2004 land rover discovery 120k
I did the mod with the inline thermostat a couple months ago and everything was great. Coolant was around 190 most of the time. Now I'm running into the issue when I'm going down the road it starts to get hot. About 221 and when I slow down it will slowly lower. I'm thinking I have a clogged radiator or bad water pump? Fan clutch is working

 

Check the simple things first, could be a defective T-stat that gave up & now it opens too late, (always use a high quality stat not cheaper ones), if stat checks out ok, then it's your radiator not water pump, because in lower speeds it can keep up with engine, but under pressure & higher heat produced when engine works harder it gives up on dispersing the extra heat,, waterpump however if not leaking is not broken, propellers don't wear out by moving water unless they work in straight water for very very long time & turn to rust, (very unlikely),, fan clutch does very little when going 45 or faster,,

 

I did the inline mod before the summer and everything was great at 183-187. Coolant temps started going up slowly throughout the summer. 189 one week, 192 a couple weeks later. Finally after it got up to 196 a couple weeks ago I tore it all apart and started over. Flushed the system out several times, replaced the almost-new heavy duty fan clutch, radiator cap and refilled with about 60/40 mix (60 being the distilled water) and a unit of WaterWetter. While the fan was removed I sprayed the radiator and condenser from the inside out and removed a bunch of dust and grass. Happy to say operating temp is down to 180 with no fluctuations and the original radiator still in use. If you start here then it won't cost you much. If you aren't using a GM Stant Superstat 180 degree, then that is what you should have. I will vouch for its performance. I think you should focus most of your attention on the flush. If you start the job with a hose and sprayer, just make sure the last 2 flushes are with distilled water and the engine getting at or near operating temp as not to leave any calcium and minerals behind from the tap water. If you don't spend at least half a day on the flush including purging the air out, you didn't do it right.
https://www.amazon.com/Stant-45358-S...0+chevrolet+GM


Flush: After draining the coolant I unhooked the hoses at the stat and ran tap water through the radiator for 10 minutes and did the same through the engine. I was using a good, lil sprayer nozzle attached and even hit the radiator from the bottom in at the outlet just to loosen up anything that may have been down there. I hooked the stat back up, filled with distilled, ran the engine a few minutes and drained. Took the stat back off and just ran tap through the radiator and then the engine once again. This is necessary as you will still be seeing the original coolant. Filled with distilled and ran the engine again. Drained, removed stat and this time instead of flushing with tap water, I use the distilled to flush the radiator and engine once again. Did the whole "fill with distilled and run engine" one last time. Drain that and buttoned the hoses and stat back up, filled with distilled/coolant/additive and was finished. Double-check all of your hose clamps so that you don't blow one off going down the highway.


Bleed: Allow the engine to idle up to or near operating temp with the overflow tank perched on top of the battery with the cap off and cabin heater on high in economy mode. (I'm pretty sure that "auto" will cut the heat when ambient temps are warm) I let it idle for 45min-1 hour and never get over 180 or even opened the Tstat so I put the cap back on the overflow tank just a few threads so that it can still breathe, set it back down so the hood could be closed and drove around the block slowly a few times to get the temp up and water moving. Parked it again with the front wheels on curb to raise the truck and let it idle for 30 minutes with tank up on battery and cap off again. Temp got down too low so repeated the drive-around with cap loose and I had zero problems with air left over. Some people say you can throttle the engine to get temps up but I live in apartments and this was at 2:00 am so I chose not to do this in my neighbors window but it may be better option for you rather than driving the truck to heat the motor and move water through the stat and radiator. I have never had problems bleeding air, even with the simple "drain through the petcock/valve" that I use in my temp gauge adapter but I have done it this other way through the overflow tank last 2 times and seems to give me more peace of mind that no air is remaining in heater core or otherwise.


Hopefully you try this before spending money on new parts because I was this / / close to buying a new radiator myself.