As Bruce said. Bulk coolant flow through the rad is like
and you're just looking for hot or cold spots really. Temp drop across the rad should be somewhere in the ballpark of 15 - 20° once the engine is upto operating temp i'd have thought off the top of my head*. You've got a larger single row core so either i was remembering wrongly (more likely) or you have a 'better' core design than the mine had
I've just dug out the old water pump from mine (kept it as it only needs new bearings & seals, otherwise it's pretty much spotless inside)... can just make out the edge of the impeller through the 'stat housing and reach it with a small telescopic magnet
Unless i've missed it we still don't know how hot it gets in heavy traffic conditions? 107°C has been mentioned... is that it or do the fans struggle to keep it below 110°?
Obviously compared to many countries the UK rarely sees properly hot and humid ambient temps but FWIW when we do get a hot spell it still takes properly heavy Sarf London traffic for mine to nudge over 100° and the fans (bearing in mind one of the leccy fans is dead) keep it between high 90s and 110ish
Personally first thing i'd do is buy a new rad cap unless it was already replaced recently... they're dirt cheap, the pressure stages get tired with age without becoming noticable like tired seals and unless the header tank is over filled that won't necessarily result in coolant being puked, just vent too early preventing the system from fully pressurising. It's pressure that's mostly responsible for raising the boiling point of the coolant... 50% antifreeze boils at ~ 107°C at standard pressure, on the open road when the engine is at higher rpm the waterpump pressurises the coolant in the engine (pumping away while the 'stat restricts flow out of the engine). At idle (and obviously after switching off a hot engine) not so much hence allowing the system to pressurise. If any localised boiling occurs around the combustion chambers things can rapidly get worse as a vapour pocket forms...
Compared to water antifreeze is pants at removing waste heat (has about 1/2 the specific heat and 1/2 the thermal conductivity), i run around a 40% mix as the freezing point is still way lower than the UK needs and it's still 'strong' enough to inhibit corrosion as long as changes aren't neglected
If the rad/temp across the core checks out then once the fan shrouds/airflow into the engine bay is sorted i'd look at the fans again... if the viscous jobby is locking up properly then instead of replacing it with one that kicks in at a lower temp i'd change the leccy fan switch first 'cause it's cheaper/easier/faster. IIRC they're available with different temp ratings and/or can be tweaked by adding a resistor to make them kick in earlier with regards to coolant temps and/or refrigerant pressure?
It's only a guess but i wouldn't be surprised if, with at least some of the hot running m104s, corrosion of coolant passages is often to blame? Even with a one owner from new example that's been religously serviced because a stamped up history is no guarentee that the work was actually done... we've all heard too many stories about filters, spark plus and the like. Paying someone to change the coolant doesn't guarentee it's been done properly/completely and drained from block as well as from the rad
* as well as engine efficiency part of the reasoning behind higher 'stat temps is rad efficiency... don't want a massive temp drop across the core and ideally the
average temp of the rad should be as high as practical because the larger the temp difference between the rad and ambient air the more efficiently the rad works
