4L80E Intermittent Leak, Morning Sickness

[JLeather]

Having some issues with the 4L80E in my '98 K2500. The truck has 265k miles on it and I'm not sure if it's the original trans but I'm sure it's got a lot of miles on it because just after I bought the truck I had to replace the pump pressure spool valve (used the Sonnax kit) due to high line pressures in reverse. If that was worn out I'm sure the trans has a ton of miles on it, but that was 3 years and a couple thousand miles ago and the truck has been pretty reliable. Here's what it does and what I think is going on. The truck has an intermittent transmission leak. Sometimes it will go weeks without leaving a puddle or needing fluid, and then sometimes it will leave a puddle while sitting or leave a big smear under the p/s frame rail after a drive. I initially thought it was the cooler fittings in the side of the trans leaking from where I had to pull it for the boost valve replacement, but now I'm not sure because of the other thing it does. If you drive the truck regularly, like at least every few days, the trans shifts fine. If you let it sit for more than a week or so the trans will not go into gear at first when you start it up. Give it 5 or 10 seconds in drive and afterwards it will shift fine all day and will be fine the next day as well until you let it sit for more than a week. Seems to me like the converter is draining back into the pan, or possibly my external leak is related and the converter is leaking fluid externally somewhere. Anyone know where the check valve would be to stop converter drainback? I'm thinking that either my converter itself might be leaking, or that I do in fact have a cooler fitting that's leaking and it's letting fluid out of the converter and onto the ground? I don't like having the converter starting empty so often, not as concerned about the mess or keeping an eye on the trans level. Maybe someone else has had an issue with converter drainback? I will also note that the pump in this trans showed some definite wear, although it continues to make spec pressure. I suppose my drainback could be through the pump, and the leak completely unrelated?

 

[Supercharged111]

I think both of mine do that. I know the dually does it. I don't notice it so much on the 1500, but have experienced it before. My driveway is inclined so by the time I back down it and stick the thing in drive it's usually good to go. The dually tends to sit in the hangar where it's flat so I notice it more there.

 

[df2x4]

If you drive the truck regularly, like at least every few days, the trans shifts fine. If you let it sit for more than a week or so the trans will not go into gear at first when you start it up. Give it 5 or 10 seconds in drive and afterwards it will shift fine all day and will be fine the next day as well until you let it sit for more than a week. Seems to me like the converter is draining back into the pan

I swapped a 4L80E out of a 350K+ mile junkyard truck into my red '97 and it does this same thing. I asked my mechanic about it and he also suggested it was the converter draining back. He told me that any time the truck has been sitting for a week or better to just put it in neutral for 5-10 seconds to engage the pump and then go to whichever gear I actually want, and don't worry about it any further. I took his advice and this method has worked flawlessly for the last few years.

Mine has never leaked any fluid though, knock on wood...

 

[JLeather]

That's good to know. I'm not especially bothered by it so long as it isn't a sign that something is gonna let go. Clearly it hasn't bothered me enough to go digging into it in the last couple years

 

[stutaeng]

My 2000 K3500 CC LB SRW does something similar, except it also sometimes bogs the engine and sometimes stalls it in reverse. In Drive it only does it on the first mile when I stop, then it's okay. 208k miles, probably original unit. Bought with 204k miles about 3 years ago as a weekend work truck for DIY projects. Was a ranch truck is all I know...that means it was worked really hard, LOL. Had a gooseneck hitch.

I'd like to figure out mine at some point. It's a little annoying.

My GMT 800 04 and 06 Suburbans don't do this, so it seems like this affects the earlier units. Dunno. But I don't think it's the converter draining?

 

[Supercharged111]

My 1500 does the bog thing. Again, I suspect torque converter. I think it may be the sprag inside pulling that nonsense. Only does it in cold weather for the first couple miles then it's fine.

 

[stutaeng]

There are some differences in the sprag, and I think it's a different break from the 91-97 years from the rear fluid cooler change.

Maybe it is 91-00? Let me look into that. There's only 2 models of sprags IIRC.


But the sprag corresponds to the drum, so you can't just swap the sprag...

 

[Supercharged111]

There are some differences in the sprag, and I think it's a different break from the 91-97 years from the rear fluid cooler change.

Maybe it is 91-00? Let me look into that. There's only 2 models of sprags IIRC.


But the sprag corresponds to the drum, so you can't just swap the sprag...

I meant the sprag inside the converter.

 

[stutaeng]

Is that the 0.195" sprag? 34 element in the '01+ models?

I honestly don't even know what a "sprag" is or does, LOL.

Edit: just saw last previous post...

 

[Supercharged111]

Is that the 0.195" sprag? 34 element in the '01+ models?

I honestly don't even know what a "sprag" is or does, LOL.

Edit: just saw last previous post...

If I'm remembering the operational theory correctly, when the sprag freewheels is when the torque converter multiplies torque. When it doesn't you may as well have the converter locked. A sprag is basically a 1 way bearing.