4r100 issues

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Jul 4, 2025 | 03:08 PM

rob6095

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5th Wheeling

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4r100 issues

Hey everyone,

I am having a couple of issues. I have an 04 f350 with a 5.4 that is acting up. When it gets hot it’s puking trans fluid out of either the breather or the front seal area. Not sure which. I’ve been doing some research and the truck spent the entirety of its life on the coast in Hilton head sc so I’m wondering if the banjo bolt has frozen up on the bypass line or if an o ring or seal on the front has just started to go out. How involved is replacing with those if I drop the trans? Never messed with a 4r100 but I’m pretty handy and have done transmission swaps in the past. Does anyone have any tips or tricks on how to narrow it down or to test if it’s a bypass or an o ring?

Jul 4, 2025 | 03:43 PM

Mark Kovalsky

Frmr Ford Trans Engr

Joined: Aug 1999

Posts: 24,730

Likes: 2,658

From: SE Florida

I'm not following how you came to that conclusion. It doesn't make sense to me.

Is the trans overfilled? That will make this happen. So will a damaged torque converter seal.

Jul 4, 2025 | 03:53 PM

rob6095

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5th Wheeling

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It’s only leaking when particularly hot or when towing or loaded heavy. Checked trans fluid and it’s clean and doesn’t smell burnt. Last time it did, it was over 100 degrees outside. Checked fluid today and it was barely on the dipstick. Trying to figure it out now. Current plan is to upgrade the cooling system but I’m trying to figure out if there’s something else going on.

When searching I’ve seen that the rubber seal does go bad sometimes and that the transmission cooler bypass can seize up. I’m looking for a good way to check and rule out different things. It’s entirely possible it was overfilled as well. It did this before when backing up loaded in 2wd with a heavy trailer and I added fluid after that.

Jul 4, 2025 | 04:18 PM

Mark Kovalsky

Frmr Ford Trans Engr

Joined: Aug 1999

Posts: 24,730

Likes: 2,658

From: SE Florida

Quote:

Originally Posted by rob6095

It’s only leaking when particularly hot or when towing or loaded heavy. .....It did this before when backing up loaded in 2wd with a heavy trailer and I added fluid after that.

Those two point to the torque converter seal. I can't see how the bypass could do this.

Jul 4, 2025 | 05:21 PM

rob6095

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5th Wheeling

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Torque converter seal sounds right. Do you know how much trouble that is to do or a good way to double check that is what it is? I wasn’t sure on the bypass but it came up in my research. Is there a way to recondition the seal and not have to pull the transmission? I’m assuming there’s no way and it’s going to be pulling the trans and replacing the seal at the very least.

Jul 4, 2025 | 09:09 PM

Mark Kovalsky

Frmr Ford Trans Engr

Joined: Aug 1999

Posts: 24,730

Likes: 2,658

From: SE Florida

I don't know of a way to recondition the seal.

It isn't that much trouble to do, All you need to do is pull the trans out, remove the converter and the pump. The pump needs to come out to replace the bushing behind the seal.

There are aftermarket seals made from Viton which is MUCH better than the stock seal.

Jul 8, 2025 | 08:19 PM

Gavin Mollard

1st Gear

Joined: Jul 2025

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Help with a Valve Body

Mark I tried shooting you a direct message but it won’t let me. I’ve got a guy that’s developed a full manual valve body for the 4R100 platform. He’s primarily a mopar guy but he’s been investing time into the 4R. He needs some insight on the pump flow and such. He would like a more direct line of contact is that possible for you to take a call to bounce some ideas off of?