Rising and Falling of Coolant Temp

[gizzardio]

Hi All

Coming back from Silverstone classic yesterday. All is well until I notice coolant temp rising towards 100degC at a steady 70mph without having previous ragged it or sat in traffic. Normally it sits about 87degC just under the white mark at 90degC.

Then after a few miles more, it came down so I thought great, just a strange quirk. But then it bounced right back up to near 100 again, then a few more miles dropped and repeat.

Pulled off at services, coolant had definitely left the cap on the swirl pot as it was dried on underside of bonnet and all down the pot itself. After cooling down for while, I take cap off and surprised to see almost full coolant, and expansion tank is also at OK level. Thinking maybe its misleading, I run engine with cap off and rad bleed valve open to burp it in case of air lock and loads off coolant starts tipping out after a few mins. So it may have been air in there so I keep topping up after it flows over until it eventually stops flowing over and I can see coolant level is fine just an inch below swirl pot top.

I also get coolant fan sensor switch spades and short them so fan is on permanently just as extra precaution to get me home. All way home the temp never gets above 90 which is good, but it keeps fluctating up and down all the way quite wildly. Its 90 then falls to say 80 for a few mins, then shoots up to 90 again then repeat.

Any thoughts? Is my swirl pot pressure cap had it? Anyone else had these weird symptoms?

[gizzardio]

Hi

Thanks, so I suppose all I can try that I haven't already done is to replace my pressure cap and more burping if its a really stubborn air lock. The V8 has a top hose which is actually higher than the swirl pot top and is upside down U shape so a perfect design for trapping air in this. It did get pretty hard to squeeze when running at working temp though so lots of water was getting through it.

Anyone recommend the pressure cap for the swirl pot I need?

[gizzardio]

Thanks. I'll take a look.

I'm leaving my arrangement as it is as its absolutely fine up til now so no need to resurrect that discussion topic..

[gizzardio]

Hope its not that. I just got it re-cored in Jan!

[gizzardio]

It was flushed and refilled when I had the rad re-cored in Jan so I hope its nothing to do with that so soon.

I will purchase a new thermostat anyway and fit it for the sake of 15quid.

Any thoughts on the lower temp opening ones against the higher? 82degC v 74degC are the ones I've seen on racetech.

[gizzardio]

Hi

So I removed my thermostat tonight and am I right in thinking that if I drop it in a jug of boiler water it should just open up?

I did that and nothing happened. But it must have been working before otherwise my engine would have just overheated, rather than just fluctuate.

Unless I've got this wrong...

[gizzardio]

Top gear - yep I've now bought a new cap and seal and will stick that on.

I was more curious than anything else to check the stat out and see it working.

Very weird then that nothing happens when sat in pan of boiling water.

[gizzardio]

Hi

Just an update as always good to share the outcome.

Replaced thermostat and rad pressure cap and it seems all is well now. Temp sitting solid,if perhaps a little cooler than before when sitting at cruising.

Cheers

[gizzardio]

I think it was both. The sticking opening of the old stat tested in in boiling water and the leaking cap prob allowed air into the system after running.

30quid fix is always nice!