2008 JK Coolant Drain and Fill.

2 gallons undiluted Zerex G-05 HOAT coolant (for my ’08)
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 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. I’ll 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.

 

With all of the recent talk about coolant, it seemed appropriate to bump this. I may be in this same boat shortly depending on my amount of free time (9hrs according to the above post).

 

Subscribed since my 2010 might need it shortly rolling at 80k.

Unrelated I found this post hilarious that it kept blanking out peti.c.o.c.k. Lol

 

What is the reasoning to drain/refill/drain/refill only a gallon at a time? If I was to drain it all, fill with distilled water, run, drain, then fill with coolant. Would that not be the same?

Honest question as I don't know too much about the coolant system besides not mixing HOAT and OAT or else it crystallizes and goes fubar.

 

There's no plug in the block to drain it. Some people pull the hoses but I wanted to avoid the mess and not get any bubbles in the block. I've heard too many stories about bubbles that won't burp out and, worse, bubbles that "trick" the thermostat into staying closed.

 

Yeah I had quite the mess with my TJ when the pet**** was stripped somewhere inside and would just spin without backing up. Pulling the hose was rather.... messy.

I figured the pet**** was the lowest point of the rad so it would drain everything. But your comment on the air bubbles makes sense.

 

I miss the "old times" when it was just pulling the hoses and thermostat valve out, draining everything and then just refilling.


First time I read about these bubbles, which I imagine are air bubbles. Is the 3.8L really so poorly designed that it does not push the bubbles out, worse, can be damaged if a few remain?

 

I should have been more clear when I mentioned getting bubbles in the system. I wasn't pointing a finger at the 3.8 specifically. But knowing of engines that have had the problem, I avoided introducing air to the system by just doing radiator drains and not pulling hoses.

Lots of thermostats have a weep hole to let air vent past it. Some engines have a specific valve to burp the system.
The 3.8 probably burps just fine, for all I know.

 

I did this coolant change over the weekend. Works great if you have a day to spend in the garage doing other things.

A couple thoughts/observations/recommendations:

At the beginning, pump out the overflow with a bottle pump if you don't want to remove it. Toward the end, some of the 9th drain fluid (or whatever number you are on for the first drain after the first new, undiluted coolant is added) could be used to fill the overflow bottle back up. The overflow bottle on the JK is not pressurized (in some vehicles they are). During the repeated draining/filling there was poor communication and thus very little mixing in the overflow bottle. Don't rely on measuring for antifreeze content from tests on fluid in the overflow bottle unless you have been driving it for a long time since the last adjustment (hours, days). I was able to see burping into the overflow bottle.

I was only getting about 2/3 of a gallon for each radiator drain. The driver's side was a bit lower and jacking up a little bit under the driver front tire might have helped. But I also worry about the missing 1/3 gallon was space occupied by corrosion. But, worrying is practically my day job.

I went through 8 gallons of distilled water bought from the local supermarket. So it was a couple more drains and fills than mentioned. Probably because I was only getting 2/3 gallon, I couldn't get the drained fluid to run clear, the best I could do was a straw yellow. I added back in about 6 quarts of undiluted Mopar Hoat fluid. I thought 7 was too much, I don't live or travel to Montana.

I don't think I will do it this way again because it did take so long and because I could only get to pale yellow. I think I will try leaving the pet**** and radiator cap open while the vehicle is running and top off constantly. There are apparently special no-spill funnels to help with this. Or pull a hose. Yes, burping would be necessary in the latter case, but it burps a fair amount anyway with the radiatory only method.