V8 00USH, I have to ask, why did you add in the Mocal brand thermostat? Why not just have the factory return line connect directly to one side of the external tranny rad and then the other side of the external tranny rad connection just connect up with the factory return line again. A simpler design no? AND then you are always cooling the tranny fluid with factory cooler and then external cooler... OR am I missing something here?
Glad it fixed your issues. I know FMugur was having similar issues but doesn't seem to have the same fix to his harsh tranny slam problem (according to another thread) sadly.

Simply because I didn't want to risk overcooling the transmission fluid - after all I went for the largest oil cooler that would fit in the space. As it is the transmission fluid now sits perfectly at the thermostat temperature - if I hadn't fitted that then more than likely it would be sat under that which then wouldn't be at all desirable for the gearbox.

So the Mocal thermostat is set to open at what temp?

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Approx 80deg C

I haven't read the entire thing. I'm about to go to sleep. But my 4.8iS threw the ole transmission overheat at me again today for the 1st time since the atf fluid and filter change and new seals.
Granted it was well over 100F today in San Antonio, but I wasn't rowing anything or driving aggressively.

So after a bunch of research and talking to a friend. I think I'm gonna bypass the OEM tranny heat exchanger and run an aftermarket cooler up front.

I will catch up on the rest of the thread tomorrow.


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OE system is more than adequate except perhaps in heavy towing in the mountains. I would troubleshoot the problem and determine the cause. Overheating may be a symptom rather than the cause.

I disagree it's adequate, but I do think something else is amiss. That said, no errors with INPA since the filter, fluid and seal change.
So whatever it is, it's not giving me a clue.
Everything shifts fine, and smooth. 1-2 downshift coming to a stop is sometimes rough when it's hot as balls out.
So I'm putting on a better cooler.


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I don't understand how you can conclude the trans cooling is inadequate when something else is amiss. Might save some time and expense to have transmission diagnosed at a trans shop.

I have over 118,000 Texas miles on a heavily modified 4.6. Trans has Dinan software and I change the fluid and filter about every 40,000 miles with ZF fluid. Never a problem with cooling or anything else. That is anecdotal but poor OE trans cooling hasn't been a common thread here to best of my memory. Just FYI.

External Transmission cooler?
 

It may not be, but I think the atf is allowed to run too hot. 200F with no towing shouldn't be seen.
ATF doesn't hold up to heat like engine oil does.
Maybe the cooler is adequate, maybe a cooler thermostat is the solution. Either way, the short life of the rubber seals and the solenoids failing. Which both lead to other issues. Do you really think that just happens because of how they're made? No, the transmission runs way too hot and essentially slowly cooks all the rubber and plastic parts which causes other issues.

Think about that. Take the average diesel HD pickup. They run 170-180F ATF driving around and only go up a tad more when towing.
With 180F ATF, can you imagine what the temp is inside the torque converter.

I'm no expert, but I truly believe most of the issues with these X5s would be avoided by running cooler. That's engine and atf thermostats.


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Snik - I am FULLY in agreeance with everything you've said above. Hence doing what I did and hence changing for an 88 degree coolant thermostat too at the same time as doing my chain guides recently.

I'm a firm believer that if the fluids were ran slightly cooler from factory then everything would last significantly longer. Only reason I believe coolant temps were designed to run at the level they are was to meet emission requirements.

Since my changes my coolant now sits at around 95degrees (13degrees cooler than before) and my transmission fluid sits between 75-80degrees and has been considerably happier for the past 25k+ miles