SIII diesel thermostat woes...

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34
Location
Wessex
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
 
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.
 
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.
 
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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.
 
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.
 
You either need a laser thermometer for testing or a capilliary temperature gauge for long term temperature checks.
 
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
 
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!)
 
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?
 
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.
 
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
 
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.
 
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.
 
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.
 
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