My 99 Disco II is in excellent mechanical condition, has been gone thru thoroughly, and had head gaskets done by reputable specialist 10k miles before I bought it. In fact, I bought it on consignment from the shop, where it had been maintained for the last 15 years.

When purchased, it had just had a new thermostat installed. Hoses etc were new with head gaskets.

I paid to have all fluids changed when I bought it. Drove it home across USA from SoCal to MD (had a running series of posts about that trip) in April.

One day it started to run warm. I added coolant, bled air out of hose, and it was fine the rest of the way home.

I have since needed to add coolant a couple of times. In total, over 6,000 miles, probably ¾ of a gallon in total.

it runs fine, does not get hot. Just every few weeks, if I check reservoir, it is low. Add a bit of coolant. Few weeks later, it’s low again.

Oil is clean. Came out after 5,000 miles looking almost new. Never lost a drop of oil on trip, or since.

Coolant is perfect. Beautiful green OAT coolant installed in CA, and used by me for adding since then.

No leaks I can see in tank. Check again today, added, warmed it up, waited for leaks, nothing. When at full temp, I loosened cap and it started to blow some coolant out of overflow. So cap seems to be holding pressure. I do not have a proper test rig for the cap. System bleed screw just burbles up coolant, no more air bubbles.

No sweet smell in exhaust. No codes. I’m confident it is not burning it.

All I can guess is the cap. It’s not cheap, but I’m guessing that the system is pissing a bit of coolant when up to temp and rolling down the road. It is low in reservoir, about ½” in bottom of reservoir every time I check, but no more air when I check the bleeder.

Never runs hotter that 203 degrees, even when reservoir gets low. It’s a higher tempt t-stat. I can watch it warm up, open and cool, and the settle into normal temp range of around 195-198 degrees on the UltraGauge. Alarm was set to 210 degrees, only went off once when I bled air bubble in Wyoming, has never been close since. I’ve winced lowered the alarm to 205, it has not gone off again.

Any other ideas? Everyone wants to say head gaskets, but absolutely no signs of head gaskets leakage or coolant burn.

I do have a 180 degree t-stat, but I haven’t seen the need to change it, since temp is in the correct range. Maybe that lets it build pressure greater than cap can manage?

If nothing else, I’ll buy a cap. Ideas and recommendations welcomed.

 

"Coolant is perfect. Beautiful green OAT coolant installed in CA, and used by me for adding since then."

OAT is almost never green. You are likely using the traditional green coolant (which is what I would do). OAT is generally orange. Still, that shouldn't matter.

If you don't smell any sweet fumes and can't find any small leaks I'm afraid to say this is going to be difficult. I'd start by looking carefully at the air intake heater which has a reputation for leaking.

 

Yes, clearly and properly marked name brand OAT coolant, and it is green. Color is more about brand, until you get into the red stuff used by Japanese and Germans.

Air intake heater was disconnected when the truck lived in SoCal, and I have not re-sealed and connected it yet. Good suggestion, but it's not connected. Thanks for the ideas.

 

Coolant generally stays 1” below cold line at least on my disco 2s. Any more and it blows it out even with new cap. You may have a rad leak or the throttle plate heater leaking. I bypassed it

 

OK, that changes things. I have been filling it all the way to the full line. I will pay a little bit more attention to where it sets and leave it alone for a little while.

as mentioned earlier in the thread it was a California car it’s whole life, and someone disconnected the throttle body heat along time ago. Funny thing is, since I’ve had it back on the East Coast we’ve had some pretty cold days, and it doesn’t seem to matter a bit.

thanks for taking the time to offer some advice on this old thread. I will watch that level and try to refrain from adding anything for a little while.