always the possibility that the trans is at the end of its life.

[Logan Miller]

Yeah the best way i could describe the smell is a rotten fish smell, Ive smelled that smell before when I worked at a dealership, I believe it was diff fluid then though. Would it be worth it to flush the fluid and see what happens you think?

[Logan Miller]

I was thinking the converter wasnt locking up perhaps, Ill have to mess with it some more to see for sure, but RPM at 80ish mph was like 3000 on the way back, which seemed high to me but then again it does have 4.10s. the shifts seem way too smooth for me to think the trans is bad but you never know i suppose. It does have 200k miles on it.

[Jsdirt]

Yep, could be a bad converter, too.

 

With all the unknowns of that tranny, I'd leave it be rather than flush it. Might create more problems, or make it undriveable. That's your call there - if it were mine, I'd leave it and plan for an overhaul.

[Logan Miller]

Alrighty I'll have to look into it some more, the truck should sit in first gear the whole time it's in D at a stop right? Or does it actually go to neutral then when you let off the brake itll go to first? 

[Jsdirt]

Nope - the second you put it in gear, it should roll in whatever direction you selected (Drive or Reverse) just by engine idle power. Should never be in neutral unless that's what you placed the lever in.

Edited November 3, 2022 by Jsdirt

[Logan Miller]

Ok so after messing with it some more I came to the conclusion that I was reading the dipstick incorrectly. I guess your supposed to read it when truck is running. With the truck running and hot the dipstick barely registers any fluid on it at all however with the truck cold and not running it reads on the cold line as one would expect. Gonna try and fill it up some and see how it does.

[Jsdirt]

That'll do it!