Car: 1993 940 NA, wagon, 144,000 mi. Coolant is 50-50 mixture. Thermostat seems to open as it should.

I am wondering if using a lower pressure cap on the coolant reservoir has lead to two overheating episodes. When I bought the car at 104,000 mi., I did most of the things listed in the 700/900 FAQs to bring the car up to spec. In addition, after reading some of the forum postings which advocated replacing the high pressure coolant expansion reservoir cap with a low pressure cap used on the 240. The reasoning was that the lower pressure cap would be easier on the hose and the heater valve. I believe the 940 cap was about 20 psi and the 240 about 10 psi or close to that anyway. Now I think the higher cap is necessary for this car. Here is what has happened twice.

Twice, when caught in bumper-to-bumper slow traffic, the car has boiled over, lost coolant, and the temperature gauge has eventually moved into the red. In both cases, the electric fan did NOT turned on. Jumping the fan switch at the radiator caused the fan to run. There has never been a problem while the car was moving. I have assumed that the problem was a faulty fan switch but I may not have thought this problem through clearly. Here is my diagnosis and I'd like comments from the forum:

1. The lower pressure cap allows the coolant to boil at a lower temperature than does the high pressure cap. According to Volvo the fan switch closes at about 220 degrees.

2. The boiling coolant expands and runs out the pressure cap at temperatures below that which would cause the fan switch to turn on the fan and cool the engine. When measured the coolant started pushing out of the tank when the temperature of the switch was about 206 degrees F. and the thermostat housing temperature was 230 degrees. The dash temperature gauge read normal (center).

3. When enough coolant has been lost, the heat gauge in the dash suddenly shoots into the red, alerting the driver. In both cases, the engine had lost about 5 pints of coolant and the expansion tank was empty.

4. I replaced the fan switch three times thinking that it was the cause of the problem. Now I am thinking the 20 psi pressure cap should have been kept.
I have now replaced the 10 psi cap with the 20 psi cap and plan to try deliberately overheating the car to see if the electric fan comes on. As far as I can tell, the thermostat is working properly but I may replace it anyway after I finish my testing. It's time to replace the coolant anyway.

Well, forum members, do you think that I am on the right track or off course? Also please have a look at a related topic which I am posting now also. Thanks much!