Speaking of regular flushing and engine block scale...
When I was chasing an overheating problem my XJ was running hot around town and on the freeway, sitting at 105*C instead of 95*C, even hotter uphill. I replaced the whole cooling system one part at a time including the radiator, viscous fan clutch, fan belt, thermostat and housing, temp sender, water pump, coolant, upper and lower radiator hoses, radiator cap, tested the thermo fan, flushed the radiator and engine block, replaced the muffler and checked for exhaust blockages. Nothing made any difference so I suspected a blown head gasket but there were none of the usual signs…no white smoke, bubbles in the radiator, or contaminated oil or spark plugs etc.
A friend offered to pull the tank off my old radiator and take a look inside to see if there were any blockages. I doubted it would be a radiator problem because I’d only just replaced it with a near new radiator from another XJ (both Adrad brand) and when I flushed both radiators out the water was clean. The attached photos show what was inside my radiator. It was 25% blocked with scale and silicon!
We gave it a good clean and put it back in my Jeep and I gave the a/c condenser a blast with the high pressure hose while the radiator was out. I was surprised at the amount of dirt that came out of it considering I always give it a spray through the front grill after each trip. My XJ’s running cool again now so the problem was the clogged radiator core or a dirty a/c condenser, or a combination of the two. It turns out the near new radiator I used only had 27 tubes compared to the 39 tubes my old radiator had. The owner of the newer radiator said it was an Adrad brand so I just assumed it would be the same as mine, it wasn't.
If you have a blocked radiator you might not be able to tell because as I discovered even flushing it out with a hose didn’t get the crap out of the tubes, and the water looked clean. I'll definitely be flushing it out more regularly in future and I'll be a bit more careful with the amount of silicon I use on the water pump and thermostat housing!