After some distractions, I finally got a chance to look some more at our Impreza's overheating problem. Initially, I thought the problem was electrical as none of the fans come on but it is something different, I think now:
The fans don't come on but can get them to come on by putting a variable resistor on the thermosensor - so the fans not coming on is not due to any of the relays, wires, bad fans, etc. The ECU clearly works ok as well.
Replaced the thermosensor - still the same problem. Measuring across the thermosensor at temperature I get 290 ohm, from the looks of it. Using a potentiometer, the fans come on at around 250 ohms.
So I am wondering if the coolant is circulating. I don't dare run the car too long, but the bottom hose does not seem to get very warm. Took the thermostat out and tested it in a pan of boiling water - it works.
I've tried burping the system by rolling the front on ramps and revving it while filling the radiator. When that didn't help, I drained the whole system, filled the engine first (using the top hose and a long funnel), then the radiator -- still no joy. I was half thinking of finding a steep incline and trying to burp the system but that just seems extreme. Is there a better way to burp the system?
How can I tell whether the waterpump is bad? I replaced that thing last year when I did the timing belt. Usually don't waterpumps start leaking when they go bad?
The car runs really well antifreeze does come out the cap eventually when it overheats.
Tia Remco