Cooling System Flush & Coolant Replacement, 1998-2002 2.3L Accord
 

The following DIY guide is for those who do not want to remove the engine block drain bolt. If you want to remove the engine block drain bolt, I put some suggestions at the end of this guide.

Parts Needed
-- Purchase a new OEM or Aisin thermostat, 19301-PAA-306 or possibly Aisin thermostat (172 degrees F) THH007 . Nippon is the manufacturer of the OEM thermostat. You do not have to replace the thermostat, but while you have the cooling system open, replacing the thermostat is a good idea. It takes no extra time at all.

-- Scour the net and find a gallon jug of the concentrated version of either the Honda Blue Coolant or Asian vehicle blue aftermarket coolant specifically designed for Hondas. In 2021, I could not find it locally. I did find Autozone still sold it online. It cost about $30 total for a one gallon jug of the Peak concentrated coolant for Asian vehicles. See https://www.autozone.com/antifreeze-...lon/720575_0_0 .

-- Six, one-gallon jugs of distilled water.

-- About five feet of 5/16-inch ID transparent plastic hose.

-- An empty, discardable jug, to hold old coolant.

The Night Before
With the engine running, switch the heater control to the highest setting.

The Next Day, Drain the Radiator
With the engine block cold, put a pan underneath the radiator drain petcock at the bottom of the radiator. Remove the radiator cap. Open the petcock and let the radiator drain. All together about 3/4 gallon will come out of the radiator.

While the Radiator Drains
Remove the coolant reservoir. Empty the reservoir's contents into a discardable container. Clean the reservoir. A coolant reservoir can get pretty scummy. Even inserting a brush and rag often will not get all the scum out.

As the Radiator Continues to Drain Remove the Thermostat
The thermostat is located on the passenger side of the engine bay, behind the engine block, at about waist level, underneath the air intake ducting. Study the drawing below. The thermostat and its gasket are parts 3 and 4. The thermostat case is part 7. The thermostat cover is part 5.Locate the thermostat cover (a.k.a. "housing"). To ease access to the thermostat cover, take a few minutes and remove the air ducting in its vicinity. Unplug the two-wire coolant fan switch on top of the thermostat cover. Unfasten any other interference. Remove the three 10mm bolts: One 10mm bolt holding a wire harness bracket, and two 10mm bolts holding the thermostat cover in place. In the drawing above, these bolts are parts 18, 18 and 17. Remove the thermostat cover. Remove the thermostat. Note how the thermostat gasket has nubs on it, around the jiggle valve, and how the nubs fit into detents (or slots) in the thermostat case.

Siphon More Coolant Out, Via the Opening in the Thermostat Case
Get the 5/16-inch plastic hose. Push one end of the hose as far down the thermostat case as possible. There's a fair amount of coolant there that you are now going to remove. Carefully suck on the end of the hose until the coolant is a foot or so from your mouth. Put a kink in the hose. Put your thumb over the free end of the hose. Guide the free end through the engine bay open spaces to a pan beneath the engine bay. There will be a good siphon. About 1/4 to 1/2 gallon of coolant will come out of the hose. Here is a photo of the set-up, just as the coolant stopped flowing:
Re-Assemble the Thermostat Cover to the Case, Using the Old Thermostat Gasket, But Without the Thermostat
Take the thermostat gasket off the old thermostat. Install the old gasket into the thermostat case, with the nubs sitting in the slots in the thermostat case. Re-assemble the thermostat cover to the case, but without the thermostat. Do not overtighten the two bolts. Connect the coolant fan switch. Loosely re-install the air ducting.

Flushing the Coolant System
Shut the radiator petcock. Put the reservoir back in place, but leave the reservoir empty. With a 12mm deep socket, crack open the bleeder valve on top of the thermostat case. Fill the radiator with distilled water until water/coolant runs out of the bleeder valve. Snug shut the bleeder valve. Leave the radiator cap off. Start the engine. Run the engine about three minutes. The goal is to circulate the now fairly diluted coolant throughout the system. At idle, googling indicates the water pump is moving on the order of 10 gpm, give or take. About two gallons of coolant are in the system. With the thermostat fully removed, the water pump should be flushing the cooling system fairly well. If you run the engine a long time, then you will have to wait longer for the engine and coolant to cool down. After three minutes or so, turn off the engine. Open the radiator petcock and drain. Again, only about 3/4 gallon will come out.

Repeat the step above until what comes out of the radiator is clear. To get to this point, I used about 4.5 gallons of distilled water.

Siphon One More Time
With the radiator drained, shut the radiator petcock. Remove interference and unfasten the thermostat cover from the thermostat case. Remove the old thermostat gasket. Again, and as described above, use your 5/16-inch ID hose to siphon as much liquid (now almost entirely distilled water) from the system as possible.

Install the Thermostat
Install the new thermostat and new thermostat gasket. Re-assemble the thermostat cover to the thermostat case. Torque the three bolts to 8.7 ft-lbs. Crack open the bleeder valve.

Final Fill, With Proper Dilution
The owner's manual gives the capacities. Here are the relevant ones for a 2.3L, auto tranny Accord:

Coolant Change Capacities, Auto tranny, 2.3 liter Engine
Change (includes radiator, reservoir, much but not all of the piping and engine block) = 1.43 gallon
Total in system = 1.82 gallon (in other words, draining the radiator, reservoir and block does not remove all the coolant)
Reservoir holds 0.16 gallon = 0.64 quart = 2.56 cups
For a manual tranny or 3.0 Liter engine, see the owner's manual. Adjust the numbers herein accordingly.

To obtain a 50/50 mix, one has to add 1.82 / 2 = 0.91 gallons of the concentrated Honda Blue (or aftermarket equivalent) to the cooling system. The draining you have done up to this point leaves plenty of room to add 0.91 gallons to the cooling system. If you had not siphoned coolant out through the thermostat case, you would not be able to add 0.91 gallons of the concentrated Honda Blue.

First, remove 0.09 gallons ( = 1.44 cups) from the jug of concentrated Honda Blue. Set this 0.09 gallons aside in a container. You are not going to use this 0.09 gallons either today or anytime soon.

Now your jug of Honda Blue holds exactly the amount of coolant you want to put into the cooling system.

From the jug of Honda Blue, take 1.28 cups and add it to the reservoir. Then add 1.28 cups of distilled water to the reservoir. The reservoir now has a 50/50 mix of Honda Blue concentrate and distilled water.

Add the rest (= 0.91 gallons less 1.28 cups) of the jug to the radiator. Now your Accord's cooling system (reservoir, engine block, radiator, pipes et cetera) has exactly 0.91 gallons of 100% concentrated coolant in it, mixed with a certain amount of distilled water.

Add distilled water through the radiator neck until the coolant mixture runs out of the bleeder valve in a steady stream. Shut the bleeder valve. Torque the bleeder valve to 7.2 ft-lbs.

With the radiator cap off, run the engine at idle. Check for leaks. You can squeeze the upper and lower radiator hoses a bit to help air removal. Idle the engine until the two fans come. The fans will eventually go off. Continue idling the engine until the fans come on a second time. Then stop the engine. Top off the radiator with distilled water. Install the radiator cap.

Feel free to take your Accord on a test drive a this time. Have a few quarts of distilled water with you.

The Next Few Days, First Thing In the Morning
Check the level in the reservoir. It should be between the high and low marks. If it falls below low, add distilled water to bring it to the high mark.





For Advanced DIYers: Removing the Engine Block Drain Bolt
From experience with other Hondas and inspecting this 6th Generation Accord's engine drain bolt, I think the engine block drain bolt can be removed without too much difficulty, and using tools that many folks already have. Here's how:

Put the Accord on either ramps or jackstands, with backups. Put eye protection on. Without eye protection, debris will almost certainly fall into your eyes while you are under your Accord. Have a 19mm or 3/4-inch socket under the Accord where you can reach it once you are positioned. On your back, crawl under the Accord, with your feet sticking out from under the bumper. Position your head roughly under where the oil filter is. The drain bolt is on the rear of the engine, roughly at the 10 o'clock position as one faces the rear of the engine and the oil filter. See the photo and drawing below.Put your hand roughly where you think the drain bolt is, then feel for the 19mm (=~ 3/4-inch) drain bolt's head. Have a 19mm or 3/4-inch socket in your hand to see if it fits on the bolt head. If it does, then you found the drain bolt. On the 2.3L 1998-2000 Accord, if you scoot back several more inches, you can see the entire drain bolt head.

I found room enough for a 19mm or 3/4-inch deep socket with appropriate extensions and a 1.5 foot breaker bar. There should be a few extra inches of clearance if you want to put a pipe extender over the end of the breaker bar, for more torque. I would PB Blaster the bolt head several days beforehand and drive the Accord around to help promote the PB Blaster soaking in, especially beneath the washer. When you are ready to free the bolt, if the Accord is a bit warm, then this will help free the bolt. In other words, consider just barely cracking the drain bolt free with the engine still warm. Before proceeding further, wait for the engine to fully cool, to keep yourself burn-free and to help ensure the engine block does not, say, warp from suddenly draining hot coolant. When re-installing the engine block drain bolt, and per the Honda service manual, apply either Permatex Ultra Grey, Permatex Ultra Copper, or Hondabond to the threads. On my old Civics, I always used Permatex Ultra Grey.

When re-installing the drain bolt, torque to 61 ft-lbs.

Part Numbers
Engine drain bolt washer (28 mm) 90401-PR4-000. This is a crush washer, good for one torquing only.

Thermostat 19301-PAA-306 or possibly Aisin thermostat (172 degrees F) THH007 . Nippon is the manufacturer of the OEM thermostat.