2000 D2 4.0. OEM replacement engine installed under extended warranty in 2009 with 65,000 miles on it. (Original engine dropped a cylinder.) 158,000 miles overall for the truck.

Over the course of every 12 to 18 months or so, I occasionally add small amounts of coolant to top up the coolant level. Average amount is probably a quarter cup: sometimes less, occasionally as much as a half cup. This comes to a gallon in total over that time period. I check the level all the time.

The need to add coolant is not consistent. I sometimes go for a couple of months without needing to add any, but sometimes it is more often.

What I have noticed is that there seems to be less coolant in the reservoir when the ambient temperature changes significantly. For example, we went from summer weather (90s) to fall weather (70s) over the last two weeks, and that change was enough to provoke a coolant loss. Same thing happens pretty much regardless of whether the temperature change is a rise or a fall.

Anyone have an idea what this might be about? Am I correct in thinking that if it were a leak or a head gasket issue, the loss of coolant would be more consistent? Or is this just wishful thinking?

Also: I have been using Prestone Dexcool 50/50. The one for GM vehicles seems to be available, but I have not been able to find the regular one for the D2. Does anyone have a recommendation for a different brand that is available? (I'm aware of the green vs orange debate. I would consider making that change next time I do a complete flush. For now, I'm just looking for a new brand for the occasional top up.)

 

btw: Replaced water pump (new), reservoir (used) and the hoses that attach to the water pump about a year ago. The loss of coolant issue has gone on for years, and replacing these parts did not have an impact.

 

@SouthByWest Check under your throttle body, the throttle body heaters are notorious for leaking the valley pan gasket is another one . Beyond that I do not think you have an external temperature issue. What is more likely is you have a small leak at a loose hose clamp or something similar, that may only leak under very specific conditions.
I had a small bubble in a rad hose that only leaked when the engine was hot and on rough roads. Basically normally the bubble was tight against a piece of plastic, so although it leaked it was tiny. Once I got on a rough piece of off road the vibration moved the hose enough to increase the leak. I found it when I did my morning engine check and you could see the water spray in dust under the hood.

 

This is what finally turned out to be the problem. For some reason, the leak never showed until about a month ago.

My mechanic replaced the heater plate, bolts and the gaskets for the heater plate and the throttle body connection to the intake, but also discovered that one of the bolts that connect the throttle body to the intake was missing, presumably owing to a prior replacement job by a prior mechanic.

Does anyone know the specific part number for these bolts?