Quick question regarding rad cap
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Safety Car
Joined: Mar 2005
Posts: 4,723
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From: Winnipeg Manitoba, Canada
Quick question regarding rad cap
I was reading my fsm lastnight and realized that it states my 90 should have a 17lbs (117kpa) rad cap. As of now my car only has a 16lbs cap. A few places that I called, all say there computers show a 90 having a 16lbs cap. Should I go and get a 17lbs or is the 16lbs fine? Thanks 
Last edited by jimmers; Jul 7, 2006 at 11:12 AM.
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Joined: Sep 2000
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From: The Top of Utah
Originally Posted by jimmers
I was reading my fsm lastnight and realized that it states my 90 should have a 17lbs (117kpa) rad cap. As of now my car only has a 16lbs cap. A few places that I called, all say there computers show a 90 having a 16lbs cap. Should I go and get a 17lbs or is the 16lbs fine? Thanks
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Thread Starter
Safety Car
Joined: Mar 2005
Posts: 4,723
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From: Winnipeg Manitoba, Canada
I'm wondering about this becaue my overflow doesnt get used. The level never reaches hot, and no coolant gets sucked back into the rad. I'm wondering if the cap could be doing this?
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From: The Top of Utah
If it is, it isn't because the spec of the cap is a pound too low. It could be because the cap is defective and won't allow the pressure to drop below atmospheric, in the radiator. Think how it works. As the coolant and the VERY limited amount of air in the cooling system heat up, they expand. As they expand, the pressure builds. Even if the cap won't hold any pressure, the greater volume of coolant (it is hot and has expanded) will over flow. As the coolant cools, it will contact and lower the pressure in the radiator to draw the coolant back in from the reservoir. If the cap won't seal, the pressure will equalize through the leaky cap seal rather than drawing coolant back in from the reservoir.
Also, if your radiator isn't full enough to force water into the reservoir, there may be no give and take of liquid, with the pressure difference being taken up by expanding and contracting air. If that were the case, however, I would expect you would see the low coolant light, on occasion.
The difference the pressure rating of the cap makes, is in the boiling point of your coolant and the strain (pressure) it puts on the cooling system components.
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Also, if your radiator isn't full enough to force water into the reservoir, there may be no give and take of liquid, with the pressure difference being taken up by expanding and contracting air. If that were the case, however, I would expect you would see the low coolant light, on occasion.
The difference the pressure rating of the cap makes, is in the boiling point of your coolant and the strain (pressure) it puts on the cooling system components.
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Safety Car
Joined: Sep 2000
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From: Hinckley Ohio
Originally Posted by jimmers
I'm wondering about this becaue my overflow doesnt get used. The level never reaches hot, and no coolant gets sucked back into the rad. I'm wondering if the cap could be doing this?
If the engine doesn't get hot, then the overflow tank doesn't get used very much......
Defective thermostat....
