2008 JK Coolant Drain and Fill.
2 gallons undiluted Zerex G-05 HOAT coolant (for my 2008 )
7 gallons distilled water
3/8 i.d. hose (about 8-12 in.)
8 push-type retainers (Rivet Retainer Chrysler 6502625, 21 x 25 x12mm)
Remove the bumper-cover below the grill and (optionally) the air dam below the bumper. Replace later.
Petcock is at lower, passenger-side front of radiator, this view (below) is from in front of Jeep, below grill, behind bumper and e-disco.
Begin with a cool engine; attach the hose to the petcock. Position the hose into a receptacle. Turn the petcock (with pliers) 1/2 turn or so counterclockwise. Turning much further will let the coolant leak from the petcock but not through the hose. Remove the radiator-cap and allow coolant to drain into receptacle (dispose of properly). The radiator holds about a gallon.
Disconnect overflow-hose from the overflow-bottle, then remove, empty, rinse, and replace the overflow-bottle.
Once the coolant (~1gal.) has drained, close the petcock and refill the radiator with a gallon of distilled water. Add some distilled water to the overflow-bottle as well. Clear away tools, etc., and start the engine. Allow engine to warm up to operating temp so that the thermostat will open. The upper radiator hose will become hot and pressurized. Shut off the engine and allow it to cool until you can comfortably hold/squeeze the upper radiator hose. Do not open a hot radiator.
According to the 2008 manual the entire system, including the overflow-bottle, holds 13 quarts of HOAT coolant. A minimum concentration of 50% is recommended. See your manual.
Considering that there is about 3 gallons in the system (not including the ~1 quart in the overflow-bottle) and that draining via the petcock only removes about 1 gallon, and assuming near-perfect mixing after refilling and running, it takes 6 or 7 drain and fills as described above to remove over 90% of the original fluid. I confirmed this using a refractometer (see below).
I took a sample from the 8th drain (the resulting mix after the 7th refill). The refractometer reading was near 0% (calculations estimated 3-7% coolant).
I then refilled with undiluted Zerex G-05 HOAT coolant. After mixing and cooling, I drained the radiator into a clean container. (This was the 9th drain save this volume, it can still be used.) (Refractometer showed the mix from the 9th drain to be ~35% coolant (calculations estimated 33% coolant). This is a very clean fraction, worth saving for later use.
Next, I refilled with a full gallon of undiluted coolant (~3.5 quarts into radiator and the rest into overflow-bottle along with enough from the 9th drain-bottle to get above the minimum level).
I then drove the Jeep for a few days to insure thorough mixing. The next sample (of the 9th refill after being well mixed) showed ~60% coolant (calculations estimated 55% coolant) providing protection down to about -60ºF. Ill check it a few more times over the next month, maybe add some distilled water to the overflow-bottle, if necessary.
I chose not to remove the lower radiator-hose even though additional volume could be drained because I wanted to avoid the mess and keep bubbles out of the block as much as possible.
Draining from the petcock took less than 10 minutes, refilling less than 5 minutes, running the engine about 10 minutes and 35 minutes to cool, so about one hour per iteration. The whole process took about 2-3 hours of work over about a 9-hour period with hardly any mess or clean-up required. Cost less than $50 for supplies and I know it was done right.
Most people measure the specific gravity of coolant with hydrometers. Hydrometers are cheap, ubiquitous, and almost useless. Unless corrected for temperature, the SG readings, even if precise, will be inaccurate. The ASTM recommends refractometers for measuring EG- and PG-based coolants. An inexpensive refractometer with automatic temperature compensation (ATC) is accurate to ±5ºF. I use an RHA-100/ATC refractometer I bought online from National Industrial Supply for $20.