4L80E Intermittent Leak, Morning Sickness

[Carlaisle]

I also have a 4L80E with morning sickness. If you drive the truck every day or even every few days it doesn't leak or lose any transmission fluid. Leave it sit for a while, and a bunch of it disappears. I attributed it to felonious squirrels, but it hasn't annoyed me enough yet to set up any CCTV to try and catch them in the act. If someone knows for sure what's happening, I'd be very interested.

One thing I have noticed is that the fluid is very high on the dipstick if you check it cold after it's sat for a long time (say, a couple of weeks to a month) but I don't have another 4L80E at present and haven't made a habit of checking cold transmission fluid levels so I don't have anything to compare against.

 

[Transamco]

The answer is actually a very simple one. . . it's converter drainback. The converter is full of fluid, right to the very top, and with the motor stopped ( and running of course ) the fluid level in the trans is substantially lower - usually a little above the pan gasket. When you switch off the motor and let it stand, the fluid in the converter will slowly drain back into the trans. Various things slow this process but wear in multiple areas increases the drainback rate. In an old 4L80, wear in the pump bushing, stator bushes and the inner pump gear (where it mates up to the converter spout ) are all going to increase drainback.
When you start the motor after a period of standing, the converter needs to be recharged before normal "drive" is acquired, and how long this takes is governed by how much oil needs to be pumped back in. To a certain extent, even a new trans will suffer from not having positive drive for 5 - 10 seconds after stading for lengthy periods.
Often the rate of drainback varies, as a number of factors can be subtly different due to minor changes in the position of the converter/pump gears each time you stop the motor.
During the drainback, the oil level in the trans increases, and can come into contact with various seals that don't normally get much oil near them, and if they aren't in great condition they can be the source of a leak. If enough drainback occurs, the fluid can reach the case breather - nothing to stop the oil flow there.

 

[Supercharged111]

The answer is actually a very simple one. . . it's converter drainback. The converter is full of fluid, right to the very top, and with the motor stopped ( and running of course ) the fluid level in the trans is substantially lower - usually a little above the pan gasket. When you switch off the motor and let it stand, the fluid in the converter will slowly drain back into the trans. Various things slow this process but wear in multiple areas increases the drainback rate. In an old 4L80, wear in the pump bushing, stator bushes and the inner pump gear (where it mates up to the converter spout ) are all going to increase drainback.
When you start the motor after a period of standing, the converter needs to be recharged before normal "drive" is acquired, and how long this takes is governed by how much oil needs to be pumped back in. To a certain extent, even a new trans will suffer from not having positive drive for 5 - 10 seconds after stading for lengthy periods.
Often the rate of drainback varies, as a number of factors can be subtly different due to minor changes in the position of the converter/pump gears each time you stop the motor.
During the drainback, the oil level in the trans increases, and can come into contact with various seals that don't normally get much oil near them, and if they aren't in great condition they can be the source of a leak. If enough drainback occurs, the fluid can reach the case breather - nothing to stop the oil flow there.

This all makes good sense, but the 4L80 is the only trans I've ever experienced this with.

 

[Transamco]

This all makes good sense, but the 4L80 is the only trans I've ever experienced this with.

Totally understand, your description of your issue that is present for the first couple of miles, indicates something completely different. Even a totally dry converter will be fully charged in less than a minute of run time, so if there's enough fluid in the unit it won't exhibit that symptom.
A sticking valve might though

 

[oldbejuanyaknowme]

I have the same trans with a different problem: If I park my truck facing down my steep driveway for a day or a month, when I start it and let it warm up it goes right into reverse and once up the driveway, and put into "drive", it pulls away perfectly and shifts perfectly. However, if I back it down my driveway and let it sit for a week, it doesn't. It acts like it is very low on transmission fluid and will slip and not go forward. Once I get it up to the street and head downhill, within a couple of miles it works perfectly. I'm confused. Here is the worst thing: I had the transmission rebuilt due to this and the new one (less than 7K miles) is doing the exact same thing!

 

[JLeather]

I will note that I always park this truck nose uphill so the bed doesn't collect water/crud. That may well be exacerbating the situation.

Sent from my moto g(6) using Tapatalk

 

[Schurkey]

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?

This is why Society invented PARAGRAPHS.

As said, your converter is draining. You need to find out what seal(s) are leaking, and fix them. This doesn't stop the converter from draining, but it should prevent loss of fluid.

I'd also be suspicious of a partially-plugged trans filter. Refiling the converter shouldn't take very long, unless the pump has to labor against a filter that won't flow like it should.

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.

Other way around.

Two common types of one-way clutches: sprag and roller. Both do the same thing--allow rotation in one direction while locking in the other direction.

The one-way clutch in a torque converter controls the stator. The stator re-directs fluid flow when the converter flow pattern is toroidial. When the converter flow pattern is rotational/rotary, the stator just gets in the way, so it's held on a one-way clutch that freewheels during rotary flow.

If a stator one-way clutch fails to hold the stator, the vehicle will have poor acceleration and low-speed performance, but can feel normal at highway speed.

If a stator one-way clutch fails so that it locks up in both directions, the vehicle will be normal at low speed, but have very poor higher-speed performance, and overheat.

Toroidial flow = locked stator = torque multiplication.
Rotary flow = unlocked stator = no torque multiplication.

https://ateupwithmotor.com/terms-technology-definitions/hydramatic-history-part-2/2/

 

[stutaeng]

According to this guy, he calls it "drain back." But he never mentioned the torque converter being the problem.

It looks like worn pump bushing, gears and the stator are the problem in this case. I think that's an excellent video.

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