It's chilly in the Pacific Northwest. I made a run to town today and my stock guages alerted me to the fact that engine temps were low long past the mile post where NORMAL operations usually commence. I never got close to the N. The heater was not blowing hot air and I could grab hold of any part of the radiator or top hose. Dag nabbit the thermostat is stuck open on my 6.9 IDI.

I ordered the replacement thermostat and I was about to order a tube of Permatex form a gasket sealer number 2 to dress the gasket. I'm old school and that's what I've used before for gasket sealer but I thought I'd ask the brain trust here if there is something else I should consider for the thermostat housing gasket sealer?

 

Seems that stock temperature gauge worked perfectly for ya, didn't it? Good thing you weren't relying on some aftermarket knee-cutter gauge tucked under the edge of the dash. You might have easily missed the problem. (Paging Dave, paging Dave. Please pick up the white courtesy phone...)

For sealer, Permatex #2 is my go-to. Goes well on crackers, too.

 

Do some research and make sure you have the correct thermostat. The old diesels take a different thermostat and the stores like to sell you the incorrect one.

 

The book says I need a E5TZ-8575-C . On line parts check for my truck says the same. Fingers crossed that OEM means original equipment manufacturer and not Oriental Equivalent Made. Part is ordered and on the way...

 

Almost forgot: Before installation, take a few minutes and test the thermostat in a pot of water on the stove. OEM, aftermarket, it doesn’t matter. Lots of subpar parts out there. Thermostats seem to be the worst for being bad from stock.

 

What will I see on the test? Will it open up slowly at 190 and higher? Be wide open at boiling?

 

I would think if it is a 190*f stat it would be wide open at that temp.
You will need a way to measure the water temp as it comes up to temp.
Dave ----

 

I see, so a thermostat is either open or closed. It's not a progressive opening with increasing heat.

 

Here's a good primer on thermostat operation and testing:

https://knowhow.napaonline.com/tempe...ps-and-tricks/


A thermostat's rated temperature is when it should start opening. At 20F above this value, it should be fully open. So if your thermostat is rated for 190F, that's when you should see it crack open. It should be fully open at 210F. Test when it opens as heat rises, and when it closes while cooling down.

The subject is near and dear to my heart as my car was giving me fits a few years ago. I had to try 3 new thermostats before finding one reasonably accurate. The worst was OEM. Second worst was name brand aftermarket. Finally tried a different brand aftermarket and that was good.

I also like to test the old one now, too, to confirm my diagnosis/hunch/guess it was indeed bad. It's more helpful to test side-by-side, as it's easier to see which one is responding when.
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Went through something similar with my truck, too. Lots of crap parts out there, sitting on a shelf waiting to trip you up.

 

I probably should test the thermostat before installing but I never have. I know the Stant “Fail-Safe” failed within a month. I’ll never buy one of those again.

 

I'm with ya. I've replaced a few in the past and never tested them. But I see in my Chilton's manual that testing is recommended. I'm going to do it this time as the 6.9 is not an easy change. I have to remove the alternator and the vacuum pump, so it's not something you want to do over.

 

Say Dave, ie Franklin, or anyone else, how much coolant do I need drain out of my 6.9 to be under the thermostat level? The radiator sits up high and the thermostat location is pretty low. I've never replaced a thermostat on a 6.9. It's not an easy one.

 

I am not sure. The last time I drained my coolant I drained it all and converted to Zerex G05. There are drains in each side of the engine block that let you get all the coolant out. Your 6.9 is not prone so much to cavitation but my 7.3 is, so I changed it over. Another reason is I went to a aluminum radiator, so I was afraid to run the green coolant with the SCA'a. It might not have as much aluminum protection with that combination. These diesel trucks never had any aluminum in the cooling system when they were built new.

 

As was stated before, the temperature rating is when the thermostat starts to open up, but it will take time to fully open up. A 190-195* thermostat will operate around 210* operating temperature.

Where people make a big mistake is they think the thermostat regulates max temperature when in fact it regulates minimum temperature. the 292 in my '56 has a OE spec 160* thermostat but the coolant temp runs a toasty 180* - 190*. The 302 in my truck however with a 180* runs on the N and O of normal in summer but in winter it will not breach the normal range and the heater just doesn't feel like it puts out much heat. It did seem to put more out with a new heater core but with my new engine I have a Robert Shaw high flow 195* thermostat considering my plan is to toss the 3-core copper radiator for a more efficient 2 core aluminum radiator to over kill my cooling system.

For coolant, Green or Global is fine if you have no aluminum or minimal aluminum. If you have aluminum, I will strongly recommend the global gold coolant it is much better at protecting aluminum components in the cooling system. I wouldn't however use long life red also known as Dex-cool, if you don't get every last bit of global gold and conventional green out of your system it will mix and turn into a sludge.

 

Rather than guess about how much I need to drain out to be below the thermostat I drained the radiator out completely through the pet ****. I have the old copper radiator and I do have the anti cavitation additive in my Prestone 50/50 green coolant so I didn't want to take a chance on losing coolant through the thermo cover dripping down the engine and on to the floor. I don't need to drain the block as the old is going back in.