Nearly overheated but radiator still cool - Thermostat fail?

[MrBonus]

So I am still doing shakedown runs on the car to sort everything out. It had been sitting for quite some time as the refurbishment took roughly a year and a half. So I had noticed the temperature gauge bounced around a bit but just chalked it up to an aging gauge but it never went anywhere above 90 degrees and generally settled in around 75 - 80.

So I took it out when it was cold last week and I am eyeing the gauge and watch it start to slowly climb to roughly 100. Fortunately, I'm 150 yards from my home and pull into the garage and shut it down. I notice the new fan we fitted was not running as it normally would.

Coolant was slowly bubbling over from the upper radiator hose that connects to the radiator but the radiator itself was nearly ice cold. Presumably, the only plausible explanation is a blockage at the thermostat housing, correct? Most likely it stuck shut? I have no clue how old the thermostat is in the car.

The car's water pump was replaced under 100 miles and a year and a half ago prior to my refurbishment and the eyeball test shows no indication of concern there.

Is there anything special I need to know about replacing the thermostat? Just drain off some coolant, put a gasket in there, and replace it, correct?

Thanks for everyone's help.

[Hawksfield]

Hi

Nothing special. de pressure, drain, remove cover lift out Stat, new gasket refit. Job done

Good luck

[benymazz]

If the car was just refilled with coolant for the first time in theory it is possible that there was air in the system preventing the coolant from reaching the thermostat but I think this is unlikely because you said the temperature gauge was functioning. To be sure of this however some members on here (myself included) have drilled a small hole (I think I went for 3/32" or 1/8") through the thermostat to provide a small amount of circulation even when totally closed to ensure the thermostat is regulating the temperature correctly and to aid in "bleeding" the system

Fortunately replacing the thermostat on an Elan is quite possibly the easiest repair you will ever do - just drain the coolant down low enough so it doesn't dump all over the engine bay when you unbolt the housing and replace the offending component

This reminds me of when I thought I had the opposite problem and my thermostat had failed open (temp gauge wouldn't move off of cold) but in fact the temperature gauge itself had failed...

-Ben

[MrBonus]

If the car was just refilled with coolant for the first time in theory it is possible that there was air in the system preventing the coolant from reaching the thermostat but I think this is unlikely because you said the temperature gauge was functioning. To be sure of this however some members on here (myself included) have drilled a small hole (I think I went for 3/32" or 1/8") through the thermostat to provide a small amount of circulation even when totally closed to ensure the thermostat is regulating the temperature correctly and to aid in "bleeding" the system

Fortunately replacing the thermostat on an Elan is quite possibly the easiest repair you will ever do - just drain the coolant down low enough so it doesn't dump all over the engine bay when you unbolt the housing and replace the offending component

This reminds me of when I thought I had the opposite problem and my thermostat had failed open (temp gauge wouldn't move off of cold) but in fact the temperature gauge itself had failed...

-Ben

Quick, dumb question: How do you drain the radiator without soaking the crossmember beneath it? I bought one of those bendable funnels and I think I can get it to pour sideways and out underneath the sump.

Also, does anyone know the torque specs for the thermostat housing?

[Foxie]

Quick, dumb question: How do you drain the radiator without soaking the crossmember beneath it? I bought one of those bendable funnels and I think I can get it to pour sideways and out underneath the sump.

Also, does anyone know the torque specs for the thermostat housing?

I stick a suitably sized bit of plastic tubing over the drain spout, mostly to save the anti-freeze.

Thermostat housing is just a water seal, minimum torque required.

[2cams70]

Just use standard torque specs for a 5/16" UNC bolt.
Around 15ft/lb or 21Nm

[alan.barker]

I would just remove bottom Hose from Rad to drain. Then remove the other end from the Water Pump and check inside the Water Pump inlet to see if it's not blocked. It can get blocked because of corrosion reaction with mixture of different Coolants. I had that happen on a 1600 Kent Engine.
Alan

[nigelrbfurness]

You might want to check the radiator itself isn't blocked. If you have the correct thermodtat fitted, even if it isn't working there is a bleed hole that allows some circulation so I would expect there to be some warmth at the radiator. All the symptoms point to no circulation as others have said.

[MrBonus]

So looking into the top of the radiator and drain bottle, there is definitely some corrosion in the coolant and on the inside of the radiator, which could understandably create a blockage. I won't have time to look at it all until the weekend (and I am awaiting a fresh thermostat housing gasket any day now).

The prior owner was not diligent with maintenance and I do not know the age of the radiator. As I understand it, these older engines can tolerate more "junk" in the coolant passages but do you think it would be worthwhile to just bite the bullet and upgrade to a 26R style radiator?

I would prefer spending the money now rather than chasing down a reoccurring issue.

Thank you all for your input on blockages. I will check the inlet/outlet to the block as well.

Also regarding air in the system, we pressurized the system when filling it so it's highly unlikely there was any air in the system.

[mbell]

I'd probably remove the radiator and back wash it off the car too see what came out. Maybe take it to a radiator shop to have it cleaned and tested.

I'd also back wash the block from the thermostat housing, with the block drain plug removed (in the lower edge of the block between 3 &4 exhaust pipe s).

I'd also consider a cooling system cleaner wash once back together just to be sure.

[jaman]

Sounds like a combination of issues including the most common ones.

Change your thermostat and antifreeze solution based on the calendar NOT the odometer. Antifreeze gels overtime even if the car is sitting in a garage and not being driven. A 24 month interval should be the Rule for changing and flushing your coolant system.

An expensive lesson learned by my Florida friend that had a Stand-by generator that when the hurricane came and knocked out the power the engine started ? ran for about 45 mins and stopped. An expensive rebuild followed. Cause? ? Anti-freeze had gelled and blocked the engine cooling system. System had not been flushed for 4 years.

[MrBonus]

Thank you all for your assistance on this. To proverbially "close the loop," there were no blockages and the thermostat did in fact fail which makes sense since it had been running just fine a few days prior. When we yanked the old thermostat, it was quite old, a bit rusty, and looked like it dates back to the last engine rebuild in the mid '90s. Let her run and idle for about an hour and she held at 80 degrees solid and the new electrical fan ran perfectly.

I had nightmares of another water pump replacement or worse.