C6 overheating mysterious cause

[Wrenchmonkey316]

I’m upgrading the transmission cooling system because it seems to be what is causing the overheating issue. When I took it to the diesel shop the radiator shop and the transmission shop they all said the transmission, radiator and engine are great.

I’ve done a compression test, timing, fan clutch, thermostat, shift modulator, new trans fluid, and it’s just slightly better but still overheating really fast under load.

I explained to all of the professionals that a short steep hill will quickly cause heat problems BUT it can go ALL DAY 3k on flat hwy. Also handles stop and go traffic fine and also around town no problem if it’s flat.
“Sounds like it’s the transmission” they all tell me.

The transmission has no shifting issues and has low miles because the previous owner had a rebuild done. I’m wondering if maybe a subpar torque converter was installed it could cause the transmission to overheat like this under load? Or maybe something else?

 

[franklin2]

How do you know it's overheating?

 

[XOLATEM]

a short steep hill will quickly cause heat problems BUT it can go ALL DAY 3k on flat hwy.

Got a few questions before we go further...

What is your total combined load you are pulling...how much total weight..?

What gear ratio do you have in the rear axle..?

Do you have a tach...? and if so...is it reasonably accurate...?

If yes to the above...what RPM can you achieve at full stall from a stop...? by powerbraking the truck...what RPM does it give you before the tires brake loose...? (DO NOT do this in reverse...)

and finally...

How do you know it's overheating?

What leads you to believe that it is overheating...? Do you have an actual temp number and how did you arrive at it and where are you measuring the fluid temp..?

Or...is the fluid blowing out of the vent, fill pipe, or front seal..?

To my way of thinking...all of this should be taken into account...

 

[Wrenchmonkey316]

It will blow fluid out the top if I get it too hot. I did I once already. It has 32” off road so the brake stand test might not work well? I have been shooting it with a temp sensor to confirm my stock gauge is still a functional dummy gauge and it’s fine, (no) is safe ( r ) is stop time (mal) is oh no please don’t die! But I’m doing new gauges for sure. The rpm works good. It’ll do 75-80 easy on the hwy at 3k. The bed has a rack and side boxes with an average load of tradesmen tools. I’ve hauled a tractor and dump trailer with a similar c6 idi rig up and down a canyon and it didn’t overheat, its just slow. So the weight is not a factor either. Any thoughts?

 

[Cubey]

It’ll do 75-80 easy on the hwy at 3k.

You need to slow down. That's the problem.

 

[Wrenchmonkey316]

I drive in the slow lane usually I’m running an overdrive unit so it’ll do 75 easy if i want to burn a ton of fuel. Doesn’t get it hot though unless you get on a steep grade. On hills it doesn’t matter if you’re going fast or 10 mph, it just heats up quickly.

 

[franklin2]

When you get the trans warmed up, put it in park with it idling and pull the dipstick. Wipe it off and then put it in again and when it's idling, what does the fluid level look like? Is it above the "H"? If the trans is a little too full, the height of the fluid can get up into the moving parts of the trans, and this will whip it into a foam and make it shift funny and puke out of the dipstick tube.

 

[KansasIDI]

F Super Duty trucks weren’t designed for freeway speeds, that’s one of the downsides of them. Otherwise I’d be tempted to get another one, but they’re just regular cabs…

On the governor in 5th gear (ZF5) I’m at 85 mph, with 5.13 gears.

Higher speeds in your auto tranny will heat things up some, but you should still be able to drive like that without getting too hot, especially unloaded…

I haven’t been thorough reading this thread, but flushing your tranny might help. Had a friend who had a truck with a 4R100, was having temp trouble, he flushed it out and that seemed to fix it, he drives it around all over now…

 

[franklin2]

C6 transmissions puking fluid out is not a common problem with those. Something is wrong somewhere. While they are fuel hogs, they are good transmissions.

 

[Wrenchmonkey316]

C6 transmissions puking fluid out is not a common problem with those. Something is wrong somewhere. While they are fuel hogs, they are good transmissions.

Yeah it got really hot once and did that. I just noticed that the 2nd fan clutch I just bought spins about 4-5 times when I shut off the engine. So frustrating. No more Napa brand fan clutch for me. Sad

 

[Cubey]

Yeah it got really hot once and did that. I just noticed that the 2nd fan clutch I just bought spins about 4-5 times when I shut off the engine. So frustrating. No more Napa brand fan clutch for me. Sad

I got a Four Seasons one via Walmart recently and it seems to work ok.

Do you have the transmission connected to the radiator cooler like it should be? Even with an extra cooler, it seriously needs it to be connected or it'll cook the transmission.

 

[Wrenchmonkey316]

I got a Four Seasons one via Walmart recently and it seems to work ok.

Do you have the transmission connected to the radiator cooler like it should be? Even with an extra cooler, it seriously needs it to be connected or it'll cook the transmission.

I swapped it for a new Napa one and it’s working, for now. I did a good long test drive today with the deep pan and rapid cooler upgrades. I took it up long grades at freeway speeds and up some really steep grades going slow on residential streets as well. It never got hot. It stays pretty cool like my son’s 1990 does with a zf5. I’m really happy. Thank you!

Yes I kept the radiator in the equation and it now has two aux coolers in the front side by side as well. It had a little one already so I linked them together. Still surprised It took 9.5 quarts to fill it back up.

 

[Cubey]

9.5 sounds about right with a deep pan and coolers.