Ran it up to temperature and tried to burp the system. I think these Mercs are good in their design to not get air locks. Certainly didn't notice any major level drops even when up to temperature with the stat open. This is where having a handheld diag tool is invaluable compared to dragging a laptop out. I've got an OBD extender cable somewhere I need to find as this will make bonnet access of the tool even easier.
It seemed to peak at around 93 to 94c when revved at which point the engine fan kicked in. Sat there at idle it seemed to sit around 87c to 90c.
Gauge is just under the 100c marker now.
Went for a test drive on the same route as before. Now not a completely fair test as, even though I had dinner and it cooled off a bit, the car wasn't completely cold as per last time. Also I reset the mpg before setting off this evening as I'd been idling it a long time on the drive.
Anyway, unsurprisingly it got far better fuel consumption numbers than before.
Engine temperature stayed rock solid on the gauge. Maybe went up or down a notch but barely noticeable. Generally sat as above. Engine temperature on the scantool seemed to be around 87c to 89c which is the stat opening temp.
As I was quite warm and comfortable where I was sat, I thought I'd go for a quick spin up and down the motorway. Some of the junctions were shut, so I ended up going much further than initially intended. However it gave a good idea of cruising fuel economy.
It was actually hitting 31.8mpg, just I lost a bit when I booted it past a load of bunched up cars/lorries so I could get to the junction exit easily.
Considering I got 30.9mpg on the long journey home and certainly never hit 31mpg, I can safely say there is a solid increase in MPG (thankfully). Biggest improvement will hopefully be town/village driving where it should warm up quicker and hold the temperature better.
Just need to go on a long run now and see what it really can get. I reckon 35-38mpg is do-able on a 60mph motorway run if I had the patience.
Another thing I've found is that the box gear changes are very old fashioned. Most modern gearboxes I'm used to will try kicking up to the highest gear possible at all times. So even at 30mph you find the box in 6th or higher on a modern unit. This however feels like a mechanical unit of old (even though it's fully computer controlled). So at an indicated 30mph cruise it is in 3rd gear and just under 2k rpm). However 31mph (exactly 50kph) it kicks up to 4th gear and 1.1k rpm. It'll stay there until about 27mph and then it'll drop back down again. So certainly careful use of speed when driving should also give quite a boost in fuel economy too.
Either way I'm massively happy the consumption has decreased - seems to be a solid 16% decrease for slower speed running. Right now it's not that far off what my very similar bhp Boxster 2.7 6cyl did (albeit that ran on super unleaded and this is regular).
If my maths is correct then a 80l full tank fill I'll be saving £19 per tank from the extra range. Basically two tank fulls of non motorway driving will easily pay for that thermostat and the replacement coolant.