SIII diesel thermostat woes...

TheYellowLandy · May 3, 2021

Hi,

A couple of weeks ago I went for a drive and within a mile or so the temperature gauge was getting very high. I managed to limp home and fitted a new thermostat, and it still over heated within minutes. However, when I removed the thermostat it runs fine. To add to the confusion both old and new thermostats open in a pan of hot water. They are both the 74 degrees type and the engine is a 1983 10J diesel. When the engine overheats the top of the radiator is hot and the bottom hose is cold, but with the thermostat out the water flows as normal and both ends are hot. Any help would be greatly appreciated as it is doing my head in!

Thanks

WoodyO · May 3, 2021

When you say overheat are you talking about it boiling over, or just relying on what the gauge says?

Sounds like the gauge is reading too hot, if the 'stat opens as it should and the coolant flow is proven with it removed, there isn't really much else to go wrong.

Blackburn · May 3, 2021

If both ends of rad are hot i would suspect rad blocked and only partly working temp should reduce from top to bottom.
Take the grill off and feel the whole width of rad to see if all even temp or buy an infared temp gauge to check it.
Is the fan slipping ie belt tight enough.

TheYellowLandy · May 3, 2021

WoodyO said:
When you say overheat are you talking about it boiling over, or just relying on what the gauge says?

Sounds like the gauge is reading too hot, if the 'stat opens as it should and the coolant flow is proven with it removed, there isn't really much else to go wrong.

Thanks for your reply, I don't entirely trust the temp gauge as the wiring is a bit of a mess. However, the engine block and surrounding area was way hotter than usual. The bottom rad hose was cold with both old and new thermostats fitted, and only warmed up with the stat removed, a classic symptom of a faulty thermostat. The gauge shows running temp around the N mark with stat removed but in the red with stat fitted. Coolant only seems to flow with the stat removed.

TheYellowLandy · May 3, 2021

Blackburn said:
If both ends of rad are hot i would suspect rad blocked and only partly working temp should reduce from top to bottom.
Take the grill off and feel the whole width of rad to see if all even temp or buy an infared temp gauge to check it.
Is the fan slipping ie belt tight enough.

Thanks, the fan belt is new and is tight enough. The rad feels the same temperature all over with the thermostat removed but only the top is hot with the stat fitted. I recently flushed the rad and I can't find any blockages.

lynall · May 3, 2021

You either need a laser thermometer for testing or a capilliary temperature gauge for long term temperature checks.

Colthebrummie · May 3, 2021

On mine and others that have posted, during normal driving on average temperature days the guage usually shows below the N. Mine only goes up to N if I'm sat in traffic on very hot days. Even though both thermostats opened in a pan of hot water. I would get another and try that. I know the heaters are **** poor but when your guage is showing hot turn the heater onto hot and see if hot air is comming out, if not, you probably have an air lock in the system somewhere.

Col

TheYellowLandy · May 3, 2021

Colthebrummie said:
On mine and others that have posted, during normal driving on average temperature days the guage usually shows below the N. Mine only goes up to N if I'm sat in traffic on very hot days. Even though both thermostats opened in a pan of hot water. I would get another and try that. I know the heaters are **** poor but when your guage is showing hot turn the heater onto hot and see if hot air is comming out, if not, you probably have an air lock in the system somewhere.

Col

Up until about two months ago my gauge also sat around/below the N mark, even on warm days and on long journeys. It has only started reading a high temperature recently but has gone back to N now the thermostat is removed. The heater definitely works (works being a relative term!)

TheYellowLandy · May 3, 2021

lynall said:
You either need a laser thermometer for testing or a capilliary temperature gauge for long term temperature checks.

I have a spare temp gauge and wiring so I will try that. Is a capillary gauge something you would be able to see when driving or is it just for tests?

divie · May 3, 2021

Late series threes have a bleed hole of around 10mm in the thermostat housing. It has been to many years since I messed with one but this comment may prompt someone with knowledge.

lynall · May 4, 2021

TheYellowLandy said:
I have a spare temp gauge and wiring so I will try that. Is a capillary gauge something you would be able to see when driving or is it just for tests?

It is basically a fully mechanical temperature gauge which works regardless of elec power or engine runninge etc.

Not cheap but they just work, and better than the std ones they work from a much lower coolant temperature, so you can really see what is going on.

https://www.ebay.co.uk/itm/301238542214?hash=item4623375f86:g:s-cAAOSwunNdly59

Colthebrummie · May 4, 2021

if it was mine, Id just run it without the thermostat. I dont do a lot of miles though and hardly use it in the winter.

Col

tottot · May 4, 2021

Water pump worth a check ?

lynall · May 4, 2021

Colthebrummie said:
if it was mine, Id just run it without the thermostat. I dont do a lot of miles though and hardly use it in the winter.

Col

Problem with that is no stat means the coolant can flow either through the rad or more likely via the bypass hose and back into the engine without being cooled.

TheYellowLandy · May 12, 2021

lynall said:
It is basically a fully mechanical temperature gauge which works regardless of elec power or engine runninge etc.

Not cheap but they just work, and better than the std ones they work from a much lower coolant temperature, so you can really see what is going on.

https://www.ebay.co.uk/itm/301238542214?hash=item4623375f86:g:s-cAAOSwunNdly59

Hmm, tempting. I have no idea how I would fit it though as I don't really want to start cutting holes in the dash!

TheYellowLandy · May 12, 2021

Colthebrummie said:
if it was mine, Id just run it without the thermostat. I dont do a lot of miles though and hardly use it in the winter.

Col

Same situation here really, I'm going to go out this weekend without a thermostat and see what happens. Come winter it'll hopefully get parked in a nice warm garage so no thermostat shouldnt be a problem.

TheYellowLandy · May 12, 2021

tottot said:
Water pump worth a check ?

I think the water pump is alright as no untoward noises coming from the bearings or pulley, and I'm not losing any coolant. Hopefully it's nothing major

nickjaxe · May 12, 2021

With out a laser temp gauge you have no idea how hot the engine is getting.

If the stat is working correctly.....the engine will soon be hot after starting.....the top of the rad will get warm/first.

And not the bottom till the stat opens at around 75C....thats hot....to hot to keep your hand on metal parts.

A laser thermometer is under £10....takes all the guess work and uncertainty of the job.

nickjaxe · May 12, 2021

TheYellowLandy said:
Same situation here really, I'm going to go out this weekend without a thermostat and see what happens. Come winter it'll hopefully get parked in a nice warm garage so no thermostat shouldnt be a problem.

It will run cool....not good for the engine.

Kev12 · May 12, 2021

TheYellowLandy said:
I think the water pump is alright as no untoward noises coming from the bearings or pulley, and I'm not losing any coolant. Hopefully it's nothing major

Still worth a check, for all you know the impeller may not have much of the fins left

SIII diesel thermostat woes...

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