losing some coolant.

bought this 1997 cherokee sport 2 years ago. the 4.0 had a cracked head and piston #1 was damaged. bought a used head and had it rebuilt. also installed a used piston. previous owner installed a new water pump and a new rad.

the engine block has a very small crack near the water pump, just under the header. I can see coolant has leaked in this area. when engine is cold i can see it's a little wet. I mean very small crack... >The engine burns off the coolant before it drips to the ground so there is no puddle on the ground.

I add coolant every few weeks and just keep an eye on it.

any suggestions?

The main problem with these tiny coolant leaks is, during the cool down cycle the cooling system goes into vacuum and the motor is likely to suck a little air in crack. The system usually (eventually) vents the air back out the top into the coolant recovery bottle.

During the next cool down cycle a little coolant is sucked back into the cooling system to replace the coolant pushed into the recovery bottle, and whatever air was vented.

Wherever you have a seep in the cooling system you have a loss of vacuum during a cool down cycle and a place for air to get into. Whether the crack is large enough to upset the balance and/or significantly affect cooling is anybodies guess.

Silver solder is the best fix, it expands and contracts with the cast iron. You have to get the block really hot (above 600 C) and any gasket or whatever that is nearby can melt. You also have to clean it well, coolant does seep/leach into the cast as does oil, you have to try and clean as much out as possible and hope for the best. Boiling water (very hot near boiling) works best, after multiple repeats you have usually leeched/flushed out enough of the coolant or oil to get a decent bond with the silver solder.

Silver solder needs a little practice to use properly, heat control and technique make it a whole lot easier. I've done a whole lot of silver soldering and if there is one thing I'm an expert at this is it.

Watch this video, https://www.youtube.com/watch?v=gRNi17RmGBo

The only thing the guy did wrong was use too much flux. After coating the material (cast iron) with the mantle flux, you can bang the rod against something hard nearby and clean the flux off of the rod. The flux tends to pile on top and keeps you from seeing how the silver alloy is flowing (the excess flux gets in the way). And he finished it, sanded and filed, until it looked pretty, which isn't necessary unless it is on a sealing surface. It is actually stronger if you pile it on a little and leave it (which requires good heat control and practice). The silver solder will flow into the crack (capillary action) if the crack isn't to big and needs little piling on and heat control isn't so critical.

Just quickly looking under the hood i happened to squeeze the small hose coming from the radiator going to the coolant bottle.
It was stiff and felt like i was crushing ice / slush.

I removed the hose. tried to blow through it but it was plugged tight!
Brought it inside and with the use of a coat hanger and a sink full of water, i was able to flush it out.
Full of rusty mud yucky stuff.

with the hose being plugged like this, the cooling system had no where to send the extra coolant as the engine warmed up. I would imagine it caused extra pressure in the system.

Any thoughts?

The system should hold around 15 PSI, depending on the year. Anything over around 25 psi and bad things can happen. Like popping seams in your radiator. I've put air pressure into a radiator on the work bench testing for leaks, you can watch it swell as you add pressure.

I found the pressure relief hole (nipple on the radiator filler neck) going to the recovery bottle tubing plugged with paint on a brand new radiator. I'm sure glad I caught that one before it caused serious issues.

You may want to use a small drill bit and/or check the tubing nipple at the radiator filler neck. Also take a close look at the spring and valving on your radiator filler cap, it may be gummed up also.

Food for thought... At work we recently had a truck go in to a mechanics shop to get looked at (I figured the block was leaking). They were able to pressurize the system and tell that it could only hold 8 psi before it started spewing all over.
If I were you in that situation, I would take it in and have them diagnose the leak under their controlled environment and proceed from there.

Thanks everyone.
My nephew is a mechanic at a small shop. I'll get him to do a pressure test on the cooling system. see what happens.