Ok so my 93 z28. Right now my situation is this. I noticed a pretty significant loss of coolant every time I would add any to the overflow. It didnt SEEM like there was a leak but eventually I did notice one. If the car was running I would see water dripping down from the underside kind of below where the passenger would put their feet. When I got under the car, it seemed to be dripping down where the pass side wheel well meets the firewall. I had noticed this before but I attributed it to condensation from the AC system because the AC lines in the engine bay get covered with water due to FL heat. Heat hasnt worked since Ive owned the car but the AC works good. So I figured it was finally time to get the heater core fixed as the amount of leakage on the passenger side was starting to increase.

She was driving until 2 days ago when I started getting DTC 36 again. I know its a little off topic. I got it before when my coolant crossover developed a pin sized hole where the metal pipe meets the rubber tube. I wrapped it in self-fusing silicone and the problem went away for a few weeks but it has returned. It started as only a minute or so of slight "hiccuping" of power when I got off work around 6am to head home(when its quite humid and moist). Two days of this and I figured, well let me take care of that opti over the weekend and I had already ordered the heater core so I figured Id knock out a radiator swap, heater core swap, and do the opti between thursday-sunday.

Ive got the old heater core out and it doesnt seem to have any holes in it though it was clogged. One of the two heater hoses had a clog as well that I cleared up. Also of note, looking down into the far left of the firewall behind the ECU, to the immediate left of the grommet where the wiring harness goes into the passenger cabin, there is a rusted hole about the same size as that grommet. If i go into the car with the heater core removed, i can see it if i position myself right.

What Im wondering is.. can I get water on my passenger side if the heater core is clogged but doesnt have a noticeable leak? Once I had it removed I took the garden hose to it and a wire coathanger and cleared up the clog.. then filled it with water and it doesnt have a noticeable leak to it. Do you think its possible that the ac evap core could be the culprit? Im hoping my coolant loss isnt a blown head gasket. I have noticed that once enough coolant has leaked out.. it eventually STOPS leaking which leads me to believe that there is some high point in the system is what has the leak and once enough fluid is lost to get below that point.. it ceases to leak. I can fill the radiator up, and the overflow to the very top.. and a few days later look in the radiator and theres obviously still coolant in there but the overflow will be down significantly.

I know this is kind of a complex situation but i figured maybe one of you amazingly helpful people here might have some insight that I have overlooked

 

If you do not see any visible leaks in the cooling system have the system pressure tested. The stuff you see leaking outside the car under the passenger feet is most likely is not coolant, but condensation from the AC evaporator core, which is normal.
Cooling system leaks are practically impossible to patch from the outside and parts and gaskets will need replacement. Take a good look around the engine after it has reached running temp and see if you can notice any exterior leaks, including hoses and hose connections, metal pipes, the overflow tank which has a tendency to crack after many years, and the weep hole under the water pump shaft which will leak water if the seal is worn.

 

You should be able to borrow a pressure tester from a chain type parts store. Leaks that can be hard to find on a hot car can show up real easy on a cold engine with 15 PSI of pressure in it.

Search Youtube for how to pressure test a coolant system once you have the tool.

 

when mine the heater core went one time i had coolent puddle in passengers floor ,i added water drove it home an right away i soaked up as much as posible with.towle.,then i put some water in there an .sucked it up with shop vac did that 2 times ,couse of the errosive effects ,then let it dry was fine ,but it is possible to leak right in the car ,mine was shot though,replaced the core nothing since,for me it was not long after a .flush ,that just stirred up the pot ,good thing your replaceing your radiator too,with my swap i replaced all the hoses radiator.an core