Hi Everyone. I have a 2000 S40 with 180K miles on it. I have some hissing that I hear after I turn off the engine after each trip. The sound is coming from the radiator overflow bottle cap, and it is a slight hissing sound. There must be some pressure bleeding off thru that cap. I understand that some bleed off may be normal. I am adding some coolant (about 8 ounces every week) because some coolant seems to be disappearing, but I do not see any leak. Could the coolant be vaporizing out of the radiator cap? How to test the cap? What is the spec for the pressure that the cap should hold? Thanks. Chris

 

Looks like the pressure is somewhere between 18 - 24 psi. Depending on the cap you have. The missing coolant could mean a crack in the head?!?

 

I test the cap and the system at the same time by Teeing into the small line going to the tank with my pressure tester. The cap could be getting weak or you could have a head gasket leak.
You could Tee in with just a gauge and watch the rate of pressure rise after you start the engine.
Ed

 

any update on this one. Sry i should have searched first, but i am having the exact same problem

 

It's either a bum cap or a cracked tank.
Ed

 

I'm going to test the cap. The tank is tight, no leaks. I have a new radiator (no leaks there), and I repaired a small leak at the little red air bleed nipple at the top of the lower radiator hose (so no more small leak there). I also repaired a small leak at my thermostat housing with some gasket sealant made for that application (aneorobic). So no more small leak there. The new radiator and the 2 small leak repairs just mentioned were done before I noticed the hissing, so it is possible that sealing those other 2 leaks mentioned above has increased the pressure to the point of the cap needing to bleed off some pressure. Is this necessarily a bad thing? Before I repaired the 2 small leaks I was losing about 8 oz of coolant per day. Now it is more like 2 oz per day. I will test the cap and be back to you. Thanks for the idea from Ed7 of "teeing" into the system. I will look into that. Can you send me a photo(s) or the spec on the tool you have to do the tee-in with? Be back to all.

 

My tester is similar to this: OTC Tools & Equipment (SPX) 7991 Cooling System Pressure Tester and I take off the generic cap adapter and use a brass fitting with barbs to tee in.
That cheap version of that tool is about $80. You could make a manifold with 0-50# gauge and use a tire pump to test the system.
As I said at the outset, at that age, you've probably got a head gasket leak and the cap may be working as advertised. 2 to 8 ounces lost a day is a lot. A little bleed off will relieve the excess pressure and slow the amount of coolant lost.
Just a gauge alone teed into the system can show the pressure rising as the engine is running.
Ed