Hey all! 2003 Discovery II here.

I'm getting temps up to 221f when I have the AC turned on and sitting at idle. I can turn the heat on and she will sit at 183 - 185f all day. I can get on the interstate and it will cool down as well. I don't run the AC and it will sit around 185 - 188 while driving in stop and go traffic. It only wants to overheat when the AC is on and sitting idle.

What I have done: Completely new cooling system (except for heater core)
New Radiator
New 180f Thermostat
New Water pump
New hoses
New reservoir cap (tank isn't leaking)
New Chevy Fan Clutch Severe Duty.
New Aux Fan Motor (installed correctly sucking air in)

System has been air bled, and the fan clutch is still like peanut butter when it spins when the engine is hot.

The engine is also a 4.6 Block from a running 1999 P38. Everything is new from the block up.

I'm really at a loss here. I don't know what else I can do

 

Have you checked that the auxiliary cooling fan is working behind the plastic grill. It will only turn on when you have the AC on. Check to make sure it spins freely and the fuse is good. Lots of times they freeze up from getting rusted.

 

I knew I forgot something. It was frozen and I kept blowing the 40w fuse. Brand New motor and it turns on with the AC.

I will edit the original post and add this

 

The fact that you turn on the heat adding that extra cooling tells me your not moving enough air at idle to cool the ac load. Maybe check your fan or you got a defective fan clutch
or go with the oem set up which works good as well

 

I had a very similar experience with my 04' DII with a 98 P38 4.6.

Would only get over 205 or so at idle with AC on. It would take it quite a while to get hot as well, I'd have to let it sit for 20, maybe 30 minutes. The hottest it got was 220 after idling with a/c on for 20 minutes. At the time my fan clutch was worn out, and my ac compressor fan was frozen. Never would have thought to look at the ac compressor fan until I noticed it wasn't coming on at 212 degrees, or with the ac.

I changed both, and it immediately got cooler. Just recently went with a new coolant tank cap and that further improved things. My truck runs sub 190 when moving over 40 or so, and between 190-195 when below 40 mph. Sits between 190-200 at idle, and will sit at that temp all day idling, ac on, etc. 180 t-stat.

Are you aware of the status of the water pump? Since you are cooling adequately at speed but not at idle it could be a coolant flow issue. Just a thought.

 

How long does it take for it to get up to 220?

 

Just checked the HD Fan Clutch at hot and it still had very good resistance. So I know it's moving air. The AC Fan or AUX Fan is new as well and turns on every time the AC is on. The water pump is new. No play on the fan clutch when I remove the belt and move the fan around.

I also checked the radiator by hand for clogs (any cool spots) and everything was hot.

I think it could be a faulty thermostat or air in the system. I just ordered everything for the Inline thermostat mod. So we will see if that helps.

It also takes about 10 - 20 minutes for it to get up to 220f depending on how hot it is outside

 

What brand of 180 thermostat ?

 

Its one I got from BP Utah or Atlantic British. Can't remember exactly. Black housing though

 

@Oasisplayer311 Assuming it is the same black as I have,I get 186 -188 hi-way climbing to 190-194 under load steep hills etc. Often dropping to 179-183 coasting on downhills. Generally 194-197 at idle, I did have an issue with my fan clutch and my temps would run up to 201-206 under load.

The Black thermostat has a hard spring so you get more variability in temperature range as compared to the grey soft spring.

My AC needs a re-charge but with it on my temps change by maybe 2 degrees

Hayden fan clutch
Everything working and new rad, no new water pump