Does this sound like a broken/stuck thermostat?

[Gregmund]

Hi all,

Returned home last night late - after driving about 10 miles - and not driving the car hard at all.

As I pulled onto the driveway the fan cut in and the Stack temperature readout was flashing - knowing this was odd - especially as it was such a cold night - I turned the thing off straight away.

Left it overnight - thinking it could be anything from a broken expansion chamber cap, thermostat failure, split hose, water pump failure to a Head Gasket Failure

This morning I checked the coolant level - which need topping up (but not by much) and fired it up leaving it to warm nicely...
Neither the coolant, oil filler cap and dipstick show signs of oil/mayo and there was no scary smoke on start-up- so I'm crossing fingers it didn't boil for long enough to cause a HGF.

Once it got hot - it went past 90deg & the fan cut in - as it should... except - the radiator and supply pipes are still cold - so the fan is just blowing cool air through. There is a return jet of water in the expansion chamber - which is driven by the water pump right? - so i guess it is not that.

So my diagnosis is that the Thermostat is broken jammed - and the water isn't circulating round the system.
Does that sound like a sensible diagnosis?

...and then - does anyone have any 'guides' to replacing the thermostat - it's a '97 standard S1.

Cheers
Greg

[Sam_68]

It does sound as though it might be a failed thermostat, but to be sure you will need to take the car for a short run. Watch the temperature reading like a hawk... it should rise steadily to just above normal running temperature (say mid-90's degrees, as it takes a few seconds for the thermostat to react, even when it's working normally), but then it should drop back to normal running temperature as the thermostat opens.

If the temperature just climbs straight past normal operating temperature as the car heats up, and carries on climbing up beyond 100 degrees , you have a failed thermostat. Do NOT allow the temperature to rise above 110 degrees, or you could cause engine damage.

[GreenV8S]

It can be useful to test the cabin heater. This is usually plumbed in upstream of the stat so it should produce heat even if the stat fails closed. If you have no cabin heat you potentially have a complete cooling system failure (no coolant, or coolant not circulating) which can lead very quickly to catastrophic damage.

[Gregmund]

Thanks Sam_68 & GreenV8S,

Now you mention the heater I'm worried I've had complete coolant system failure - the heater wasn't working very well (if at all) on my short run home yesterday - though the coolant level was low.

If the water isn't circulating is it likely to be the water pump - can it be anything else?
Having topped it up I need to try it again with the heater on I guess.

Fortunately I haven't cooked it beyond 105 deg for a brief moment (yet) - so hopefully no major damage.

[Sam_68]

Complete loss of water is hopefully unlikely, if there is a return feed coming in to the expansion tank, though.

It's possible you have a hell of an airlock somewhere in the system, I suppose?

In answer to your earlier question, changing the thermostat is a bit of a bastard, if it has failed... it's in a housing tucked away under the inlet manifold.

[Scuffers]

if your description is accurate, then the pump would be screwed...

[Sam_68]

If the pump has failed completely, would you still be getting the return jet at the expansion tank, though, Scuffers?

[GreenV8S]

If the coolant level has been low, it's possible that you have developed an air lock. An air lock in the heater circuit could be enough to stop the heater working, so lack of heater isn't solid evidence of a major problem. However, it's a bad sign and I'd be worried by the evidence of no flow through the radiator and simultaneously no evidence of flow through the heater. It all points to no flow, period. What's the pump like on that setup - is it vulnerable to impeller failure, how well does it self-bleed? Is the drive belt still in place?

[Scuffers]

GreenV8S said:

If the coolant level has been low, it's possible that you have developed an air lock. An air lock in the heater circuit could be enough to stop the heater working, so lack of heater isn't solid evidence of a major problem. However, it's a bad sign and I'd be worried by the evidence of no flow through the radiator and simultaneously no evidence of flow through the heater. It all points to no flow, period. What's the pump like on that setup - is it vulnerable to impeller failure, how well does it self-bleed? Is the drive belt still in place?

Pumps don't fail that offten, and as they are driven of the cam belt, I think you can assume it's not that...

with issues like this, it's hard to be accurate without being infront of said car...