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!

I have not heard of this problem on 940s. Overheating is more of an issue in 240s, some due just to the thermal compensation board, some real.

I have switched over to the 75 kilopascal cap on the family fleet, the turbos, NA, the 1993 plus the 240. No problems. I believe that relates to about 11 PSI. I think Volvo Dad, if I have that name correct, one of the more experienced "Dads", had a brief and convincing argument for the lower pressure cap. Something like let the cap blow off the excess pressure rather than a cooling system component. And the system pressure should never exceed 7 PSI.

I have experienced a red line hot only once in the 1992 240 and it has a good thermal compensation board. 100F and going 25MPH could not cool the engine. Heater on and shortly after a faster speed corrected the overheating. No thermostat replacement yet.

Clean out the radiator fins, replace the thermostat, I think you use the hotter T stat. Now I question the condition of the radiator installed thermostat although I have not heard of failures but a carbon build up in the contacts due to the higher amp fan motor is possible, just not common.

Tom

I will replace the thermostat. The thermal switch in the radiator is brand new from the Dealer. The overheating problem ONLY happens when the car sits at idle or in very slow traffic jams. Even a speed of 25 mph is enough to push sufficient air through the radiator to keep the temperature down.

The lower pressure cap works most of the time except as I mentioned above. If I figure out a way to get that fan to turn on at a lower temperature, all should be well.

Thanks for your input.

http://www.brickboard.com/RWD/volvo/464094/740/760/780/expansion_tank_hose.html%0A


.............I can't speak to the hose issue and Ringlee seems to have covered it head to tail, but I've been looking into this cap issue on my own 1985 745t and can tell you what I know.

The green cap has a pressure rating of 150kpa, while the grey cap has... 100kpa? Not sure, although I know the black cap is 75kpa. Anyway, switching to a higher pressure cap means that it will take more system presure to trigger an oveflow condition (out of the cap) than it does currently.

The benefit of a the green cap is that with each +2 kpa you get a reduction of 1 degree f... or something close to that. Basically, as you increase the pressure you decrease the heat and increase the efficiency of the cooling system.

The risk of this is that, if your cooling system is older and your radiator hasn't been replaced with a heavy duty jobber, you run the risk of blowing a hole in it from the pressure increase. Additionally, even IF your radiator has been replaced you run the risk of blowing out any of the 10 or so hoses that play a role in the cooling system. Worst case I can imagine is that the premolded coolant channels in your engine have weakened structurally over time and you blow through a sidewall somewhere. I don't think this last item is likely given what little I know of volvo engine's durability, but I like to account for every contingency.

I personally favor replacing the radiator with a Nissens 3-row, rewiring the aux electric fan to come on whenever the radiator hits 200 degrees (f), and going with a lower pressure cap if you can find it (I've only seen the 150kpa green ones). Volvo cooling systems are incredibly efficient, and my experience has been that the 75kpa cap is sufficient as long as the rest of the cooling system is up to spec. I have a black 75kpa on my 745t right now and it works fine...

good luck,

I agree that the black cap will work fine if the fan comes on at 200 deg. F. How would you do this? The OEM switch in the radiator comes on at a higher temperature and so do the switches which I bought at CarQuest and others. Is a lower temp switch available?

I'm also considering a change to my AC circuitry which turns on the fan at low speed whenever the AC compressor is on and the car is idling. Volvo kit (P/N 971008-1) does this to improve AC efficiency at idle speeds. I think that it could be redesigned to turn on the low speed fan whenever the engine is at idle.

jim this seems like overkill to me but have a look anyway

http://www.ronfrancis.com/prodinfo.asp?number=AR-24

http://www.ronfrancis.com/prodinfo.asp?number=AR-35

Interesting. I wonder how that fan thermostat is mounted. I can't tell from the image. Years ago I installed an electric fan in a Pinto. That thermostat was pushed between the radiator fins. It worked.

the easiest way to resolve the problem if turning on the fan at 200F is what you really want is to wire the switch to be manually operated by a small toggle under the dash. i had to do this years ago on a capri convertible my wife had to have would not give up no matter what. keeping this australian made oddity running was a challenge.

other than finding a 200 degree switch for something else and retro fitting this is the cheapest and easiest. just make sure you wire in it in such a manner it will not run when the ignition is off or your battery will die after you forget to turn off the toggle.

One word of experience from a former black cap user: they can fail and lead to overpressure in the cooling system, if the aftermarket cap is used. I have seen Volvo black caps, but never apart from a running vehicle (though I was tempted to steal them anyway).

Ari

when ever i make a trip to the junkyard i always grab any black cap i find. the yards nearly give them away and oyu can assemble enough to last a life time of oem black caps for a few dollars