P38A Alternative Thermostat - V8

After my recent overheating issue, one the part I'm changing just in case is the thermostat.
I've just gone for an aftermarket one.

I was disappointed that I couldn't get one that operates at a lower temperature without buying something like this aluminium cased one for £400. Also it's out of stock:
RPI Thermostat

Which got me thinking, could I piece my own together?

I'm thinking about a universal low temp inline thermostat like this: Universal Unit
That's £70. I'd want to 33mm one.

Then buy a couple of 32mm / 18mm tee pieces like these at £15 a piece: Ebay Link

Then some jiggery pokery like this.



I would then have a serviceable thermostat where I can select the temp I want all in for a £100 plus a few extra jubilee clips.

Looking at my new aftermarket heart thingy now, even when the thermostat is open (I tested it) the top three pipes are all connected so I think my idea would operate exactly the same way.

My only hesitation is I've struggled to get seals when using these aluminium T pieces before. But other than that, any thoughts on this "upgrade"?

The reason thermostats were set to higher and higher temperatures was for emissions targets. I don't know what effect running it at a lower temperature would have on that. I'm guessing it would probably be OK as you can adjust your new setup up or down.

Montyjohn said:

After my recent overheating issue, one the part I'm changing just in case is the thermostat.
I've just gone for an aftermarket one.

I was disappointed that I couldn't get one that operates at a lower temperature without buying something like this aluminium cased one for £400. Also it's out of stock:
RPI Thermostat

Which got me thinking, could I piece my own together?

I'm thinking about a universal low temp inline thermostat like this: Universal Unit
That's £70. I'd want to 33mm one.

Then buy a couple of 32mm / 18mm tee pieces like these at £15 a piece: Ebay Link

Then some jiggery pokery like this.
View attachment 298896


I would then have a serviceable thermostat where I can select the temp I want all in for a £100 plus a few extra jubilee clips.

Looking at my new aftermarket heart thingy now, even when the thermostat is open (I tested it) the top three pipes are all connected so I think my idea would operate exactly the same way.

My only hesitation is I've struggled to get seals when using these aluminium T pieces before. But other than that, any thoughts on this "upgrade"?

Click to expand...

A working thermostat is a pretty reliable piece of kit and lower temperature devices are available but IMO will not stop an overheat. Overheating occurs due to a problem either with the cooling system or the engine itself so to me your complicated arrangement serves no purpose. You could always try running with no stat or one that is jammed open.

You can run at a standard 82c with no issue at that point the fuel map is standard, not on the cold/rich 'choke ' map.
The difficulty is the bypass valve that opens and closes with the thermostat, which allows coolant to flow when the thermostat is closed, then closes at the thermostat opens, to force water through the radiator

Datatek said:

A working thermostat is a pretty reliable piece of kit and lower temperature devices are available but IMO will not stop an overheat. Overheating occurs due to a problem either with the cooling system or the engine itself so to me your complicated arrangement serves no purpose. You could always try running with no stat or one that is jammed open.

Click to expand...

I drilled a small hole in mine so there was always some flow through the radiator. It took a bit longer to warm up but on a hot day out would reach normal operating temp if left to idle to which defeats the point

kermit_rr said:

The difficulty is the bypass valve that opens and closes with the thermostat, which allows coolant to flow when the thermostat is closed, then closes at the thermostat opens, to force water through the radiator

Click to expand...

Are you sure the thermostat is supposed to operate this way. I assumed it would, but I can confirm that if I heat my new one that arrived today, the top bypass inlet does not close off and remains open.

Montyjohn said:

Are you sure the thermostat is supposed to operate this way. I assumed it would, but I can confirm that if I heat my new one that arrived today, the top bypass inlet does not close off and remains open.

Click to expand...

Certainly on the Thor one they should be physically connected, don't know about gems

Datatek said:

A working thermostat is a pretty reliable piece of kit and lower temperature devices are available but IMO will not stop an overheat. Overheating occurs due to a problem either with the cooling system or the engine itself so to me your complicated arrangement serves no purpose. You could always try running with no stat or one that is jammed open.

Click to expand...

I do believe I no longer have overheating issues, however I would feel more comfortable knowing I had a larger buffer in my engine temperature.
This engine seems very sensitive to any cooling defect and I wonder if it's because it runs so hot as standard.

Montyjohn said:

I do believe I no longer have overheating issues, however I would feel more comfortable knowing I had a larger buffer in my engine temperature.
This engine seems very sensitive to any cooling defect and I wonder if it's because it runs so hot as standard.

Click to expand...

I noticed that when i had a blockage problem it would get to around 98, then start going up much quicker, compared to the increase from 80-90

kermit_rr said:

Certainly on the Thor one they should be physically connected, don't know about gems

Click to expand...

I think I'm not explaining myself properly.

The top three pipes are physically connected, I just assumed that an open thermostat would block the bypass hose off (top middle one). But on the one that just arrived today doesn't do this, which means water will still bypass, as well as go through he radiator.
Not sure if this is a defect or not.



I was also surprised that to open the thermostat, a gentle rolling boil wouldn't do it. It needed to be violently boiling. So appears to be closer to 100 deg, rather than the low 90's I'm sure I read somewhere.
So I could easily see the top hose being 110 degrees during normal operation.

Which might be fine, but seems high.

If i recall, it's supposed to start working at 86 and is fully open around 96 which is essentially boiling. It was somewhat slow to respond when i tried it in a pan
That top one should be normally open then start to close as the thermostat starts to open. It should fully close when the thermostat is fully open but like i day that takes a while. I daresay in normal driving, both are part open

Montyjohn said:

I think I'm not explaining myself properly.

The top three pipes are physically connected, I just assumed that an open thermostat would block the bypass hose off (top middle one). But on the one that just arrived today doesn't do this, which means water will still bypass, as well as go through he radiator.
Not sure if this is a defect or not.

View attachment 298899

I was also surprised that to open the thermostat, a gentle rolling boil wouldn't do it. It needed to be violently boiling. So appears to be closer to 100 deg, rather than the low 90's I'm sure I read somewhere.
So I could easily see the top hose being 110 degrees during normal operation.

Which might be fine, but seems high.

Click to expand...

I would say that stat is faulty.

What he said ^^^^^
Drill it for piece of mind

It wouldn't surprise me if there's always a small flow to the rad. There's always flow through the matrix. Older cars ran at about 83C but they pushed that into the nineties to hit emissions targets.

Grrrrrr said:

It wouldn't surprise me if there's always a small flow to the rad. There's always flow through the matrix. Older cars ran at about 83C but they pushed that into the nineties to hit emissions targets.

Click to expand...

The thermostat I have seals the rad flow pipe really well. When closed, nothing gets through. I'll get the old one off this weekend and see if that one seals the bypass when the thermostat is open.

biketeacherdave said:

What he said ^^^^^
Drill it for piece of mind

Click to expand...

Drilling the thermostat won't prevent bypass however, which is my concern.

Is your car GEMS or Thor ? The flows are different, but not sure about the stat.

The bypass always requires some flow. When cold the stat blocks flow to the radiator. Warming coolant flows from the heads towards the top of the thermostat and back to the water pump. There's a separate flow from the head to heater, which returns via the top of thermostat and also back to the pump.

Once the thermostat opens, most of the hot coolant flows to the radiator, and back through the bottom of thermostat. The heater return joins the radiator flow back to the pump.

The key difference between THOR & GEMS is the bypass hose. On THOR it's part of the main hot flow to the radiator, but on GEMS it's a separate bypass pipe from the heads. When the thermostat is fully open it's the bypass pipe that gets most blockage, or ensure all the hot coolant goes to the radiator.

What exactly do you want to prevent?

pwood999 said:

When the thermostat is fully open it's the bypass pipe that gets most blockage, or ensure all the hot coolant goes to the radiator.

Click to expand...

This is where I think my new Thermostat isn't doing what it's supposed to do. Unless, boiling it at 100 degrees isn't fully opening the thermostat.

kermit_rr said:

What exactly do you want to prevent?

Click to expand...

I'm worried that when I fit this thermostat, when the engine is hot, I won't get as much flow going through the radiator as there is a large amount bypassing as it's unrestricted.