Difflock Guide to checking and renewing vehicle coolant and thermostat

The cooling system is a critical part of your car and neglecting it can cause serious engine damage.  Whatsmore, such a failure will most likely occur during the first real onset of winter or on the annual long, hot summer journey to your holiday destination since these extremes of temperature aid either freezing or overheating.

This is because the coolant used in the system comprises at least 50 percent water and this, without special additives, can either freeze solid or cause internal corrosion leading to leaks or blockages.  Luckily all the right additives to prevent freezing and corrosion are contained in modern Antifreeze DF261 mixture and this will be added to the vehicle from manufacture.  However its properties degrade over time and for this reason the antifreeze and water mixture in the system must be renewed every 2 years without fail!

To help you maintain you vehicle in tip top condition, we’ve produced this comprehensive guide to checking and renewing your engine coolant.

Check the antifreeze strength

You can check that your antifreeze mixture is not too dilute at any time of the year.  All you need is an inexpensive AntiFreeze Tester DF320 and access to the vehicle’s coolant.

Check that the engine has cooled so that it is no more than lukewarm to the touch of your bare hand (or the coolant may be scalding and under pressure!).

Draw solution up into the tester several times to equalize the temperature of the instrument and then take a reading of the float level and coolant temperature.  These two figures are then cross referenced on a simple chart to find the point at which your coolant solution will start to freeze.  It couldn’t be simpler!

Draw up coolant solution into the Tester and read off the float level and temperature to work out the freezing point of your coolant.

DIFFLOCK TIP: Finite Nitrile gloves (shown in pictures above) are great at protecting you from chemicals but poor at protecting you from cuts and bruises or burns. When you encounter a nut or bolt that is very difficult to undo (or near a hot item such as the exhaust), pause and swap your synthetic gloves for a pair of thick Kevlar Gloves. That way if your hand or spanner slips or the nut releases suddenly you will be protected from harm by the thick gloves. Do the same if you are swinging a hammer or using a hacksaw and you are sure to finish the day with the same amount of skin as you started with! Don’t forget to swap back to the Finite Nitrile gloves to ensure your protection from chemicals

Draining and Renewing Coolant

If your coolant needs to be renewed, or perhaps drained off for maintenance, check that the engine has cooled (no more than lukewarm) and then follow these simple steps

Locate the radiator bottom hose and engine drain plug.

Locate a 17 litre Drainpan DF265 and Extra Large Driptray DF421 under the radiator bottom hose and using either a flat bladed screwdriver or better still a 1⁄4” drive, 6 point hexagonal socket (if the clip has a hexagon head) loosen the hose clip that retains it to the bottom of the radiator.  Gradually work the hose loose (but not completely free) until you can direct a steady flow of draining coolant into the tub. 

Be careful here, if the hose is stuck then try rotating it backwards and forwards to break the seal.  Its very easy to find the hose has flown off the radiator coupling and coolant is gushing out everywhere, including over your good self!  Only when you have steadily drained out as much coolant as possible should you then release the coolant filler cap to release any remaining air locked fluid. 

DIFFLOCK TIP: Only buy hose clips that have hexagonal heads and replace any that have round slotted heads..  Hexagonal heads are far, far easier to fit and undo than simple slotted heads and you can use UJ's and extension bars on sockets to reach really awkward ones.  A hex head also allows the easier application of torque (via a ratchet handle) especially on stiff or seized plated mild steel clips.  Remember though that you will need a matching 6 point hexagonal socket to be sure not to 'round' the hex head of the clip.  Also use a 1/4" drive system if you can to prevent inadvertent over tightening.  When replacing hose clips spend a little time thinking about where the hex head will be positioned once the clip is tight.  Try to ensure that it is positioned in the easiest place for you to subsequently undo the next time around.  If you can obtain them, stainless steel jubilee clips are ideal, but put a small dab of silicon grease on the screw. Never use chassis grease, which could harm the rubber hoses.

Once the radiator has drained down you can then reposition the bowl and undo the engine drain plug.  Here you may find some spillage occurs as the coolant drains down so mop it up immediately if this happens.

Replacing Thermostat

Thermostats are absolutely vital parts of the engines cooling system and you should never run an engine without one, even in the Tropics!  This is because they regulate the flow and direction of coolant around the engine at all operating temperatures, from freezing cold, through warm up right to full operating temperature.

Not all Thermostats are created equal however and we strongly advise that you only ever buy Genuine Approved Thermostats from the vehicle manufacturer since these are usually far more accurate in response and regulation than cheaper aftermarket parts.

Sometimes Thermostats are placed inline in the large coolant hoses themselves.  However most engines have them mounted in a dedicated housing attached to the cylinder head.  With this type of fitment it is imperative to note the direction the thermostat is fitted.

Undo the hose clips and remove the radiator top hose both from the thermostat housing and the radiator itself.  Using an appropriately sized socket undo the three bolts that secure the thermostat elbow (to which the hose was attached).  Remove the thermostat elbow and thermostat and remove all traces of the old gasket.

Here the thermostat housing cover has been removed and the Thermostat can be seen inside – Engine = Land Rover 200TDI.

DIFFLOCK TIP: Old gaskets often harden and ‘bond’ together the parts that they are sealing.  If this happens use a soft faced mallet (preferably Nylon) to gently tap any stuck parts to break the seal formed by a hardened gasket.  Be careful though, such parts are often aluminium or iron based castings and these are brittle and easily broken by overzealous levering or hammering.

Once separated all traces of the hardened gasket must be removed prior to reassembly with a new gasket.  Use a purpose designed scraper for this or one of the special gasket softening compounds available.  Again take care not to score the mating surfaces since this may affect the seal.  If you do score the surface then use a continuous bead of silicone based instant gasket instead of a ‘paper’ based gasket for resealing since it will gap fill to a certain extent.  Note that score marks on surfaces subject to very high pressures (such as head gasket mating surfaces) cannot be recovered in this way, nor can those on surfaces exposed to fuel.  If you want to recover a scored or slightly warped surface in a cast aluminium component then you can do this with a sheet of plain glass (modern float glass not glass from very old windows – its not flat!) and a few sheets of 600 and 1200 grade wet and dry abrasive paper.  Wet the paper, lay it flat on the glass and rub the scored surface backwards and forwards steadily over it.  Don’t ‘rock’ the component as you move it or the surface will not remain flat.  Do this with the 600 grade paper first and then the 1200 grade until the score mark has gone and the whole surface has a uniform appearance and colour (you will need to wipe it clean to see this).

Examine the thermostat.  It should be corrosion free, completely closed and the small ‘toggle’ free to move.  Check the temperature rating that is stamped on it; it should be as per your vehicle manufacturers recommendation:

If the temperature rating is wrong or if you have any doubts about the thermostat then replace it since it is a relatively inexpensive component.

DIFFLOCK TIP: You can do a very basic test on your thermostat by immersing it in boiling water.  It should be completely closed when cold but open fully in the very hot water, closing again as the water cools down.  However, for the small cost involved its wiser simply to replace a thermostat whenever its housing is opened.

Flush the system

Now take a hosepipe and flush water through the engine block via the thermostat housing.  Keep flushing until clean clear water emerges.  If you can back flush the engine up the bottom radiator hose (use old rags to pack out the gap around the hosepipe) then do this too.  Repeat the flushing procedure for the radiator, again back flushing if possible. If it possible to remove the radiator easily do this, turn it upside down and reverse flush it this way.

Locate the heater hoses and disconnect them both from the engine.  Use a hosepipe adaptor to join one of the heater hoses to your hosepipe and flush the heater out until clean water emerges.  Make sure the heater control is set to ‘hot’ during this procedure.

DIFFLOCK TIP: If your coolant emerged as brown, muddy water then the system has been badly neglected.  You should buy a radiator flushing kit  and flush the system chemically, following the kit manufacturers instructions.  Do this before refilling with coolant.

With the system fully flushed, reassemble the heater hoses, thermostat & housing and the top and bottom radiator hoses.  Use new hose clips if the old ones are rusty or very stiff to turn. Refit the engine block drain plug and tighten to the correct torque.

Refill with fresh coolant mixture

Make up a mixture of 50% water and 50% antifreeze (for UK all year climate) to refill the cooling system.  The antifreeze should be Mono ethylene glycol (MEG) based and be suitable for use in engines containing aluminium.  You need around 12 litres to refill the system but if you mix a couple of extra litres this can be stored and used for future topping up.

If your vehicle is an older design, it should be filled with fresh coolant via the radiator cap and this pressurizing cap left off temporarily after refilling.

In more modern vehicles the radiator has a filler plug but the system is topped up via an expansion tank.  Using a funnel refill the cooling system via the radiator filler plug hole until the radiator is completely full and replace the filler plug, tightening to the correct torque.  On some engines such as Land Rover 300TDI’s the coolant must also be filled via a filler plug in the thermostat housing.  Check that the coolant level in the expansion tank is level with its MAX indicator, top up if necessary. Some engines may have one or two other plugs to be removed or loosened when filling, this is to prevent airlocks.

Restart and watch for leaks

Check the system for leaks and if all other work on the engine is complete, restart the engine.  Continue to monitor the coolant level in the expansion tank or radiator filler cap neck (as appropriate) while the engine warms up.  You should let the engine warm up until the Thermostat has opened and warm coolant can be felt flowing through both top and bottom radiator hoses.  WATCH OUT for the cooling fan and belts since these are extremely dangerous when turning!

Keep monitoring the coolant level and top up if necessary.  Continue to check the level daily for 5 days and then weekly thereafter.  Note that Diesel engines are particularly susceptible to low coolant levels and the injectors are rapidly damaged by too much heat caused by poor cooling.  Never, ever start any engine without coolant, even for a few seconds.

Make sure you check only when hot OR cold as the manufacturer suggests. Failure to get this right may lead you to think you have a leak, because the fluid level can drop considerably when the engine is cold.

DIFFLOCK TIP: Watch carefully for leaks since fresh anti freeze has a ‘searching’ action and will often expose small leaks in the cooling system.  This is especially true if a chemical flushing agent has been used to purge a neglected system.  Never top-up with ordinary water because this will dilute the special antifreeze in the coolant, promoting corrosion inside the engine and increasing the likelihood that the engine will freeze up in winter.  Either buy or make up your own ready mixed solution of one part antifreeze DF261 to one part water.  If you need lots of coolant to top up the reservoir then you may have a leak somewhere or the filler cap may be faulty.  Go back and look for those tell-tale signs of antifreeze crystals around the system.  A serious or persistent coolant leak MUST be fixed as soon as possible.

DISCLAIMER: Repairing and servicing vehicles should never be attempted unless you have appropriate professional training and are suitably qualified. If you are in any doubt about the work you intend to undertake you should consult an appropriate professional vehicle mechanic. Additionally you should always read and follow any relevant manufacturer’s manuals and safety instructions in full BEFORE attempting any work on a vehicle. To the extent permitted by English law, Difflock Ltd accepts no liability for any loss, damage or injury arising as a consequence of any of the advice provided.