No Thermostat and Split Davies Craig water pump = Blown Head Gasket?

David I would hate to see someone replace a head gasket only to realise it was an issue with the coolant system, or a cracked block.
 
So here are some notes I made about testing the cooling system a while back. Some notes are borrowed from a guide by Ian Morrison (I hope you don’t mind Ian) posted in the MR2 Club Victoria magazine some time ago, and the rest I recorded myself based on my own experiences and discussions with other MR2 owners, some of which are members on this forum. Ian has a sticky guide which is in the technical forum.
 
When refilling the MR2 cooling system, the motor should be switched off and cold. Do not attempt to run the motor while you are adding coolant unless you are a professional mechanic. Place a tight fitting funnel in the filler neck to help with filling and slowly pour new coolant into the filler hole until no more will go in.
 
This may take some time and you may start to wonder if all that coolant you measured when draining out will ever go back in. But remember it needs to work it’s way in to fill all of that plumbing, especially through the bleeder rivet in the thermostat, so just stick with it and have patience.
 
You should eventually see coolant in the bleed hoses at the front and some air bubbles. If not, open the bleed valves a bit more until you see coolant. The coolant level in the hoses should come up to the same level as the coolant level in the filler funnel. Also note you can bleed air from higher in the system by slightly opening the bolt on the thermostat housing, the bolt that takes a 17mm socket (I think).
 
When there are no more bubbles and no more coolant will go in, leave it sitting for a few minutes. You may find the level has dropped further over this period. With the system full, close bleed valves, replace the pressure cap and tighten it to the first click only as recommended in the BGB (do not completely tighten).
 
Start the engine and let it idle for about 2 minutes. Then stop the engine, take off the pressure cap, open the bleed valves again and top up while checking for bubbles in the service hoses. Air pockets can remain around the radiator area and squeezing the heater hoses either side of the radiator below the bonnet a few times will dislodge the air bubbles that otherwise might stay there.
 
When there are no more bubbles in the service hose, close the bleed valves and repeat the previous sequence again with the pressure cap still on the first click.
 
Do this as often as required until you reach the stage where you have added almost the same amount that you originally drained out, then fully tighten the pressure cap, and take the car for a run for at least ten minutes.
 
When testing the system make sure that the thermostat has fully opened. You can tell that the thermostat has opened when fluid entering the radiator (left hand passenger side) is a few degrees hotter than the other side. If the other side (right driver side) does not get hot then you may have an air bubble blocking flow through the radiator. As I have a coolant temperature gauge, I disconnect the radiator cooling fans to make sure the coolant gets up to 95 degrees meaning the thermostat is or should be fully open, you just need to keep an eye on the temperature to make sure you don’t overheat the engine with the fans disconnected.
 
You will need to switch the motor off and wait for it to cool down before taking the pressure cap off to check the level, but at this stage if you have managed to refill the same amount as was originally drained out you can be fairly sure that it will be close to full. Top up at the filler cap as required. Don’t keep opening the pressure cap and topping up that little last bit over a few days as most people do, you need to let the system stabilise over a number of days. Changes in ambient temperature and atmospheric pressure can make a difference to the coolant level behaves.
 
The coolant in the plastic overflow bottle should be siphoned out and refilled with new coolant up to the ‘full’ mark on the side of the bottle. The mark is approx 300 mm below the top of the filler neck.
 
You should also check that the overflowed coolant when the engine is hot returns to the system when cold. Let the car fully cool and then check that the cooling system is full, it should draw back any expanded fluid back into the system from the overflow bottle. When the engine is all topped and cool, remove the overflow bottle, empty it and clean it thoroughly so you can observe the level of fluid in there afterwards – note the overflow bottle is a **** to get out but knowing the fluid returns to the same level each time the engine is cool is a critical sign of a well functioning system.
 
You would be better off trying to clean the outside face of the bottle while its on the car, you can reach it from top and bottom to give a good clean although often the internal surface of the bottle can be very dirty due to years of having coolant in it so you cant see the coolant level even after you have cleaned the outside. If you do take out the bottle best way to clean the internal surface is with some water and sand/gravel and give it a good shake, making sure you get all the sand/gravel out afterwards! You would be surprised how much crap builds up inside the bottle.
If it the system is not drawing the coolant back, try threading a thinner hose (I used a hard plastic hose) through the existing hose. The smaller diameter should help the fluid to be drawn back in easier. A bit like sucking fluid through a straw rather than a pipe if that makes sense.
 
If the system is still not drawing fluid then it there is most likely a leak somewhere as the system is in vacuum it is sucking in air rather than fluid.
 
Start by taking off the radiator and having it checked and new o-rings on drain and bleed taps. Take the fans off and then the radiator. You are better off cutting off the rubber hoses otherwise you might bend the end tank hard lines trying to get them off. When the radiator is removed, bridge the gap with a couple of hoses and hard pipe and run coolant system cleaner through the system. Thoroughly flush with tap water, bleeding the system as best you can without the radiator in place.
 
Empty the water from the system and then with garden hose thoroughly flush the radiator lines and block. When this is done use a cooling system pressure tester before you put new coolant in. Any leaks will show up more with air rather than water. Spray water on hose/hard line joints, radiator and fittings to engine to see if any air is bubbling, if significant you might hear the air bubbling the water. The system should hold pressure for 2 hours at least, otherwise you may have a leak.
 
When fitting radiator, use new hoses. Put the hoses on the radiator first and then fit in place. If you are having trouble getting the hose on the hard lines, the hard lines can be loosened to give you a bit of wiggle room.
Get a coolant funnel or lisle spill free funnel, they are great. Also a coolant temperature gauge (or ECU if you can dial in from laptop) is important to understand that thermostat is working properly i.e. hoses either side of radiator are hot. Also pressure test your pressure cap, they should be replaced every 2 years.