I've often wondered how much pressure the pump in Zf6 will make. Could be only. Leaks when it gets a little pressure on it.

Are you pulling the block drains when you flush coolant? If you don't pull the block drain there still 1.5-2 gallons still in block and everytime you refill it it just dilutes the mix down.

 

You'll be able to get a lot more of the polluted coolant out or the engine by removing the block drains. One on each side of the engine. One behind the starter, the other one directly on the other side of the block on the wall of the block a couple inches above the oil pan "gasket". Very easy access on the left side of the engine, the other side, you do not have to remove the starter, it's not impossible, just tight quarters.

 

And Simple Green poured straight in is about the only thing that works. I've used it and it makes a LOT of foam to rinse out, but it cuts the oil.

 

In my searching around, I had read the ZF pump puts out as much as 16 psi, which is higher than what the rad runs at (12 psi cap?). I hadn't thought that maybe the trans pump is blowing out a passage while underway, only for it to "heal" itself once the pressure decreases. Could explain what I'm seeing.

Not pulling block drains. Bad weather is here and I can ill afford a problem. Last year I did the whole flush thing on my e99, plugs included. The drained fluid was cloudy, but no where near what I have going on. It wasn't so cloudy that it obscured the coolant color, like this is. But I also wasn't dealing with oil in the coolant either. There is no trace coolant color in the drain liquid. This is a real ugly gray. It is true that the detergent will help emulsify the oil, but the lack of separation after sitting is puzzling, if it is still residual oil. Could be it's the first drain/clean in it's 20 years for all I know.

 

I'm using 1.5 cups of dish detergent per nominal 6-ish gallon of water. The drained liquid is still pretty soapy when it comes out.

And by "poured straight in", are you putting multiple gallons of that in there?

 

I would have to be at least a little suspect that it wasn't a coincidence that this trouble kicked up 6 miles after leaving the shop that just worked on that part of the truck, although I don't have any theories other than they did something stupid like blow 100 psi shop air through the cooling lines to "clean them out" and damaged something. (Oops, I guess I do have a theory!)

IDK why the trans oil wouldn't be contaminated with coolant. That cooling core is immersed in coolant at all times and a hot cooling system will stay pressurized for quite awhile after the motor is shut off, so you'd think water would push it's way in. Are you sure it's ATF and not motor oil from bad oil cooler rings?

 

I thought the cooling system was right around 15 or 16 psi? Or its supposed to be assuming the cap is working right.

 

Yeah, what I was working on was a VW that blew an oil cooler. Same idea, water to oil heat transfer and they would occasionally fail. Oil gets pumped into the cooling system. I removed the thermostat, bought enough Simple Green to fill the entire cooling system and let it run for a while. Drained and flushed. Took a lot of flushing to get it all rinsed out, but it eventually cleaned up with no oil residue whatsoever.

 

100% positive on ATF. When I first noticed the prob, I extracted some off the top of the degas bottle before doing anything else. It had been sitting all night so it was nice and separated. Color and smell=ATF.

 

I'll have to guess they did something then. Maybe they removed the cooling lines on the radiator end as well as the transmission end to get them out of the way of the transmission R&R and accidentally broke one of the fittings on the cooler either getting it off or putting it back on. Or maybe they didn't even know they did it. Doesn't really matter at this point because I doubt they'll be fessing up to anything and buying you a new one two weeks after the fact and you've already starting digging into it yourself. If it's the original 23 year old radiator a new one wouldn't be the worst thing anyway. I would strongly consider doing the oil cooler as well at the same time if it's never been fixed since a lot of that labor overlaps. Maybe even the water pump since again, you're in there anyways and a lot of that labor overlaps.

Depending on what the lines looked like I'd maybe even consider replacing those while you're at it. Thread about those including pictures and part numbers here. As an aside, I found one of mine rubbing pretty hard on the front sway bar the other day trying to rub a hole in itself...this might be an issue or quirk specific only to trucks with the ZF6 and the 2wd front end, which there aren't that many, but worth a quick look under there. I was under the impression these were sold as an assembly since the lines on mine are banded together with these little c-clamps but apparently they are sold individually. The clamp I took off was part of the problem...removing it definitely freed the line up from rubbing as hard, but it's still touching. Other one clears the bar with a couple mm.

 

The transmission cooling lines are sold separately, and always have been.

I replaced all of my transmission cooling lines 21 years ago, in order to add an OTW cooler. My truck was not originally built with one. I had to order each line separately.

When installing the new cooling lines, I bought a little bag of those line clips from Ford, and used all that were in the bag, along with reusing the original clips that I pulled off of my old lines.

With clips every six inches or so along the parallel runs of those two lines, they have stayed in their intended position over the last 21 years, without rubbing on anything.

 

I stand corrected on the cap rating. I think 16 is the standard, not 12. Either way, the pressure had to be much lower than this given my starting conditions were around 40F when the leaking started. I was not under heavy load, or any long runtimes for that matter. If the ZF6 transmission pump runs at/near 16, then in all likelihood it was ATF leaking into the coolant - under running conditions.

Now what is happening when things cool off and pressures equilibrate? I am now less inclined to think the coolant heat exchanger "sealed itself" once the trans pressure backed off to the point no coolant would leak into that system.

 

If you only drove it an easy 6 miles in 40* weather then probably the cooling system wasn't very warm or highly pressurized at all...takes awhile under decent load for the motor to get hot enough to reach the cap pressure. On the the other hand if the trans pump starts pumping oil right away at 16 psi as soon as the motor is started, that would explain it. It might have stopped pushing ATF into the water when you shut it off and it's just a tiny crack, with oil and water not liking each other anyway. With the truck off and cold the pressure on both sides of the crack would be nearly equal and not cross contaminating much or at all.

 

It’s pretty much impossible to flush this cooling system without removing the block drains at each flush interval and running tap water through the open drains.

 

I am going with this, because it is the only theory that explains what went down. It is either that or at the shop they mistakenly poured in a couple of quarts of ATF into the radiator.

​​​​​​Fortunately, my plug and cap effort has worked so no continued issues at the moment.