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Here is another possible "simple to implement" option, to get more coolant flow through the back of the head's water jacket.


It would involve simply replacing the existing mechanical heater water ON/OFF valve, with just two (2) ports, to a four (4) port heater FULL bypass valve.

When the heater is "ON", the hot coolant would pass through the heater as normal.

When the heater is "OFF", the hot coolant would bypass the cabin heater, & return to the inlet side of the water pump, via the existing return coolant line.

Consequently, coolant would flow out the back of the head, at all times the engine was running, whether the heater was ON or OFF.


The only issue with our Rollas, is that the area where the existing "ON/OFF" valve is located, is a bit tight, so the new 4 port valve may have to be located adjacent somewhere close, & a few 16mm  I.D. hose connections made.

The area between the back of the head & the firewall, is also tight, but at least, there is only one hose connection there.

Here is a cheaper plastic one, that is cable operated & comes in two (2) models.

25-1018  Pull to Open,   &    25-1019  Pull to Close.


They work like this . . .



Only trouble is I have been unable to find a local source here in Australia or S.E. Asia on-line.  From what I can see, they are only available on line in the UK & USA.

Give us a yell, if you know where I can source one locally.

P.S.  In researching this, I found out why they are so available in the northern "colder" hemisphere.  Apparently, in many older European cars, the coolant flows through the cabin heater coil all the time.  The heat is controlled not by regulating the water flow at all, but by the air flap, in the heater box.  When the heater box air flap sponge rubber seals deteriorate, hot air leaks into the cabin, when the flap is closed, & in July (their only warm month) it gets too hot inside the car, so these valves have become popular as a mod on older cars, to stop the water flow altogether, when heating is not required.  Modern European cars apparently, sometimes have the heater on constantly, & it is turned off, when the A/C unit is switched on.  A bit different to our conditions, here in Australia.

Cheers Banjo




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Hi Jeremy,

                   Yes, it is cable operated, but is just a 2 port valve (ON/OFF). However, it is not on the engine side of the firewall.  It's under the dashboard, close to the heater coil.  You could just keep the old one open permanently, & add this suggested one, in the engine bay.



Below are pics of the heater coolant return path to the inlet side of the water pump.





Cheers Banjo

Edited by Banjo
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That's the cleanest non show spec engine bay I've seen yet.

I usually go to aliexpress for this kind of thing, (prototyping and inventing). I just had a look, but yeh the problem is like u say, no cable operated ones. Only electric or vacuum actuated. Or two port cable operated :(


Nope I can't find any

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  • 3 weeks later...

I had a bit of trouble doing this rear head coolant temperature measurement.  I was hoping to insert a temperature sensor in the water jacket itself, in the rear of the head, so that the sensor was screwed into the head itself, just like the one fitted below the thermostat.

There are no screw plugs at the rear on my 4K-U head, but it does have the small plate on the rear of the head, for the heater inlet connection, into which I could have fitted a ferrule, & screwed the sensor into it.

I was reluctant to remove this plate, as the threads in the head are not blind, & the bolts become rusted, & invariable snap off, trying to undo them; especially the lower RHS one.

There is also only a couple of inches to work between the head & firewall, so any issue, would have resulted in the head coming off, which I wanted to avoid.

I decided to buy an inline hose adaptor to take the sensor (gold bit).  I found a suitable bit of hose, with two appropriate bends in it, at SCA, which was designed for a VR Commodore 3.8L V6 Water Valve to Heater Inlet (P/N CH1779).

The pictures below, indicate how I mounted it all, with the sensor as close as possible to the point where the coolant exited the rear of the head, on it’s way back to the water pump inlet, as it’s a heater return line.







Got any air out of the system, by running it for 10 minutes, with the radiator cap off, & it was ready to go.

A week ago here in outer SW Brisbane, it was in the high 30s, but last night, couldn’t have been more different, with air temperature in the low 20s, probably 15 deg C, below what it had been 7 days previously.

So my daughter needs picking up at the bus stop about 15 minutes drive away.

I drive to the bus stop, wait less than 10 minutes for her to arrive, & straight back home again.

The coolant temperature measurements are shown on the graph below.


Unfortunately, the night was so cool, the ram effect of the ambient air, wouldn’t allow the thermostat to open. From previous tests, I’ve done, I know this thermostat opens at approximately 88 deg C, as it should. In this case, the temperature never reached that temperature.

However, the rear head coolant temperature, was actually cooler than the coolant just under the thermostat, at the front of the engine, by just a few degrees, which is a great result, as I know previously, I had measured head surface temps up to 10 deg C greater at the rear. Some of this could be because the sensor was not in the head proper, & quite likely, the fact the water was moving over the rear sensor, as it doesn’t return to the top of the radiator, on the inlet side of the thermostat.

So a few more tests are required.  Firstly, I’ll take the plastic fan off, and run without it.  If that does not help increase the coolant temperature, then maybe a piece of cardboard over the front of the radiator, will assist.

I may also plumb the heater return line into a spare stop plug in the thermostat housing, just below the thermostat, so coolant exiting the head from both ends, goes to the same point.

Not all Rollas have the plate on the back of the head with the heater inlet port.  If you have a 3K big port head, you are out of luck, & it may take a bit of machining to remove the “bung” & make a suitable plate.


If you have the plate without the heater connection, it does provide a situation, where you could fit an outlet facing to the dizzy side of the engine.


If you have ever had that plate off, at the back of the head, you can see why this overheating problem occurs so easily, with low flows.


The passages in the water jacket are quite small, & with low flow & “gunk” buildup, it’s easy to see that you could have a head with minimal flow at the rear.


If this mod works as well, as it appears it will, it might also be a good idea, to either buy a head gasket with no water jacket feeders between block & head "blocked off". Alternatively, as most head gaskets now block the front feeder holes off, to direct all coolant to the rear of the head, maybe these could be drilled out, in the head gasket.

So I’ll do some more tests, & see whether I can’t generate some more heat into this coolant, & see what transpires.

Cheers Banjo


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Hi Keith,

              Just about to pull the fan & radiator cowling off, & take a run into town in about 90 mins. & see what changes that makes.  Another cool day today, which is pretty unusual for SE Queensland, at this time of the year.


Love the way the heatsink puts both head temps up when you turn it off, but the hose temp goes down.  Very distinct.

I love data logging, as it shows you dynamically what is happening, especially when you measure & record several variables at once, that each have an effect on each other.

Sometimes it throws up things, which are quite the reverse to what you imagine is going on.  It's always eye opening.

They say, "What Gets Measured, gets Managed", & there's another one that says "Without Data, it is Just a Matter of Opinion".

It's a great way of quickly solving issues, instead of the "trial & error" method.

I'll throw up the graph without the fan, & cowling, when I get back.

We will get this issue sorted, I'm sure, but my idea is to change one thing at a time, so you can quantify the results.

Cheers Banjo



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I don't beleive those readings. 
Is it possible to swap the temp sensor probes from front to rear around and test again, just in case the rear one is calibrated differently? or are they different threads?
Then again, coolant that gets to the front, has in theory been exposed to the heat for longer with the new modified gaskets as it goes from the rear to the front of the head. 
The coolant going out the back has just come up from the block and gets to exit before it gets the opportunity to suck any more heat out the head.
Its the difference in the actual head temp (between front n rear) that concerns me. notreally the coolant as such.

It think... all the holes in the gasket should be drilled out, and the rear of the head should have another thermostat and coolant exit the same size as the front one. haha

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Interesting thoughts Jeremy.  It occurred to me that the real issue is that the cooled coolant from the lower radiator hose enters the water pump, & therefore the block, at the front of the engine.  The heated coolant, exits the top of the head at the front of the engine.

Best results must be when the coolant enters the engine at one end, & exits at the other.  That way you get consistent & uniform flow through the whole engine, however, it does create a temperature gradient along the length of the head, with the rear still being hotter than the front.  This assumes that all the water jacket holes in the head gasket between block & head are opened.

An even better way, would for coolant to enter the block at the centre of the engine, & exit from both ends of the head.  Bit impractical in our case.

However, the first scenario, is possible.  Leave the water pump where it is, but remove the thermostat enclosure from the front of the head, & replace with a blanking piece. Weld a larger spout to the plate on the back of the head, & direct coolant back to the top of the radiator, via a thermostat. Only trouble is there is not a lot of room between the back of the head & the fire wall.

There are some car engines that are designed that way.

Anyway, to things current.  I've been driving around today, without a fan at all, & with no fan cowling.



The results are quite different, & I did get the coolant temperature up over the 90 deg C mark.


The average ambient temperature was around 26 deg C.  I left home at 10:20am, from a cold engine start, and you can see it took about 10 minutes for coolant to reach 70-80 deg C.

The next 10 minutes to 10:40am, was on open road @ 80 klm/hr without traffic, & you can see how stable the temps were.  From thereon in, you can count every set of traffic lights I stopped at, once I hit town, with no fan to draw air through the radiator to cool it.  Just before 11:10am, I stopped at the P.O. for 4-5 mins, and you can see head coolants, front & rear rise sharply, with the heat sink effect, but the lower hose coolant temperature actually dropping.  The interesting part is that it there is only about 1.3 deg C, between coolant temperature entering the engine, & the coolant exiting the rear of the head. In fact, just looking at the graph, in places it looks like they are on top of each other.  This is a bit of a puzzle.  The other one, is why does the temperature at the bottom of the radiator drop so suddenly, as soon as the engine stops.  Hot water rising ??

You can really see why you need a thermo electric fan in city stop start driving, only switched on, when needed.

The sensors used under the thermostat & in the lower radiator hose are identical, and were calibrated together, in the same data logger, in boiling water.  Graphs of the temperatures of both probes, when the car has been stationary over night, where engine & coolant are the same as ambient temperature, also show both temp sensors reading exactly the same.

I might see if I can't "replumb" the return coolant line to the underside of the thermostat, rather than back to the inlet of the water pump, which is where it currently is. That may display a different coolant operating characteristic.

Might pop the radiator out at the weekend, & see how easy or difficult, it is going to be to fit the Echo radiator with integral thermo electric fan.


Cheers Banjo



Edited by Banjo
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I don't think the fan need a cowling, the cowl just blocks airflow for the other 90% of the time.  It only needs to remove a small amount of heat from an idling engine, and a slight breeze through the rad seems to do it.

Weigh both rads empty and measure the volume they hold as you do it. The alloy rad I bought is lighter than the stock, but holds more water to end up as the same total weight. I'm always worried about sticking more weight in the nose.

Overall thermoswitched fans are great! A quieter motor, extra engine power, and they hardly ever come on.

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Hi Keith,

           I tend to agree with you about the need for a cowling.

I did a bit of research, & basically the cowling has two functions.

1.  To direct air flow

2.  To prevent cavitating & looping of the air at the tips of the blades.

" The most efficient movement of air occurs when a fan has a cowling. A cowling prevents the looping and forces the air forward, moving much more air for the same power."


However, although some could argue that with an enclosed space under the bonnet, all air will pass over the engine, & through the engine bay & exit down under the rear of the car, whilst ever it is moving forward, when the car is stationary at lights, in traffic, there is no forward movement, so maybe  the air needs direction more onto the engine itself.

Debatable ?

The argument against a cowling, is as you say   .   90% of the time the fan is not required at all.

The best way to decide this is for me to try it out, once I have fitted the thermo electic fan & radiator.

I can then take measurements & see what effect removing the cowling has.

I will definitely make the weigh & volume checks you suggested when swapping over the radiators.

Cheers Banjo

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