Hey We're the best, F the rest!. You know what I'd do now is take out the stat housing/stat/fan belt & fill the housing neck up to the brim with coolant & start it & power brake it & see if you see bubbles or the coolant starts to rise. You only have a brief window of several minutes give or take till heat expands the coolant the same way (wouldn't be bubbles tho) but if it bubbles or rises right off the bat within several minutes then that is a dead giveaway for you know what. Good point missed earlier & that is to put on a non recovery cap then pump it up to 15 then you will have a sealed system that ain't going nowhere unless there is a leak in the rad, in the hoses, in the pump (all external) or internal/inside in the head gasket (BB) or head gasket and or intake gasket (SB) & alot of those recovery systems dont work right in the first place (they dont end up drawing the coolant back to the rad) so pumping up a recovery cap system overnight ends being a valid test by default

Unfortunately, the 4.7 thermostat and housing sits at the bottom where the lower rad hose meets the motor. If I removed it, all coolant would drain out. The non recovery cap comment, I don't understand. Sorry, but this is where the newbiness shows. LOL Wouldn't the pressure tester's cap be considered a non-recovery, since it has no valves? Currently I'm pumping it up to 21 PSI, since that's the max pressure on the cap.

Yes you are correct! A brain fart on my part.

Were the best, F the rest.....LOL

You guys are a trip... I think I'm going to like this forum.

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

Well, guys.

I can definitely rule out the overflow tank and hose as having a leak or being faulty. The only thing left is head gasket, but I think I can almost rule it out too. I went through some steps from the service manual, including hooking up the tester, pumping to 16 psi and revving in place. Manual says if gasket was perforated, the revving would have caused pressure fluctuation. The needle was rock steady, and rising slowly with heat expansion. Furthermore, I smelled the exhaust and not a hint of coolant. If anything, a little water vapor with the cold weather. Heater core is dry too and there is not a single drip anywhere. Still, the coolant does not seem to be returning from bottle, so it must be losing the vacuum effect somewhere.

I'm totally baffled at this problem. W-T-F? LOL

headgaskets can leak into the oil pan and not the combustion chamber, how's the oil level look? See any milky residue at the fill cap?

Does you new cap work properly when the system cools downn, creates a vacuum and tries to suck coolant back into the system?

I'd get it hot & pump it up to 15 & let it set overnight. If it ain't leaking externally then it has to be leaking inside the eng

Cap is a week old. Nothing changed when I replaced it. I just took this....

I pumped it up hot just now. Pressure drops as it cools, which seems normal. If I pump it up cold, pressure holds for a long time. I will pump it up and leave overnight.

actually not normal. when pumping it up when hot then the pressure starts to drop not because it is cooling off but because the HG is leaking. pumping it up when cold would highly likely leak if let set up overnight (just takes longer to develop when the eng is cold). Supercuda has you covered in his last post

And no oil in coolant...

Oil and coolant look good, so that leaves the new cap. Does your tester also have a vacuum function to test the cap and make sure it'll draw in coolant under a vacuum?

If so test it, if not I would suspect the cap of not doing it's job even if it is new.

I'm sure if I leave it overnight, it will lose some pressure, but according to everything I've read, the system is not expected to hold perfect pressure with a tester attached all night. I've left it for an hour and lost less than a PSI (cold). Most say if the pressure does not drop much in 20 minutes, there is no leak. Remember, cooling overnight should create pressure drop enough to create a vacuum.

If I pump it up while hot, I'm combining thermal expansion with air pressure from the pump. As it cools, pressure will drop due reducing thermal expansion, till the expansion is no longer a factor and that would leave only the air pressure...so there would be a considerable pressure drop.

With the tester attached, I found a minute leak which I never saw under normal operation (last week). I fixed those with heightened hopes, only to be disappointed.

Nope...wish it did.

Alright how about this, pump it up stone cold to 15 & let it set overnight & if it drops to say 10 then we have lost 5 lbs of psi somewhere & now that might be 5lbs of air at the top of the system or coolant in the rest of the system or a combo of both but we ARE losing coolant & tho you and me neither want it to be a HG issue, that 5 psi loss (& some if not all is coolant cuz we are losing it) then it has to be going somewhere

Hate to leave a thread hanging so I thought I'd come back and update and to let you know you guys were right. I finally ran a chemical test on the block and it failed. Video below.

I ran a second chemical test yesterday with the truck hot, and the fluid quickly changed to yellow without me having to raise the RPMS like I did in the video.

I still don't understand what caused the issue though, as I never overheated this truck and checked fluids anally. Again, truck runs and idles perfectly. No smoke and no overheat. No oil in coolant or vice-versa and pressure tests were passed. No bubbles in coolant, etc. Obviously though, exhaust gases are in the coolant. So it might be a cracked block, cracked head or bad HG.

So there it is... "head gasket" failure. I was driving the truck daily till diagnosis. Now, I drive it only a few miles on weekends to keep it from sitting too long. Waiting until the weather gets warm to work on it. Don't want to get worse. Hate to say this, but I'm going to try using block sealer first. I think with such a mild condition it may work. Will bypass heater core when I do this. Got this truck cheap and the miles are pretty high. Don't know if digging into it and possibly finding a cracked head or block is worth the investment. Plus I'd have to change the timing chains too.

I just saw this thread. As soon as the pressure tester went to 23psi there was your answer. How can a 15lb cap seal 23psi? It can't. Thus the coolant fills the overflow bottle. After working in dealerships over 27 years I can tell you head gaskets fail on a regular basis w/o being overheated. All types of engines.
Doug

I don't understand your theory. The pressure tester's cap replaces the vented cap. The pressure tester has no vent to expansion tank. The pressure can rise till the hoses (or something) bursts by either pumping up enough air pressure, or coolant thermal expansion when running engine. BTW, the cap is 21 psi on this engine so 23 is just a tad over.

More pics of the oil and coolant. I also tried a sizzle test. I dripped the oil onto a hot skillet and there was zero sizzle, so there is no water in the oil. Coolant, you can see for yourself.