if there's a clog in the block then HG aren't going to fix it and I doubt you pull the motor to get it hot tanked. How do you know it's the HG and not something else? Very easily could be a bum t-stat or failed radiator.

 

Almost impossible for a D2 to overheat with no thermostat and full of coolant, even with a blown hg, as long as you run the expansion tank cap one full turn loose from full tight. It will drink coolant, kill your oxygen sensors, get steadily worse, might even get so bad as to hydrolock the engine overnight so be careful. But the physics are that it is impossible to overheat if it is full of coolant, pumping, and does not have a thermostat installed.

 

Ok then, I'm stumped. I just installed a 180 degree thermo (which made no difference, possibly got worse) the cap is not fully tightened, and I'm certain there are no air pockets. So either there's a serious clog, or the water pump impeller has disintegrated.

I know it's the HG because I did a chemical block test - combustion gasses in the coolant.

 

I'd start by replacing radiator, doing inline t-stat mod and replacing water pump. Just the HG alone should not cause crazy high temps as Extinct was saying. Pull the plugs to see if you have an internal hg leak or check back corners of motor for an external leak. I drove for months with an externally blown hg and just kept the fluid topped off until I had the time to do the work myself.

 

Thanks both, that's very helpful. My feeble understanding was that exhaust blowing into the coolant alone would make it overheat. The block test was clear - it turned very yellow, so I think we know there's an internal leak. The radiator has a 2015 date on it but who knows....

If I could just get the overheating under control and buy a bit of time that would be fantastic.

 

@desmorider You literally are going to do further damage if is is head gasket leak, O2's and cats may have to replaced. At the temps you are seeing you have a significant leak

At 235 you are just 10 degrees from overheat light coming on, 244.6 is it, you may very well warp one or both of heads constantly running that hot.

Having said that when my head gasket started to fail I went up to 196 degrees but overpressure, no indication of combustion gases in the coolant, but is was a hairline crack.

 

@Richard Gallant what's your recommendation? I'm aware of the danger, so since I saw it go to 230 I haven't driven it except for the 5 MI to the shop, and I went through three cooldown cycles to get there.

I don't want to a HG just for the sake of it. What I'm hearing is that it's probably a blockage which is causing the high temperature, but the block test indicates I have a head gasket failure anyway, and that if I can cure the overheating I could probably get away with the head gasket for a little while. Am I understanding all this correctly?

 

Do you have any other signs of a bad head gasket, white smoke, coolant in the oil etc. It could be a worse case cracked block.

 

The oil looks normal. There *might* be a little white smoke at start up, but if so its so slight I'm not 100pct sure. But the chemical block test failed...

 

Hmmm... well that is odd then. You best bet is to do a clean flush forward and backward forward on the cooling system. Is the Disco new/newish to you ? It maybe someone tried rad stop leak and gummed up the system.

You can do a basic pump test, by observing the coolant tank, there should be water movement a lower rear. That is the water flowing from the manifold to the tank.