2001 7.3 with 180,000 miles. Replaced Water Pump twice and installed a filter.

Truck runs great.

But I noticed that my coolant dropp from the max line down to just under the min line on the daggus bottle. I had driven about 500 miles.

I have looked all over externally and can not seem to find any leaks.

No white smoke.

Oil level is good and does not seem like there is anything in it. However I just ordered a kit from Blackstone to make sure. Do not have results yet.

Any thoughts?

 

You will soon see a lot of 7.3 owners (including myself) have the same fluid level drop, despite filling the fluid over and over.

I have come to the conclusion this is normal and the level has not changed at all since I stopped adding to it.

 

There can be a little oozing at hose connections which dissipate by the heated engine environment, leaving no real traces or minimal traces which are hard to locate. I've seen that happen at my Ranger heater valve connections, but that also translates into a much slower leak than you're describing.

I've also seen what Sous has concluded... once the level hits a certain low point in the reservoir, it seems to hold it's own for quite an extended period of time.

 

Pressure test the system for 15 minutes cold / hot. No loss in pressure, you're probably okay.

 

Good thought on the Blackstone kit.

 

Mine does the same thing and it has been that way for atleast 10 years now every time I have tried to top it off it goes to that same level and stays there I havnt ever found the leak but I guess it is not hurting anything 250k on the truck still running good knock on wood the addative I put in mine has a dye in it that I could try and find the leak if I wanted to really do it but I'm not to worried about it I'm guessing it's a small leak where the two halves of the degas bottle connect. But when I take the cap off I still have the pshhh sound.

 

I took the time once to roughly figure out the volume of thermal expansion when heating an 8 gallon, 50/50 mix of water and ethylene glycol from 50F to 200F. My calculations showed the volume it expands exceeds the space available in the degas when it's filled to the max fill line, so when it heats up, out it goes.

 

When do you lose it? When I first start the truck in the morning the level is at the lower mark. Once it's warmed up and pressurized it goes to the high mark. After the truck cools off it goes back to the low mark.

If you think you might be losing some check around the heater hoses. Any residue on the engine block? Also is there a smell in the cab (possible heater core issues)?

I have some residue around my cap where I get a little weepage every now and then. I also recently had the overflow line connection on the radiator break on me. I never saw it coming as it is on the top passenger side of the radiator under the degas bottle and radiator support.

 

The difference (between your calcs and actual) might be explained in how the second temperature you're heating too is under a higher pressure than when the system is cold. That's why when you loosen or remove the cap at HOT conditions, the coolant will spew out everywhere and potentially burn anyone nearby.

 

After many years of having 7.3's, and still owning 2 BTW, I came to the conclusion a long time ago that as long as it's within an inch, or there abouts, of the low level indicator, and no drips can be found under the truck, it's fine.

 

I wonder if it could also be because the boiling point of a 50/50 mix is 223F at standard atmospheric pressure. Then, when the system pressure is relieved, there are parts of the engine above that temperature, causing an instant boil in those areas. Purely hypothesis, of course.

 

I think it's an instant boil throughout the system, if all the coolant is over the boiling point and it all depressurizes simultaneously. I did that once when I was younger on my $100 64 Lincoln. Always ran a bit hot and one day I absentmindedly pulled the cap off, had a 5 foot muddy geyser. Extra dangerous because it took a second or two to spout, just enough time for a guy to put his eye down there to take a look. Luckily the Lincoln has a forward tilting hood so not easy to take a quick peek. On the plus side it was an effective way to flush the system! After refilling it no longer ran warm.

 

For sure, that is certainly part of the thermodynamics taking place when the pressure is relieved.

 

The 03 -06 (although 5.4L and 6.0L with a different design) had a sticker mailed to all owners to stick to the bottle to correct the problem of filling to the max line for just this reason. We all had about 20 ea. given to us to put on them when they came in for service and had to affix to all the inventory as well. Loved those easy money days.


Maybe you should have performed the calculations for Ford

 

I sold them the labels. Made a fortune . . .