Thanks for all the info guys! I'm going to leave the cooling system as is for now. I'll charge the A/C in the spring and see what happens. The highest I've seen since my rebuild has been 203 at idle.

 

Although I agree with most of Best4x4 above post, this part is not technically accurate. The Disco ECU goes in to closed loop fueling mode very quickly after startup, long before it is warmed up to normal operating temp. What that means is it is keeping the fuel mixture at stochiometric (spelling?) based on the oxygen sensors. You can see this if you monitor the oxygen sensor cycling with a scan tool and you will see the voltage cycles within the first 5 minutes even on a cold day. The ECT does not affect that air fuel ratio, even at very cold temperatures. The air going in to the engine might be slightly cooler at 170 vs 180, but it is affected more by ambient air temperature than ECT.

What running at lower temps does do is reduce the expansion of the cylinder walls and pistons. Because pistons are aluminum and cylinder walls are steel, they expand at different ratios and the clearances are different cold versus hot but it is a linear ratio, so a ten degree difference creates the same dimensional change at 170 as it does at 220. Because of the different expansion ratios, the clearances are tighter at higher operating temps than they are at lower operating temps. The engineers designed the clearances based on the planned operating temps, so when you operate below those temps the clearances are slightly larger. This can affect things like blow-by past the pistons of fuel, oil, (common in boat engines, where the thermostats are typically 140 degress to prevent salt precipitation in the cooling system) and in some cases it can create piston slap (most common in early Chevy LS engines where they ran the clearances a little loose cold).

The other thing that this expansion effect does is change the interference fit between the cylinder liner and the engine block, which is why I run the 170's when I can. The aluminum block expands faster than the steel liners, so hotter temps mean the fit gets looser. I have had several 03's with the dreaded slipped cylinder tick that would not make the tick as long as the thermostat was a 180 stat. I like to run the 170 stat for a little extra head room to reduce the possibility the cylinders slip.

Not to disagree with most of what Best posted, but I wanted to make sure the forum members had a correct understanding of the effects of a lower temp thermostat.

 

 

There's no way around that either is there?
And if I remove the SAI, then I'll keep getting a check engine light too, right?
I have an 04, and will be doing an inline thermostat, I also still have the SAI.
So, I guess it's use the SAI and a 180 thermostat...or clear check engine lights regulary.

 

If you did an SAI delete & got an ECU that isn’t SAI = no more lights. As long as the pump & vacuum lines are in good shape the SAI system isn’t to bad. I just prefer non SAI’s due to less clutter under the hood.