[mikael82]

Hi
Just before I purchase a thermostat for my 94VT, I would like to confirm that the NAPA number THM 399198 or THM 148 is the correct number. Can anyone confirm this please?
Kind regards

I can not see Napaonline in Europe so I can't see what those numbers are and for what cars they go.

But this is original Diablo thermostat:
Behr B.476.74

 

[mikael82]

This is not so simple thing...

What temperature should thermostat have? Factory says 74 celsius.
Fans will kick on att 80 celsius and off again 74 celsius.

There for temperature for thermostat should be 74 celsius, am I right?
(I am asking this because Paul has refered temperature rage being 79 celsius and that is too high in my opinion.)

 

[mikael82]

Original temperature for opening of Diablo thermostat is 74 celsius.

I also have 79 celsius thermostat that is aftermarket. (infact my car did not have thermostat att all! and you can't run it without one.)
I have couple of options coming in to replace:

-Original 74 celsius that is sceary expensive.
-80 celsius unit that is too high temperature.

Issue is that there is no right sized alternative to replace original, but I try to see if I can modifi one to fit.

(Issue with thermostats for me is that here up north they only sell "winter" thermostat that are always little higher temperature, so I have to order all "summer" thermostats from central Europe.)

 

[mikael82]

If your thrermostat is working and no issues; do not change it.
There are only old stock thermostats left and when those are gone, there are no more.

Thermostat should work with radiator fan switch, so it gives radiators full change to cool down engine and that is with thermostat being fully open if this does not work radiator fans will cool it down.
This is how system works:

Radiator fans work:
80 celsius "on"
74 celsius "off"

Thermostat works:
74 celsius start to open
86 celsius fully open 7mm



Radiator fans keep car att operating temperature.

If you have thermostat higher:
Thermostat opens 79c (and fully open?)
Radiator fans start 80c

In this case: when thermostat opens radiator fans will be "on" same time.
Radiator fans will cool coolant to 78c and thermostat is closed and inner "warm up" sicle is open.
Issue here is that thermostat is never fully open, because radiator fans will cool it closed imidiatly so cooling system does not fully open (or close inner "warm up" side)

This is why you should use 74 c thermostat;
cooling system is fully open (and inner "warm up" side closed) when radiator fans work, coolant flow is maximum because it is not just thermostat operating temperature it also to do with coolant flow.

 

[mikael82]

Can you advise where to purchase the thermostats you are talking about Mikael?

Ebay is best source for now, I think Lamborghini don't have them anymore.

I am not having a problem with mine but it the correct thermostats are few, then I would like to purchase one before there are no more.

There is no dirrect replacement that I know.
Couple of issues are are size and temperature, but also flow, how much valve opens.

I have one good looking replacement with 75c but more of this when I project going.
here are couple of options I have, do note plate size, there are one ebay seller that has for sale thermostat that is top left in picture, it does not fit, but I will try to see, if I can cut down larger plate to fit Diablo thermostat housing, but it is 75c thermostat wth only little bit too big "plate" size.

Brass thermostat on lower side of picture has too low flow and would need adapter, it does not work.

But I write it again here; Do not change original thermostat if it is not broken!
With eny other car I would throw away thermostat and replace it, they are cheap, but with Diablo, because there are no parts... Basicly good guality thermostat do not brake, don't change them because you can.

 

[mikael82]

Here is current selection and that is it.

- Upper one is good, seems original.
- Middle one does not work, plate is too wide and it does not close inner circule. Do not buy this one.
- Lower one seems to be good, similar to upper one.

 

[mikael82]

I can not see Napa website, because people outside US are blocked; what are specs for that thermostat?

 

[mikael82]

180f (no mention what is starting temperature and what temperature thermostat is fully open) thermostat is not ideal and would like to know what range it is. I did try that "european" BMW thermostat, mention on other site, but radiator fans did run more often with it.

If your thrermostat is working and no issues; do not change it.
There are only old stock thermostats left and when those are gone, there are no more.

Thermostat should work with radiator fan switch, so it gives radiators full change to cool down engine and that is with thermostat being fully open if this does not work radiator fans will cool it down.
This is how system works:

Radiator fans work:
80 celsius "on"
74 celsius "off"

Thermostat works:
74 celsius start to open
86 celsius fully open 7mm



Radiator fans keep car at operating temperature.

If you have thermostat higher:
Thermostat opens 79c (and fully open?)
Radiator fans start 80c

In this case: when thermostat opens radiator fans will be "on" same time.
Radiator fans will cool coolant to 78c and thermostat is closed and inner "warm up" sicle is open.
Issue here is that thermostat is never fully open, because radiator fans will cool it closed imidiatly so cooling system does not fully open (or close inner "warm up" side)

This is why you should use 74 c thermostat;
cooling system is fully open (and inner "warm up" side closed) when radiator fans work, coolant flow is maximum because it is not just thermostat operating temperature it also to do with coolant flow.

By runing higher temperature thermostat will mean that driving, with natural airflow and high coolant speed, you will loose that part of cooling where radiators do the job and end up constant runing with radiator fans.

 

[mikael82]

Nope, that's it...

One option is TX 23 75D , but it is not dirrect fit, I have grind one down, that I am using now and it work great.

My Diablo thread, fixing and service

So it hapend; I got my self a Diablo, next to Countach. Fresh from flatbead as I can not register this car in my country before it is 25-years old to avoid huge car tax, so two years I have time to get this car going.

www.lamborghini-talk.com

btw, you can dismantle Mahle thermostats and I have done so, but even so I did not find any combination that allow two different thermostats to be made as one that works with our cars.

 

[mikael82]

Never run these cars without thermostat; Engine works with two circuits.
1. Inner circuit, open while engine warms up, but closes when temperature is reached
2. Outer circuit, closed while car warms up, that opens after temperature is reached

If run without thermostat, waterpump mixes inner and outer circuit, so you end up roughly half cooling used.
(My car was without thermostat because it is custom to do so in middle east and issues with VDO gauge)

 

[mikael82]

You got very clean machine work on that thermostat housing. Next time I have inlet manifold out I proably machine mine flat as yours, helps a lot for finding thermostats.