Thermostat stuck shut?

[Tofufi]

Hi all,

Went to take the Rover for a spin today, and noticed that within a mile or so the coolant temp was above normal on the gauge... it normally sits right in the middle of the number 85 on the gauge. Within another half a mile, it was getting towards the red, so I pulled over and stopped.

After leaving it (5 hours) to cool, went to drive it home again, and within a mile again it was getting past 85 on the gauge.

Coolant level is normal, and it's not had any issues previously with overheating.

I'm wondering if it might be the 'stat stuck shut, although the pipe from the 'stat housing was warm-ish to the touch, the inlet manifold was a fair bit warmer.

Any thoughts? I'm not really used to these odd engines which have water to cool them

It's a 1970 3500, with an LT77 manual gearbox, standard fan and cooling system and a Holley 390 carb/Edelbrock manifold. It's previously been fine even in the hottest summer days. It had a new thermostat last time I serviced it (within 12 months) as the previous one stuck open after less than 2 years.

 

[harveyp6]

At the point the stat opens the top hose should be too hot to hold, so if that isn't happening, and the temperature is rising, then the stat isn't opening. You could run it up to the point that the stat should open and then gently tap the thermostat housing around the joint with the manifold with a hammer to encourage the stat to open. Sometimes they just get a bit sticky, especially if the car's not been used.

 

[Tofufi]

At the point the stat opens the top hose should be too hot to hold, so if that isn't happening, and the temperature is rising, then the stat isn't opening. You could run it up to the point that the stat should open and then gently tap the thermostat housing around the joint with the manifold with a hammer to encourage the stat to open. Sometimes they just get a bit sticky, especially if the car's not been used.

Thanks for the advice

I thought the fluid travelled through/round the system the other way? (i.e. through the stat and then through the top hose into the radiator?)

When I first stopped, I gave the thermostat housing a tap with a wheel brace and it didn't appear to have any effect. Car hasn't been started in 3 weeks before today as I've been working away

If it's going to stick *ever*, I'd rather just replace it for the sake of less than £10.

 

[SydneyRoverP6B]

Hi Jim,

If your thermostat is remaining shut but your engine temperature is climbing, then the top hose to the radiator will remain soft. Only when the stat opens and coolant flow through the radiator is achieved will the pressure within the hose increase making it much harder to compress.

Ron.

P.S : It does travel into the top hose and exit via the bottom hose.

 

[harveyp6]

I thought the fluid travelled through/round the system the other way? (i.e. through the stat and then through the top hose into the radiator?)

It does, that's why when the stat opens the top hose is too hot to hold.

 

[Tofufi]

I thought the fluid travelled through/round the system the other way? (i.e. through the stat and then through the top hose into the radiator?)

It does, that's why when the stat opens the top hose is too hot to hold.

Ahh, got it! - thanks.

I forgot to add, the heating was coming through warm by this point.

Would this happen with a stuck thermostat?

 

[harveyp6]

I forgot to add, the heating was coming through warm by this point.

Would this happen with a stuck thermostat?

Yes, because the heater is fed from behind the thermostat. But if the stat were jammed shut I'd expect the heater to be roasting, not just warm, once the temp started rising above normal, unless of course the heater matrix is blocked.

 

[Tofufi]

I forgot to add, the heating was coming through warm by this point.

Would this happen with a stuck thermostat?

Yes, because the heater is fed from behind the thermostat. But if the stat were jammed shut I'd expect the heater to be roasting, not just warm, once the temp started rising above normal, unless of course the heater matrix is blocked.

Thought so, thanks. I didn't let the temperature go much higher than normal, so hopefully a new 'stat should sort it.

Thanks again!

 

[westOz74P6B]

Sounds like it is thermostat, check the new one in boiling water to see that it opens fully before fitting it and do the same with the old one to confirm failure otherwise it may be something else, look forward to your update.

Regards,
Scott

 

[ghce]

In a list of likely's probably the thermostat, 2nd on the list would be headgasket, 3rd water pump failure.

Graeme

 

[Tofufi]

Fingers are crossed it isn't head gasket or water pump, I'm meant to be using it on Saturday :shock:

New thermostat (Genuine Land Rover part this time, rather than the two before which were cheap QH ones from Halfords) has been ordered, so hopefully that'll arrive. I'll keep you updated, but I won't be able to fit it until the weekend :roll:

Thanks

 

[SydneyRoverP6B]

Tofufi wrote,...

Fingers are crossed it isn't head gasket or water pump

Hi Jim,

As far as I am aware, water pumps fail in one of two ways, through bearing failure or the seal breaks down. The result in both cases is coolant loss through the weep hole on the underside of the pump, and given that you have no coolant loss, then that would rule out the pump. Head gasket failure will also see coolant loss to one degree or another.

Thermostats are in theory supposed to fail open, but there is always one that didn't listen in class... :wink:

Ron.

 

[ghce]

Cheap thermostats abound! make sure you get a reputable brand.

Graeme

 

[Tofufi]

Well, I got home early today to swap the thermostat - took less than half an hour.

However, the temperature went up and sat at the borderline between green and red on the gauge...

It stayed there when idling for prolonged periods, and didn't fluctuate into the red.
I tried another dash cluster to confirm it isn't the gauge out of line - the second one shows broadly the same temperature.

Both old and new thermostats were 88 degree ones, although the new one is genuine Land Rover.

However - the old thermostat when put into boiling water, did open, probably around 3-4mm. To get it to open, I had to replenish the boiling water after a minute of sitting in the first lot of boiled water - it certainly didn't open readily, but I don't know what is 'normal'.

Having let it cool for a few minutes, I took it for a drive to my friendly local garage in search of a means of measuring the engine temperature (infra red thermometer or similar). On the way there, the gauge stayed pointing to the number 5 (roughly where I'd expect), nowhere near the red :roll:

They didn't have one, and on the way back the needle stayed around 3-4mm from the red area. :roll:

I'm wondering if my temperature sender might be misreporting?

It doesn't appear to be pressuring the coolant, or losing any either. Oil pressure is normal.

Edit: pictures:

Thermostat quality broadly clear - the RH one is genuine Land Rover, the LH is a cheap one from Halfords.

Trying a spare (LHD) dash cluster:

One of the many (constant) readings the proper gauge showed on a test drive.

 

[harveyp6]

The senders can fail, as can the voltage stabilisers, but when they go the higher the RPM, the higher the temperature, and the more fuel you have...

A sender would be the next logical step if the guage readings are stable.

 

[Tofufi]

The senders can fail, as can the voltage stabilisers, but when they go the higher the RPM, the higher the temperature, and the more fuel you have...

A sender would be the next logical step if the guage readings are stable.

Thanks Harvey

I did wonder whether to order a new sender while buying the thermostat, but thought it probably wouldn't be needed.

I'll take it out again this evening and see what it reads then.

Did you spot the pictures I edited into my last post? They're probably not that informative to be honest!

 

[harveyp6]

Did you spot the pictures I edited into my last post? They're probably not that informative to be honest!

I've just seen them. The original (Halfords) stat didn't have a jiggle pin in it. And no hole drilled either.

 

[SydneyRoverP6B]

Hi Jim,

I had a sender fail on me many years ago. I recall starting the engine and almost immediately the needle went up to the red. Feeling the radiator hose it was soft and stone cold, so I replaced the sender and all was well.

The photos that you posted of your thermostats. It is not entirely clear, but the L/H stat, is there a bleed notch showing at the 12 o'clock position? If there is, it does not need a hole drilled in it to prevent an air lock.

When you tested the thermostat, did you suspend it by a piece of fine string so that it was immersed but well above the bottom of the pan? If you use a cooking thermometer to monitor the water temperature, you can both see and hear when the stat opens. It will just open a mm or so at the rated temperature, and as the water temperature increases so the stat will continue to open until it is fully open around 10 degrees C above the opening temperature.

Ron.

 

[Tofufi]

Did you spot the pictures I edited into my last post? They're probably not that informative to be honest!

I've just seen them. The original (Halfords) stat didn't have a jiggle pin in it. And no hole drilled either.

Hi Jim,

I had a sender fail on me many years ago. I recall starting the engine and almost immediately the needle went up to the red. Feeling the radiator hose it was soft and stone cold, so I replaced the sender and all was well.

The photos that you posted of your thermostats. It is not entirely clear, but the L/H stat, is there a bleed notch showing at the 12 o'clock position? If there is, it does not need a hole drilled in it to prevent an air lock.

When you tested the thermostat, did you suspend it by a piece of fine string so that it was immersed but well above the bottom of the pan? If you use a cooking thermometer to monitor the water temperature, you can both see and hear when the stat opens. It will just open a mm or so at the rated temperature, and as the water temperature increases so the stat will continue to open until it is fully open around 10 degrees C above the opening temperature.

Ron.

There was a small bleed notch, although probably not quite the size demanded. When I fitted it I considered it to be enough, and didn't have a problem.

I didn't dare try it in a pan, I used boiling water from a kettle - I know this is less accurate, but reduces the risk of upsetting others

However, from your description it would seem the (old) 'stat is probably OK.

I just went out (in the dark) and removed the sender unit. Took some boiling water out in a plastic beaker, and got a reading from that - gauge read just below 85 degrees. Unfortunately I don't have a cooking (or otherwise) thermometer to measure the temperature more accurately...

Thanks for the thoughts - I may borrow the IR temperature measuring thing from work next weekend if I don't get to the bottom of it.

 

[rockdemon]

maplins sell ir thermometers for £20 iirc - that's how i know PAE is actually ok despite reading just below the red in general usage...

Rich