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Hi to all.
So I have this weird issue. My roadster (engine same as 450) is overheating, temperature is jumping around 90-110C. I have replaced the thermostat and issue is still there.
If it idles longer at a stop or moving slowly in heavy traffic I see the temperature rising slowly to 110C and above (cooling fan kicks in at 100C). I know it's an air pocket in thermostat housing because temperature drops to 90-100C quickly if I give it some higher RPMs (5000-6000 RPM) while in neutral or running several seconds in lower gear, so higher RPM forces the pump to push some of the coolant through the air pocket.
I end up releasing the air via radiator bleeder and temperature sensor at the thermostat once a week or so.

All of it would point to shot head gasket, however in this case I would expect to see my fluids mixing or level dropping. However it does not require any top ups, oil is fine and clean as well. I only have to release the air once a week and it's good as before, for several months now. It's almost like it sucks the air in somehow, or as if it boiled.
I use a pre-made G-12 yellow coolant mix.

Anyone have any ideas before I try introducing some kind of sealing gunk in the coolant?

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It sounds like you know how to bleed the thermostat housing, have you blead it from there yet?  It is the best way I know of how to get all the air out  of the system. Some will say to tip vehicle up so the radiator is at the highest position and do it that way, why when the thermostat works 99% of the time imho.    
Have you tried the site Evilution.uk.co yet? Pay their lowest donation charge to get deep into their knowledge banks.  Worth every penny IMHO.....seriously a wealth of solid information and it's in your own back yard so to speak. The owner is a straight forward person who will help if he can...from what I have learned from asking him questions myself. Kane is his name.   The only issue with his site is it doesn't have a chat section for members to discuss issues and solutions. Other than that it is the best knowledge base for most Smart related issues.  It also takes a few minutes to navigate easily through their set up. Simply keep X-ing the cars in the top left box you do not wish to speak about until it leaves just the one you have, then search. Good luck.

But I expect your answer will be the same, bleed the system from the thermostat location...fingers crossed.

 

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I do my fair share of bleeding each week. Radiator and thermostat, both on flat surface and tilted. Problem is, the air accumulates somehow again.

I've paid evilution some time ago. Better than nothing, but pretty basic stuff really. And not useful in this case. 

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Then it is introducing air as you say into your system and I'll bet it's from where you think it is. Head issues, either gasket or a cracked head maybe? How do you drive it? Aggressively or sensibly so to speak.  I ask as I used to suffer issues like this when I was younger...lol. Now I drive like my age 62...lol.   OR.....and I highly doubt this especially coming from Jolly Ol'England......could it have frozen...?  Yes i saw you have coolant not water.  Has it always happened since you owned the car? Are you the first owner? IF not maybe someone else beat the daylights out of it before you?   Just throwing things against the proverbial wall so to speak.

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Hmmmm.....IF you can afford maybe it's time to do a rebuild from ground floor. I have done it and you can get everything required from the Germans...lol SW-SMART   I think is their website.  Not pushing them but they are the only ones I could find everything except crank bearings  so I was forced to use my still good bearings. You can buy already rebuilt engines from them or simply the parts you require. IMHO, far better than trying to find a good used non beaten engine.  To each their own of course.  I had a good running CDI 450 which showed zero signs of wear or issues. But after opening it up I discovered badly warped bores caused by whatever ...you can pick the cause that best fits..?  I think it was used for very short city driving not allowing the engine to get fully up to temp then shut down resulting in warping the bores.   It ran perfectly.  I replaced everything but the crank, can and connecting rods and crank bearings. Sorry I also machined the head and block.  Not cheap, but I now own a like new engine which i know to be perfect.  How much does a used good engine cost? Add the cost of swapping it, then compare it to a rebuilt or the parts.  Then you decide.  Again...it's not cheap...lol.

OR, just take head and rebuild it....could be as simple as a head gasket...?  To a crack between valves..?  You won't know anything unless you peek in and see. If you can turn a wrench you can do it yourself IF you have the place to do it.

Hope this helps.

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