I don't have the rushing water noise from the air in the heater core. I'm fairly sure that the air is out of the system.

It could be a faulty coolant temp sensor, I get a reading that increases as the car warms up, so at least its doing something. I used my IR the other day to measure the metal that the temp sensor is seated in, it was within 10 degrees or so if I remember correctly. Just for kicks, yesterday I also used a thermocouple that I taped to that same piece of metal (just to the drivers side of the t-stat) using an aluminum tape to try and get a normal heat transmission. It was reading pretty consistently 14-16 degrees lower than my OBD readout. I'm not sure if I should attribute that to the heat loss between the inside of the metal (where the sensor sits) vs the outside face, or if my temp sensor is actually reading wrong.

So certainly possible that the sensor is wrong, but based on what I'm seeing I'm not 100% convinced yet.

The fan shroud is also intact and fits well.

 

Yes. Unfortunately there's no fool-proof head gasket test. Try pulling your spark plugs and see if one of them is cleaner than the others. If it is, coolant is getting into that cylinder and steam cleaning it, meaning you have a bad head gasket. If not, sadly it doesn't mean you -don't- have a bad head gasket, just means it may not be presenting in that manner.

As far as diagnosing blocked coolant passages, if you can confirm that coolant is circulating through the engine, that's as far as I can take you. I'm not sure if there's only a singular coolant passage winding its way through the engine, or if it branches off. If it branches off, you could still have flow and a blocked coolant passage. I'm sure someone on here more knowledgeable than me can share wisdom on this. There's a method for flushing the coolant passages that involves hooking the garden hose up and running water through the engine to see what, if anything, comes out, but I don't know the details.

I would definitely replace your temperature sensors. You have two of them, they're right next to each other and look completely different. One reads temperature to the ECU, and the other reads direct to your dash gauge. Replace both, they're super cheap. That way you're sure you aren't getting a false overheat read. I would have done this first before I did any of the other stuff you tried.

 

So my $0.02 worth is a little more severe. Before I go into my nay-saying I will say that my truck was getting hot as you describe and I found a huge pile of oily leaves, bugs and junk between the radiator and the AC condenser. I cleaned that out and it made a difference but still didn't cool the engine down to the 190-200 that I thought it should be at. So I did all the things you are talking about as well as installing a gauge that drilled directly into the thermostat housing so that I could see my temps in real time while I was driving. The temps on the gauge, from OBDII and a laser thermometer all showed the same temps within about 8 degrees or so. Still high and at times (hot days on sierra freeway hill climbs) would still creep to 225-235 and I didn't like that. It is important to note that the needle on the dash doesn't move until like 235 degrees or something so that's how you know its starting to cook.

What I did that actually made a difference was replace the timing chain and gears. That dropped my temps quite a bit. I would say 10-15 degrees. Also I am completing a rebuild of my motor and there were 3 lobes gone off my cam and several others short and fading. Like having a slack timing chain this can translate into higher temps as the exhaust gas cannot be blown out without the valve opening all the way at the right time. It may also have to do with running rich, same problem just for intake? Maybe... When I got my Disco it had 174k miles so its possible that you could be experiencing the same kind of internal wear that these engines (and any central cam and lifter motor) are so well known for.

 

I would suggest an alternative explanation for why your temps dropped after changing your timing chain and gears. To get to all that stuff, you have to remove the water pump and front engine cover. The front cover is gasketed on both the block and the water pump side. On the block side, the gasket retains oil from escaping, coolant from escaping, and keeps oil separate from coolant. It's quite possible, if not likely, that you had a leak in this gasket, which allowed coolant to escape internally into the oil, not only reducing volume in the cooling system but also introducing air.

That being said, when my engine was stripped down for the head gasket and front cover job due to overheating, the timing gears/chain were absolute toast, so maybe it's contributory The world may never know.

 

There was no air, oil or water leak from my timing cover unless it was absolutely minute. I too did the test for combustion gases in coolant and came up negative as well as inspecting the pistons for that steam cleaned polished effect and there was none. When I get this all back together I think it is going to feel like a very different engine in most part because of the top-end rebuild. I also very strongly believe that the days of 215 degree normal running temps might be behind me because my valves can open all the way, for the right length of time and with the correct oil pressure. So Dirty97disco, if the timing gears are something you have not dove into yet it may be a good thing to replace and you can do the oil pump gears while you're in there. All good things improving the life of your motor.

 

I appreciate the suggestion. Unfortunately Ive already done oil pump gears, timing chain, and water pump along with cleaning the oil pickup. How long ago did you rebuild your top end? have you run it long enough to actually notice a difference?

Im sure that i have some lobe wear, but honestly digging that far into the engine is pretty daunting for me. I've done just about everything else on my cars aside from deep engine work. Maybe I just need to stop being so scared of it. Who knows.

 

Any brakes dragging? Handbrake, calipers etc. Can cause overheat by dragging on the engine and tranny. Are the tranny fluid lines clear to radiator, can cause warmer trans temps and add to radiator heat.

 

huh. thats actually a good question. I know the brakes are good since ive replaced them all..... but the tranny is a new possibility. Thank you sir

 

I agree it is daunting, I just finished getting everything back together and running yesterday (Sunday 8-19) so I am still trying to break it in and get some bugs worked out. But so far the temp has not gone above 190 on my 15 mile drive to work today. I should have also said that I took my radiator to a shop to have it hot tanked and inspected while the motor was out. They cleaned, re-rodded it and repainted it which could have a good bit to do with cooling down better too.

 

So you asked if I drove it long enough to see if it made a difference and the answer is unquestionably yes now. I work at 1500 feet elevation and live at 2600 feet of elevation and have to drive up highway 80 in the mountains to get home with real steep and some long climbs. On one grade that is short and steep the temperature climbing that hill before my rebuild was about 215. On Monday when I drove home from work and hit that same climb it was 95 degrees outside and the temperature barley hit 190. I would hit 215 climbing that thing before when it was 40-50 degrees outside! Theses temperatures are taken from a coolant temp gauge installed in the thermostat housing. That is a big difference, real big. Again look into getting your radiator inspected and cleaned. Mine was about clogged and may be the biggest reason why my temps dropped by 30 degrees.