As many of you know I have had a cooling issue since my head gasket change. I have had a number of threads re this but thought I would collect everything in one Troubleshooting thread that might help someone else.

It is the Rad by the way completely clogged.

Prior to head gaskets the Disco ran 188.6 to a max of 201 - other than load slow offroad uphills in low range a 5kph reached a max of 215 over 20 minutes - dropped to 176 as soon as I was moving quickly

Post head gasket I did do a cooling system flush - Prestone and water

Running Zerix premix AF

The issue

• The Disco runs fine at highway speeds temps between 188.6 and 194
• Any load increase temps generally to 197.9
• Idle temps run up to 206
• adding A/C makes thing worse - recent test (today)

Troubleshooting steps

1. Confirmed hoses were not rock hard and no coolant being expelled (catch bottle on overflow line)
2. Bled cooling system fully so there are no air pockets
3. Checked belt routing just in case I managed to get it wrong
4. Checked the fan,clutch and shroud
5. Did an A/C test quick increase in temps

Probable causes

• Heater core
• Water pump
• Radiator
• Thermostat

Diagnostics

This is always difficult with cooling system as you can not see what is going on, I purchased an IR Thermometer. reaching past a moving fan is not a good idea

1. Start the disco and check the coolant tank for movement of AF, the throttle body coolant line will cause the AF to move in the tank, if it does not your water pump may be the issue. In my case normal movement, so the water pump is likely still working well
2. Next check the hoses for firmness while the engine is running and at temp they should be firm but squeezable, firmer than cold.
3. Next check the hoses with the engine running, you will not get perfect temps with the IR but you will get differentials. I started with the heater hoses, 10 to 15 deg F diff with min fan. There may be some clogging but nothing earth shattering.
4. Checked the main lines with IR top hose to rad 150 bottom hose 62 there is a big problem. Engine temp was at 197.
5. Shut down the Disco checked the lower rad hose by hand - cold to the touch upper hot to the touch
6. There is a final test you can do the check the water pump, but you need an assistant. Pull the top hose, extend it so you can catch the coolant and crank the Disco over. You should get a good pump of water out. DO NOT RUN THE TRUCK FOR MORE THAN A SECOND, IF IT STARTs SHUT IT RIGHT DOWN

Updated to include Thermostat

It was noted by @shanechevelle and @redwhitekat that a compromised thermostat can produce the same issue on a Disco. This makes perfect sense given the location of the thermostat on a Disco, however mine was working perfectly prior to the work done. It is in the realm of possibility the cooling system flush has dislodged material which is preventing the correct operation of the thermostat. I find this unlikely but I will investigate once the system is apart.
My reasoning here is once the Disco has hit the open temp for the thermostat, it is open and coolant should flow normally, I would expect yoyoing of temps rather than an continuing increase in temps. None the less they both have a valid point, and thermostat needs to be confirmed to not be the issue.

While I may well flush this rad, to see if things improve I will be ordering a new one. If the old one cleans up it will become an emergency spare.
.

 

That's great news, a simple and inexpensive fix!

 

This sounds like my temps exactly with a 180 stat. Just upgraded to the inline mod and the temps are way down. Went from idle at 204 to idle at 184. Radiator was brand new about 18 months ago and the temps have been like that ever since then.

 

Sounds like your thermostat
if you have a second person get the disco to operating temps and increase rpm to 2000 I bet that bottom hose will get real hot when things are flowing and thermostat fully opens

I had the same issue. It was the thermostat

 

IR guns are a great tool. They take the guess work out of things. You can get exact temps, in areas that are hard to reach or just to hot to handle. They are very reasonable too, price wise and recommend everyone owning one. Glad you got this sorted out.

 

Lower hose was 62 and it's the rad?

If the temps were normal before the gasket change and messed up after how can it be the Rad? It didnt just clog like that overnight.

Borrow a bore scope.
Empty the rad.
Take off upper hose.
Insert scope down into rad.
Post pictures of the core.


I would sooner guess the little holes in the thermostat would be clogged before the radiator core.

190-208 is acceptable.

 

@shanechevelle it could be the thermostat, it is on my list of things to verify when I pull it apart. You may have missed the part about doing a rad flush after the head gaskets, once the thermostat fully opens the engine should cool normally, it does not. It is possible the thermostat became compromised, I have an inline kit coming as well. But I expect the top of the of rad to be the issue.

I will update this thread once everything is checked

 

@shanechevelle well I got things apart today and there is definitely something up with the old thermostat, When I used smooth end of a drill to check the 4 holes in my grey softspring it took a bit of force on a hot thermostat to move things it was like the spring was bound up. It also does not seem to open correctly warm.

The old rad "seems" ok, but I suspect some blockage but I have a new one and it is going in. The Sai sensor was covered in what looked like "mud" cleaned up fine though.

I have both a Black 78 and Extinct's in line kit, I am going with black of now as I have some ideas on mounting the inline I want to play with and the switch over is only an hours work.

 

Record the temperatures with both, it will be interesting to see how much lower it is with the inline. I am going to do the same on one of my trucks, but was going to wait for summer to really stress test it.

 

Well started her up 183.5 to 185 ish running the black 78C LR thermostat idling for 20 minutes - no driving to build temps yet.

@Extinct I will - I will log engine temps for the next little while. Even if stay with black LR for a couple of months your kit will be my trail spare, I have hoses sitting waiting to be used for the inline install.