au3 overheats and misses

[bodhisattva]

Hello, I have an AU Series 3 auto standard six cylinder, petrol, no mods, 178K, and it's generally good except for a few annoying things which someone may be able to help with please.

1. It seems to overheat easily – not while driving, the temp gauge never goes above between the N and the O in NORMAL, but I notice that when I stop my car after going up my long and steep driveway, the coolant overflow often boils and bubbles, and sometimes overflows, or at the very least I can often smell the hot coolant vapours. I notice that the radiator fans don't seem to stay switched on when I turn the engine off, which I would have thought they should if the radiator/engine is hot (all the other car makes I've had do this). I took it to an auto electrician and we checked the fuses and they looked okay, and when the engine is running he said the fans were working (he used a torch to see in the back of the fan shrouds) so how come the coolant overflows or bubbles and the fans don't stay on when the engine is off to cool it down? The car has been serviced regularly since new and the coolant appears nice and green and clean etc, although I haven't had it flushed yet. Any ideas please? Why does the engine seem to run so hot?

2. Second problem is a miss that comes and goes intermittently, but mainly seems to happen when engine is warm/hot. I don't know if it's related to the problem in 1. above? It's not major banging and coughing, but enough so that I can feel it, but only when the engine is under reasonable load like going up reasonable hills. When it's not missing (not often) it will go up a particular hill in Drive in Economy mode towing a trailer at 100kph easily; when it's missing, I have to drop it back to 3rd in power mode to reach 90kph with no trailer! So here's what I've done so far: changed all the plugs to Iridium ones, got a mechanic to check on oscilloscope for signs of plug lead shorting etc (no signs of that), changed air cleaner, ran fuel injector cleaner through the tank (seemed to make a small difference for a while) and then the big one – changed the fuel filter. The filter was full of jet black grunge that looked like used sump oil. With the new fuel filter the car ran brilliantly for a few days, then the misfire returned. So, my theory is that more grunge might have been introduced from the tank into the new filter, so I'm having that changed again this week and we'll see what colour the petrol looks like this time. If it's black again, then we have to suspect a load of dirty fuel in the tank, and will probably need to flush the fuel tank. If filter seems fine, then then I'll have a new filter again anyway, and the problem will have to be looked for elsewhere. Any ideas on this one also please? Many thanks.