Lars L Posted September 4, 2007 Share Posted September 4, 2007 What difference does the cooling system pressure actually do? Reason for asking is that I have converted from 2,5 NAD to a 200 Tdi from a Disco and have kept the original expansion tank. The original cap is marked 8 psi and the cap on the Disco's plastic tank says 15 psi. Caps for V8 seem to be 15 psi as well, so I could just change the cap on my present tank. Question is what practical benefit I would see if I changed? Quote Link to comment Share on other sites More sharing options...
BogMonster Posted September 4, 2007 Share Posted September 4, 2007 Increasing pressure increases the temperature at which water will boil, simple as that. If you run a cooling system with no cap on you might/will get localised boiling in the cooling system if the engine is working hard, and that ain't good Quote Link to comment Share on other sites More sharing options...
Les Henson Posted September 4, 2007 Share Posted September 4, 2007 A pressurised cooling system prevents the water from boiling within the normal operating temperature range/cooling ability of the engine. If you were daft enough to remove the radiator/header tank cap while the engine was at operating temperature - the sudden drop in pressure would cause the water to instantly expand and blast out at beyond boiling point. If you were in the way it would instantly remove the skin from your hands/face etc. Severe pain/really bad scarring/plastic surgery would result. I've seen melted skin from this type of burn - it looks really bad. Use the cap that should be with the engine (assuming it's standard) Les. Quote Link to comment Share on other sites More sharing options...
western Posted September 4, 2007 Share Posted September 4, 2007 use the TD's 15psi cap on your metal oval shaped header tank, as the others have said pressuring a liquid raises it's boiling point. Quote Link to comment Share on other sites More sharing options...
Lars L Posted September 4, 2007 Author Share Posted September 4, 2007 Yep, more or less what I have concluded myself. I have run it with the 8 psi cap for about 10.000 kms since the conversion and all seems to be well. Temp is only rising when the engine is working hard or when driving at walking pace and the fan kicks in to cool it. OK, I'll find myself a 15 psi cap and switch over as it has the correct relief pressure for the engine. Wonder if that will make the poor gasket on the block plug leak even more? And yes, I have worked at service stations for several years and also opened caps on boiling engines. Using the necessary caution... Quote Link to comment Share on other sites More sharing options...
Guest diesel_jim Posted September 5, 2007 Share Posted September 5, 2007 And yes, I have worked at service stations for several years and also opened caps on boiling engines. Using the necessary caution... what, the apprentice? "oi, you... go and open that cap...." Quote Link to comment Share on other sites More sharing options...
Aragorn Posted September 5, 2007 Share Posted September 5, 2007 if the cap hasnt popped then you arent going to increase the cooling system pressure by fitting a 15psi cap If it exceeds 8psi you'd have known about it, the cap starts spewing boiling water everywhere Quote Link to comment Share on other sites More sharing options...
Lars L Posted September 5, 2007 Author Share Posted September 5, 2007 I thought the idea of the expansion tank being there was to avoid that? As it's only half full (or empty for the pessimistic guys) the air compresses when the pressure builds up and avoids water being pushed out. Quote Link to comment Share on other sites More sharing options...
Les Henson Posted September 5, 2007 Share Posted September 5, 2007 The air in the header tank expands as it gets hot too, so I would imagine one cancels the other out. A transparent header tank water level doesn't vary much between cold and operating temperature. Les. Quote Link to comment Share on other sites More sharing options...
Lars L Posted September 5, 2007 Author Share Posted September 5, 2007 Oh yes, the air expands as well. But it's at least compressable, something the water isn't. Quote Link to comment Share on other sites More sharing options...
JimAttrill Posted September 5, 2007 Share Posted September 5, 2007 You should read up on this stuff re Google or whatever. Having ethylene glycol in the coolant raises the boiling point by a few degrees, only about 3c. Pressurising the coolant to 1bar or 14.5 psi raises the boiling point by 26c!! That is why you should not release the pressure quickly. This is also why your engine will overheat quickly if a hose blows. Quote Link to comment Share on other sites More sharing options...
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