Thread: Discovery 1 V8 thermostat behaviour

Originally Posted by disconc

Hi everyone,

I have a 1998 D1, "rest of the world" 3.9 EFI hotwire, no cats and no O2, which we have just finished rebuilding and has about 1000 miles since then.
As I like to know where I am going, I have just added some VDO gauges to it, oil pressure and water temp.

I never thought it was overheating (well no evidence of it), and in fact it is not "really", but this is what I discovered :
From cold start, it warms up normaly up to the supposed thermostat opening temp, which should be about 88°C/190°F, but then it continues to warm up quite fast (well faster), up to about 215°F, and then, as if the Thermostat opened suddenly, immediately drops back to around 190°F and stays then after that all the time (never goes beyond 200).
In slow traffic,I sometime hear the aux fans quick in, but not for long, and the stock gauge never goes beyond the middle (accurate or not ?)

So I can not say it overheats, but I found it strange that it needs to get that high before the thermostat opens !

Thermostat is brand new, but from your experience on the D1 guys, is it behaving as it should or should I replace it ?

Thanks for your valuable feedback.

Erik
Noumea
New Caledonia.
---
1998 Disco tdi300
1998 Disco 3.9 V8 "rest of the world"

The way a thermostat senses temperature is of course by absorbing heat from the coolant passing by. If the coolant flow is particularly slow then the coolant at the temp sender may be higher than at the thermostat. As it remains mostly stable after that I would say the thermostat remains slightly open allowing plenty of flow. So one known method is to drill a 5mm or so hole in the thermostat skirt so water flow is improved. It may also be a case of not enough flow via the heater circuit, which takes water from behind the thermostat and flows it back to the pump.

Hello Erik,

In the past when my V8 engine displayed temperature behaviour as yours appears to be doing, I replaced the thermostat. After doing so, the problem was solved.

As bee utey advised, make sure that there is a 2mm ( not 5mm..way too big) hole at the 12 noon position in the thermostat. It usually takes a couple of days for the air to bleed completely out of the system once the thermostat has been replaced. I don't have an expansion tank, so others may have different experience in this regard.

Ron.

Hi Guys,

thanks for your answers.

bee utey, you talk about the heater circuit. I indeed installed the VDO temp sender there using a kind of special tee (19mm heater hose adaptor to provide a 1/8 NPT thread for a temps sender), as I read that the heater circuit should have a permanent flow of coolant out of the engine, and I though it would be a good place to measure the coolant temp.

Do you think this could explain the behaviour described ?

I am not indeed measuring the temperature at the same place as the thermostat, and when it opens, then the coolant can flow to the radiator, and the temp I measure imediately drops to the one expected...

RoverP6B, maybe I should also try to change the thermostat indeed as the part is a Britpart and I read in various places that the quality of these were sometimes.... crapy.

Thanks for your help.

Erik
Noumea
New Caledonia.
---
1998 Disco tdi300
1998 Disco 3.9 V8 "rest of the world"

Hey mate, i've got the same motor and had overheating problems even though the temp gauge said everything was good. I could always smell that hot watery smell, the sensors i feel are less than accurate. The thermostats in these motors are exactly the same as the ones used in holden 202's and the like so repco or bursons or whoever will have plenty and will cost a few dollars. I put a new one in and everything has been great since.

I have the same problem when theres an air lock in the system. Exact symptoms - 85 county v8, the temp sender is directly below the thermostat.

Easy way to tell if you have trapped air in there, is to run the system up from cold for a few minutes - then pop the cap, you'll get a water spout spewing hot water goodness everywhere if air is trapped in the system. Get the missus or the kids to do the popping of the cap. Much safer

Originally Posted by disconc

Hi Guys,

thanks for your answers.

bee utey, you talk about the heater circuit. I indeed installed the VDO temp sender there using a kind of special tee (19mm heater hose adaptor to provide a 1/8 NPT thread for a temps sender), as I read that the heater circuit should have a permanent flow of coolant out of the engine, and I though it would be a good place to measure the coolant temp.

Do you think this could explain the behaviour described ?

I am not indeed measuring the temperature at the same place as the thermostat, and when it opens, then the coolant can flow to the radiator, and the temp I measure imediately drops to the one expected...

The water in the heater circuit is flowing from some modest distance away from the thermostat? The small bypass hose from the thermostat area should normally allow the hot coolant to reach the thermostat and any air bubbles return to the radiator so providing it is running freely the small hose should make the two areas similar in temperature. It is of course possible that your thermostat is sticking, a different one may help.

Thanks again for your answers guys.

For peace of mind, I will try to change the thermostat when I can source one.
In the meantime, I will also try some other measures with an IR handheld thermometer, just to see what it reads...

Hardchina :-))
That air lock thing bothers me. You mean start from cold, let it warm/pressurize and then take of the expansion tank cap right ?

Is there any other tip to avoid airlock in this system ?

Cheers

Erik.

Originally Posted by disconc

Thanks again for your answers guys.

For peace of mind, I will try to change the thermostat when I can source one.
In the meantime, I will also try some other measures with an IR handheld thermometer, just to see what it reads...

Hardchina :-))
That air lock thing bothers me. You mean start from cold, let it warm/pressurize and then take of the expansion tank cap right ?

Is there any other tip to avoid airlock in this system ?

Cheers

Erik.

Yeah mate, if there's air in there it will want to expand a lot more than water. Make sure the system is as full as possable. if there's no air lock you'll just get a little overflow when the cap is popped. If you have a decent amount of air, it will sound like a hippo farting, lots of big gurguling noises and lots of water overflowing when popped. Just be careful - dont let it get too hot. Its a quick test - go and try

also most thermostats have a 2-3mm hole already drilled, for some reason they put a crimped dongle in the hole to block it off ??? I just cut this off with sidecutters and place the hole at 12 oclock as sugested earlier.