: Transmission Fluid Under Pressure at Cap?

05-01-05, 03:34 PM
I went to check the tranny fluid today, and found something a little odd. When I opened the cap, it let out a 'hiss.' The fluid was where it was supposed to be, color & odor were ok. When I started the car, the fluid level quickly rose above the filler, and would then just as quickly back down. There is a small "return" port visible through the filler neck, and fluid which seemed bubbly & semi-foamy would periodically be dropped back into the tranny through the port. When I revved the engine a bit, the fluid level would ride above the neck, and spill over.

The transmission doesn't slip, doesn't bang into gears, and seems to operate normally. There's a little bit of 'lash' when you depress the gas from a stop, but that seems no worse than it has been in the past.

This sound ok?

Thanks for the help!

'97 ETC w/ 80,000 miles

05-01-05, 05:28 PM
First off, you are supposed to check the trans warm and idling in park. If you were "full" with the engine off, then you are likely overfull. It will rise when you start the car. I believe that is normal. Look just to the right rear of the TB when lookin over the left fender. There is a rubber tube sticking up with a metal cap attatched to it. That is the trans breather tube. Make sure the little cap on top is "loose" so it can breath. It is not removable. I am not positive but I don't think the "hiss" is normal.

05-01-05, 06:27 PM
I found the breather tube, which wasn't properly connected to its bracket by the TB. The top of the breather tube is free, but had a buildup of gunk all around it. I cleaned it up a bit, and reattached it to its clip. It doesn't seem to be blowing any fluid out the breather. I can't see how the car could be overfilled, since the fluid and tranny haven't been touched since the car was new. The fluid looks clean, smells okay, and doesn't have any debris in it. Is there a way air could be getting into the tranny and causing it to 'burp?'

Maybe I'm being too concerned - it still operates flawlessly!

05-01-05, 06:53 PM
Air in the tranny causing it to burp does not sound logical but I don't know that much about it. See what Bbob has to say if he chimes in on this one.

05-01-05, 07:21 PM
FWIW, I'm just horning in and making guesses. It stands to reason that if there is pressure in the transmission the breather has to be stopped up somewhere. Maybe the high fluid level when checked with the engine off is from drain back or the pressure from the stopped-up breather is forcing the fluid up the dipstick tube.

05-01-05, 07:29 PM
If it's foaming, either you're overfilled, or underfilled. Either one is not good. The clogged breather is what gave you the hissing, but wouldn't cause the fluid to foam. Take the tube off the transmission, and clean it with soapy water or brake cleaner to get all the crud off (if you haven't already), otherwise you are risking seal leaking.

05-01-05, 07:39 PM
I guess I would check the breather to make sure that it is open. You could pull the little cap assembly off the end of the hose to make sure it it venting and that the hose is open to the trans (blow thru it with the filler cap off) but I don't think anything at all is wrong. It sounds like things were cold when you checked it and started the motor up...??

My suspicion is that the little cap on the transmission vent is venting pressure fine but may be sealing up on suction. Those caps are the same caps as are often used on rear axle assemblies and canister purge fresh air lines and I know there are instances of them not venting vaccum very well...(don't ask how I know this). They were originally designed and intended to vent pressure so venting vacuum is a secondary requirement. The little cap can suck back onto the top of the tube it is staked onto and seal if the tiny slots in the sides of the tube are closed over by deposits or flashed over by plastic from the die. It is no big deal as the trans vent is there for venting pressure as the unit heats up. As it cools down, pulling a slight vaccum inside the case might happen if the cap on the vent line didn't allow vacuum venting....but....who cares. Having a tiny level of vaccuum (the hiss you heard) is nothing at all to be concerned about.

When you have the little cap off of the hose try sucking thu it like the trans would to see if it is restricted in that direction. You should be able to blow thru it, as in relieveing pressure, very easily.

The burp of fluid you see starting is normal. You are not supposed to start the engine with the trans cap off. Always check the trans fluid HOT and idling in park. That is the only time that taking the cap off matters.

The 4T80E is a dry sump transmission. There are two oil pumps. One simply scavenges oil from the bottom pan thru the screens and dumps the oil into the side cover for storage and to feed the main line pressure pump. The variable pressure/variable displacement line pressure pump sucks from the side cover reservior (that you are looking at thru the cap) and delivers line pressure to the trans.

When you started it cold with the cap off you saw the scavenge pump pull all the oil that had drained into the bottom pan and dump it into the side cover. Since a lot of the oil that is circulating thru the trans normally had settled to the bottom pan it takes the line pressure pump a few seconds to refill the trans itself. So...all the excess oil scavenged from the bottom pan "overfilled" the reservior in the side cover momentarily until the line pressure pump equalized the level by pumping oil into all the "vacancies" inside the trans.

I would expect to see exactly what you described under those conditions.

Check the level hot and idling in park. Don't start the engine and check it...drive it until hot, stop, remove the trans dipstick and check the oil level while still idling.

Everything sounds perfectly fine with your trans......

05-01-05, 08:32 PM
After following the correct procedure for checking the level, it checked out okay! Now I know not to be opening the cap while starting the engine - I have NO idea what inspired me to try that.

Thank you all for the responses - Now I'll be able to sleep tonight!


05-02-05, 01:55 PM
Now I know not to be opening the cap while starting the engine

Man, you must have some loooong arms! :)

05-02-05, 08:37 PM
Well, I don't drive a caddy because I'm short.... at 6'8", my choices for cars are limited!

...plus, long arms made adding the blinker fluid a little easier!