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98-SLS, 01-SLS
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Discussion Starter #1
After doing all kinds of things to cure a coolent loss/overheating problem (water pump, cover gasket, belt, belt tensioner, thermostate, upper hose) I replaced the presure cap with one I got at the dealer. Didn't pay attention to the new cap until started to lose coolent again. Could not find any spec listed in either the owners manuel or the service manuels. Did find a picture in the owners manuel showing a cap with 15lbs on the decal. The dealer replacement cap is 18lbs. Turns out that the upper hose was leaking from a loose clamp. Maybe caused by higher presure but maybe I just didn't tighten it enough when I installed it.

So my question is what is the correct presure cap limit and if it is not 18lbs should I replace with the correct presure cap?
 

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Cadillac Owners Connoisseur
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Windrider said:
After doing all kinds of things to cure a coolent loss/overheating problem (water pump, cover gasket, belt, belt tensioner, thermostate, upper hose) I replaced the presure cap with one I got at the dealer. Didn't pay attention to the new cap until started to lose coolent again. Could not find any spec listed in either the owners manuel or the service manuels. Did find a picture in the owners manuel showing a cap with 15lbs on the decal. The dealer replacement cap is 18lbs. Turns out that the upper hose was leaking from a loose clamp. Maybe caused by higher presure but maybe I just didn't tighten it enough when I installed it.

So my question is what is the correct presure cap limit and if it is not 18lbs should I replace with the correct presure cap?
We need to know what year and model car you have. They changed from 15 to 18 pounds at some point somewhere between 1996 and 2004. My dad is rebuilding a 2004 DeVille right now that has an 18 lb. cap, but my 1996 has a 15 lb. cap. I don't know if they have changed something about the cooling system to accommodate the higher pressure or not. Three pounds is significant enough, in my admittedly ignorant reasoning, to create leaks where none existed before. One pound is probably OK but three pounds is 20% higher than the original rating. Unless the cooling system has the structural fortitude to withstand 20% more pressure, it will leak. Maybe someone else will chime in here with more educated thoughts, but my initial impression is that if the manual says 15 lbs. or the original cap was 15 lbs., an 18 lb. cap is 20% too much.
 

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White Diamond '03 DHS (with DTS floor shift)
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Just a guess here but, I would think the entire system is designed and tested to withstand much more than what is recommended, be it 15 or 18. That said, I would use the proper cap, whatever that may be.
 

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Ranger said:
Just a guess here but, I would think the entire system is designed and tested to withstand much more than what is recommended, be it 15 or 18. That said, I would use the proper cap, whatever that may be.
I agree, Ranger. My only point is that, especially in a cooling system that is 8 years old, things such as hoses and seals may have degraded due to normal aging and wear and tear to such a point that the extra pressure could cause a failure to occur immediately that might not otherwise occur for many more miles. There probably is some margin of safety built in. Combined with the above and given the importance of cooling system maintenance on Northstar engines, I would consider it a priority to abide by the recommendations made by the engineers.
 
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