Your cap or any cap for that matter is designed to bleed at a given pressure. In your case 21 psi. If your coolant system runs more than 21 psi it will bleed coolant into the over flow until the pressure is below the target amount. You've been driving around over 21 psi, thus the coolant loss. If there is no vacuum on the engine side coolant is never drawn back into the engine. The only way coolant will back feed into the engine is if the pressure in the closed side of the system is below atmospheric.
Doug

Doug, everything you just said is correct, except

"You've been driving around over 21 psi, thus the coolant loss."

The expansion tank should be above max line then. It's not. It was actually running a bit low when I last checked it. The coolant is being burnt in the combustion chamber, I believe. The same perforation that expels the coolant, prevents the vacuum. Overall, I'm experiencing loss.

I agree the gasket leak is why it doesn't recover. In your 1st post you stated that the over flow jug filled up. So it's either it can't recover, filled more than it should have due to excessive coolant system pressure, or a combination of the two. Now time for a gasket.
Doug

I just wanted to come back and say thanks for all the help and to update you on my final results, almost a year later. Thought you guys might like to know how the fix went down.

As earlier confirmed, with a chemical tester, there was a small HG leak, or a cracked block. I think the former, given the mileage. I scoped the cylinders and it turned out that cylinder 5 had a small amount of coolant in it. The plug also looked a little cleaner than the rest.

Anyway, I used this K&W block seal, making sure I isolated the heater core first. The first time, I followed the instructions and idled with it for 20 minutes, but it didn't do squat. The leak wasn't big enough for the product to make it's way into the fault area so quickly. I tried another bottle, this time I ran it in the motor for 500 miles and that did the trick. Almost a year later and the coolant stays filled to the neck.



Thanks again!

I always thought that stuff was snake oil, that's great that it worked good for you & good info that sometimes it takes much longer in there to take care of it

what did that stuff look like out of the bottle ? was it a liquid only, or did it have some chunky, pepper looking [size wise] stuff suspended in it ?

[quote=Dodgevity] I thought I'd see pics of my ex-wife

A few pics of the old battle axe that won't quit.

(slightly) off topic. I remember reading about the pepper in the radiator trick from someone (several times) way back in the Slant six club of America, well one day I had been nursing a fairly small leak in an OE 65 dart radiator & I stopped by the quik shop one day on a lark & bought a tin of pepper & poured it in (some people sure gave me some funny looks) & I drove home & when I got there (a mile or two) the leak had stopped & stayed stopped for a year till another leak sprung in another location. recently I tried it again on another OE copper/brass radiator & mistakenly bought coarse ground pepper (had bigger pieces), I figured they would seal better! & the next day they had swelled so much that it immediately started to overheat & they were embedded securely in the tubes & it was a lost cause (don't remember if the leak stopped or not).

It was watery liquid with what looked like copper dust floating in it. It was added with water after all coolant was drained and motor flushed with water. I did that via a hose nozzle hooked up to the heater hoses and thermostat removed.

Supposedly the stuff has nano particles, but who knows. The idea is that it flows into the crack/leak and the copper melts into the crack. Since the head is the hottest area, it melts there, but won't elsewhere. Additionally, the liquid is supposed to harden like glass, once it's exposed to air and left to dry.

I didn't want any of that crap in the heater core, so I disconnected the heater hoses and looped the heater lines from engine with a single hose. Furthermore, once I was done driving the 500 miles, I drained the radiator and flushed it with a hose, to get it all out of there. Only the motor had it left in. Heater, radiator hoses and thermostat were removed. I left it to dry for a few days. That's when the stuff hardens.

Its not snake oil, but it's probably not meant for a leak as small as mine was. I'm glad I caught it early. If you search on this stuff, you'll see it worked for many, including one in a jeep forum with a very bad HG leak.

Note: There are many brands out there but do NOT use the ones that are mixed with coolant and are not meant to be drained. Those are the ones that will clog things.

PS: I recall reading somewhere that this stuff is good for a final seal after a HG job.

4.7s are notorious for slight head gasket leaks. we see this almost daily at my shop, and your symptoms are classic combustion leak. you may even be able to smell a slight whiff of exhaust in the coolant...

I wasn't able to smell it. The leak was tiny, but the chemical tester sure recognized exhaust fumes in the coolant. I pulled all the plugs and scoped each cylinder. I found out that number 5 cylinder had coolant in it. At first I used a UV light and saw the coolant glowing in that cylinder, then I stuck the scope down in and took this crappy pic. Not a high quality image, but you can see the green coolant pooled from about the center, going diagonally to the right corner of the pic.



To be absolutely sure I was seeing coolant, I rocked the truck back and forth...the movement was enough to see the liquid shaking back and forth in the cylinder.

Anyway, I'm glad that I didn't have to pull the head off. I'm pretty impressed with the 4.7L. This thing is running excellent for having 271K. I drive it daily and it doesn't burn a drop of oil and the coolant stays up to the neck now.

that is quite a success story. I will add that stuff to my list of legitemate out of the bottle fixes, the other one (the only one) I believe in is seafoam 16oz "motor treatment" in every tank of gas.

Yeah, just if you use it, be smart about it. Don't let it flow through, or dry in areas that don't need it. Meaning, disconnect heater core before using and flush radiator before drying.

The first bottle didn't work. I had even run the engine without #5 spark plug, so there would be no back pressure impeding the incoming liquid. None of that crap worked. I just had to drive with it for 500 miles with no thermostat. Of course, you'll want to do that in the summer when you don't need the heater core. I blocked off a part of the radiator with cardboard so it would overheat slightly and keep coolant pressure high, to push it through. I also disconnected the overflow hose (didn't want the stuff in overflow bottle) and ran the overflow hose into a glass bottle (wont melt), which I stuck down near the manifold. Once a day, I'd pour the overflow back into the radiator, so it would keep filled. Even after the 500 miles, it didn't seem to work completely at first. I was still losing a little coolant, albeit, much less. After about a week, it completely sealed up. Weird stuff.

2003 Dodge Dakota Quad Cab SLT
4.7L V8, Automatic (545RFE), RWD
263K

Hey, Guys. Just saying hello. Truck still runs great and losing no coolant 40K miles later. Still my daily driver but a Tesla is calling my name.

From the frying pan into the fire.