6.9L & 7.3L IDI Coolant Flush

Coolant Flush & Cooling System Service Procedures for 6.9L/7.3L IDI Diesels

Applicable Models:

1983 - 1994 Ford F-250, F-350, F-Super Duty

Applicable Engine(s):

International 6.9L IDI V-8 diesel
International 7.3L IDI V-8 diesel

This do-it-yourself coolant flush procedure is lengthy, but thorough. You'll want to set aside a solid day in order to flush your engine cooling system properly. The premise of this coolant flush, which flushes the cooling system with distilled water, is simple - get as much of the old coolant/water out of the engine as possible, and replace it with new fluids. There are alternative methods, and you are free to perform the procedure that you find fit. If using standard Motorcraft/Ford green fluid, we recommend a coolant flush once every 30,000 miles or every other year, whichever comes first.

6.9L & 7.3L IDI Cooling System Service Parts List

Part Description

Part Number(s)



Motorcraft RT-1049


Thermostat housing gasket

Motorcraft RG-531


Thermostat housing

Obsolete part


Radiator cap

Stant 10229


Upper radiator hose

with AC

Continental 61326


without AC

Continental 61324

Lower radiator hose

with AC

Continental 61323


without AC

Continental 61321

Engine coolant overflow tank

Obsolete part


Engine coolant

Motorcraft VC-5



Motorcraft VC-8


SCA/DCA test strips

Fleetguard CC2602A

[1] Factory 190 degree (F) thermostat; opening temperature between 190 - 192 °F.
[2] Obsolete part, no suitable replacement available. Aftermarket housings claim compatible fitment but we've yet to find a suitable replacement. Recommend checking with dismantler.
[3] Obsolete part, recommend checking with dismantler.
[4] Standard Ford green engine coolant.
[5] Supplemental coolant additive/diesel coolant additive required to combat the effects of cavitation in the engine cooling system.

Engine Coolant Selection & SCA/DCA

Your engine cooling system uses coolant and distilled water in a 50/50 ratio in order to remove heat from the engine. Coolant contains corrosion inhibitors, amongst other things, which help prevent corrosion in the engine water jackets and various other channels in which fluid flows. Engine coolant is often referred to as "anti-freeze". Since coolant has a lower freezing temperature and a higher boiling point than pure water, it prevents the cooling system from freezing in cold weather and helps prevent the engine coolant from boiling during operation. So why not skip the water and just use antifreeze? Water is the better working fluid in the cooling system - it has a higher heat capacity, so it is more effective in removing heat.

A few important remarks about coolant flushes and engine coolant selection:
1) Stay away from universal fluids unless there is an emergency and you need to limp the truck home for repair. These are fluids, sometimes pre-mixed, that are available at every gas station and auto parts store. The quality and compatibility may be questionable.
2) Avoid premixed coolants - they won't work with this flushing technique and you'll end up with an incorrect antifreeze-to-water ratio.
3) There is nothing inherently wrong with Motorcraft VC-5, ethylene glycol based engine coolant (Motorcraft green). It's readily available at any Ford dealership and is 100% safe for the IDI diesel so long as the proper SCA/DCA is added correctly.
4) There are many high quality engine coolants, such as those made by CAT and Fleetguard, that are popular alternatives for the IDI diesel. The benefit in converting to these coolants is extended service life and the fact that they come precharged with SCA/DCA.
5) Avoid the "tap water" flush. Tap water does not belong in an engine cooling system. Only distilled water may be used in your engine, and tap water is far from distilled.

Cooling System Flush & Service Procedures

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Draining the radiator

• Locate the radiator drain petcock on the driver side bottom of the radiator. Place a suitable collection container below the petcock, then open the valve to begin draining the fluid. Remove the radiator cap to speed up the process.

• Once the system has been drained, tighten the petcock and fill the radiator and overflow tank with distilled water, then reinstall the radiator cap.

• Start the engine and allow it to reach operating temperature. Since these engines have a reputation for running cool, it may be necessary to drive the truck.

The goal is to allow the fluid time to circulate, and for best results we need the thermostat to open, which only occurs when the engine is warm.

• After the engine has reached operating temperature, shut it down and allow the engine to cool - releasing the pressure in a hot cooling system can result in serious injury. We allowed the engine to cool for 1 hour between flushes.

• Repeat the flush process two more times - drain the fluid, refill the system with distilled water, allow the engine to reach operating temperature, shut it down, let the engine cool, repeat. You should notice that, with each flush, the fluid becomes clearer and clearer with each drain - we're essentially removing all the coolant and replacing it with distilled water.
• If the fluid still has a green tint to it, do not worry at this point as we're going to do another flush after we've replaced our components.

• Drain the fluid before continuing to the next step.

Removing the alternator

• The alternator and vacuum pump will need to be removed in order to access the thermostat housing, there is simply no way around it.

• To remove the alternator, first disconnect both negative battery cables, then remove the wiring from the top of the alternator and push it aside.

• Loosen the bolt to the right of the alternator that holds the tensioner bracket, then remove the upper and lower bolts (red arrows).

• Slide the belt off then remove the alternator and set aside.

Removing the vacuum pump

• The vacuum pump is held on by two bolts with 10 mm hex heads. There is also a hose in the back, held in place by a basic spring type clamp.

• Remove the mounting bolts, then remove the hose and set aside the vacuum pump (Note - the hose is likely fragile, take care when removing the clamp).

Removing the vacuum pump bracket

• Finally, remove the bracket for the alternator/vacuum pump, which is held in place by three bolts (red arrows in image at left).

Removing the thermostat housing

• There is now sufficient space to access the rear thermostat housing bolt.

• Remove the upper radiator hose, which is held in place by clamps at either end.

• Remove the thermostat housing, which is held in place by two bolts with 1/2 inch hex heads.

Cleaning the thermostat housing

• You'll want to clean the thermostat housing inside and out, especially the mounting surface. Be mindful of the check valve built into the housing. The small metal ball is held in place by a composite (cork?) material that is relatively fragile. Check to see if the ball moves freely - a shot of WD40 will not hurt anything if it is frozen, just don't get carried away and be sure to wipe away as much excess as possible.

• The thermostat housing mounting surface must be clean before continuing - use a gasket scraper or putty knife, then a fine sandpaper to remove all remnants of the old gasket.

Replacing the thermostat

• Stuff a clean shop rag into the thermostat mount of the engine block, then use a gasket scraper and fine sandpaper to thoroughly clean the mounting surface. The rag will keep debris from falling into the passage of the engine block.

• This surface needs to be absolutely clean and dry before proceeding, or the seal may be compromised in the next step; don't forget to remove the rag.

Reinstalling the thermostat housing

• Apply a thin coat of silicon sealant to the thermostat housing mounting surfaces (on the engine block and the thermostat housing). Do not go crazy with the silicon, very little is needed and you do not want excess sealant entering the thermostat or check valve opening.

• Install the thermostat and thermostat gasket, reinstall the thermostat housing, snug the bolts, then torque to 20 ft-lbs. Make sure the thermostat is installed in the correct direction!

• Replace the upper radiator hose (it is best to use all new clamps when replacing the hoses).

Replacing the radiator hoses

• Replace the lower radiator hose - it is fairly easy to access the clamps from below the truck.

• Replace all intermediate hoses, which run from the engine block to the heater core. Replace the radiator cap with a new one.

• Once the thermostat sealant has had time to cure, as per the manufacturer's instructions, fill the system with distilled water and perform another flush (per the prior procedures).

• Drain the system via the radiator petcock - if the fluid comes out clear (little to no signs of antifreeze remaining in the system), continue to the next step. If the fluid still has a sharp green tint, repeat the flush procedures as necessary.

• You now essentially have a cooling system that is purely distilled water - drain the radiator one last time, then close the petcock.

• Add your SCA/DCA per the instructions, unless you are using precharged antifreeze. We're using Fleetguard DCA4, a popular choice for the IDI diesel, but there are many additives for your to choose from including Ford's Motorcraft VC-8. With DCA4, add ~24 oz of additive - we'll test it later.

• Add 4 gallons of engine coolant (half of the approximate total capacity in order to obtain a 50/50 ratio). If you can't fit 4 gallons of coolant in at this point, do not worry; there's air in the system, simply let the engine run for a bit, cool, then continue to add.

• Once you have added 4 gallons of coolant, top it off with distilled water (to maintain our desired ratio) until the system is full and measures appropriately on the overflow cold fill line.

• Finally, check for leaks and repair accordingly.

Adding SCA/DCA to the IDI diesel engine

• The last step is to test the SCA/DCA level using a set of test strips. NAPA and Fleetguard test strips seem to be the most readily available. Testing should occur only after the truck has been driven and the coolant has mixed thoroughly. Follow the procedures included with the test strips.

• It is worth noting that SCA/DCA test strips have a limited shelf life, so do not intend on stocking up for any period of time; buy them as you need them. We also advise using test strips offered by the manufacturer of the SCA/DCA brand you choose.