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4L80E top vent tube question

24K views 10 replies 4 participants last post by  ChevyTruckGuy 
#1 ·
I was looking around my 2500hd and found and open vent line that dumps out over the transfer case. At first I though I had a leaking seal. But it coming from the vent tube line. Now the line goes to the top of the trans to a vent line I assume its a vent line. Then splits in to a why which goes to the top of the trans and a check valve. I also assume there should be one on the other end. Any help as to what supposed to be there would be great thanks.

Craig
 

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#2 ·
both tee into each other ones for trans other for t case run the vehicle and check transmission fluid if over full it will vent out or if the trans is over heating but a common issue is the input seal on the t case goes bad letting transmission fluid to fill the t case or t case fluid filling the trans making a overfull condition
 
#3 ·
Do you have a temperature gauge on this transmission? I ask because one of the more common failure points we see on the 80E's is the torque converters. If you have a gauge you will see high temperatures when driving at speeds of 60+ for extended periods, or you may see high temperatures just driving around town. Either way, fluid coming out of that vent is a bad sign and you need to figure out why it is happening. I would put a gauge on the trans immediately and watch what the temperature does. If it is running high, it's time for a converter swap, flush the existing cooler(s) extremely well OR put in an external cooler, and pray you got it in time before the failing converter contaminates the transmission with metal & clutch debris. Typically by the time we see them, the converter has dumped it's guts back into the trans requiring a complete rebuild. The symptoms you describe are what the customer describes as happening shortly before it stopped pulling in any gear, leaving them stranded.
 
#5 ·
Yeah I have the LT package so it has all the bells and whistles. The trans temp gauge read between 100- 150 on the freeway In the mountain about 160-180. It has an oil cooler. It more like sweating out fluid over a weeks time. the t case looks like it might of been repaired the bolt head have been painted black. As far as the converter goes the shift points feel soft and you have to step on it to get it to kick down. I don't know if it was tuned or adjust this way. It was a fifth wheel rig at one point.

Craig
 
#6 ·
the temps are normal the 4l80s do shift smooth from the factory if its not slipping or flare on a up shift your ok there
the t case vent is toward the front drive shaft
if yours is 1996 or newer get a programer i use Hypetech E CON a lot not only does it tune the engine it rasies transmission pressures and the shifts are quicker and i recomend to do a transmission service once a yr (filter and fluid replacement no flush but use a synthietic ATF like Amsoil or Moblie or Royal Purple
 
#7 ·
I will check for the vent tube in the morning It a 2001 chevy 4x4 crew cab 2500hd. I need to do the service soon when you buy a used truck you have know Idea when or if at all anything was done. 145,000 miles when I pick it up. 147,000 now. I have been debating on what fluid to use.

Craig
 
#9 ·
I can't say why but what you have is normal for an unmolested stock setup. Why a an open drain at the bottom and a vent at the top is a question for the GM engineers I guess.
By the way, those aren't check valves. They are just vented caps. Around here if you leave any sort of small, dry round hole open a dirt dauber will be along shortly to build a nest in it and plug it the hell up. Those caps work great to prevent that.
 
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