TH400 blowing ATF @dipstick

Discussion in 'GM Powertrain' started by TomW, Jul 1, 2008.

  1. TomW

    TomW New Member

    The TH400 transmission in my 1986 Suburban C20 with 7.4 liter engine shifts fine even when towing a 5,000 lb travel trailer. The ATF color is good, and there are no leaks.

    A month or two ago, I noticed ATF coating the dipstick tube. A week or two after that, I checked the oil level (level surface, engine warm but not hot, go through the gears, etc) and had to add two quarts. Now I knew I had lost some ATF during two different radiator change-outs, but was surprised the level was so low. I also verified the vent tube on top of the tranny was not blocked.

    Last week, we took the travel trailer out of state, and the Suburban ran & shifted fine. Then an odd thing happened. We were set up at our campsite one morning (unhitched) when I remembered one of the trailer's propane bottles was empty. The campground had a refill station, so I loaded the empty bottle in the Suburban, cranked the cold engine up, and drove the block or so to the refill station where the engine was then shut off.

    Around ten minutes later, the motor was started back up, and I drove back to the campsite. After backing into the site, I saw a path of oil drips which corresponed to my back-up path. Popping the hood, I found oil running down the ATF dipstick tube & splashed on the exhaust manifold.

    I have heard of ATF boiling out under heavy load, but this was a cold engine. And, when towing, I somewhat baby this truck because it is old and I like it.

    The transmission has around 60,000 miles on it since being rebuilt around 10 years ago. Like I said, it shifts fine, and the ATF oil color is not dirty.

    What could cause ATF to blow out of the dipstick tube on a cold engine?

  2. tbplus10

    tbplus10 Epic Member Staff Member 5+ Years 5000 Posts Platinum Contributor

    You've looked at the leading causes, blocked vent tube and over filled trans.
    When was the fluid and filter changed last, even though the fluid isnt dirty you could have a clogged filter or the fluid could've boiled at one point and is now heating prematurely. Another possibility is with 2 qts low you could've been cavitating the pump and foaming the oil, foamed oil will expand and overflow the dipstick tube (like shaking a soda can before opening it) this will also boil the oil.
    I'd probably have the fluid and filter changed, maybe even a full flush, then keep an eye on it.
  3. TomW

    TomW New Member

    The master plan is to take the truck back to the guys who rebuilt the transmission and get their opinion. Luckily, since the Suburban is not my daily commuter, I've got a little time.

    I don't know too much about automatic transmissions, but I am baffled as to what the problem is. I hope the transmission shop has seen this before.

  4. Crawdaddy

    Crawdaddy All hail the Mad King!! Staff Member 5+ Years 1000 Posts Platinum Contributor

    This is sometimes an issue that occurs when these transmissions are installed in race vehicles, and they sell special locking dipsticks for that, but you shouldn't have this issue. I have no clue what's wrong other than tbplus's suggestions...

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