aTOMic

I wondered because mine pulls a strong vacuum when the cap is loosened after it's been run and allowed to cool. Replaced the cap - same thing. I would think that the cap would allow air IN (as it cools), but not OUT (except the ~22PSI it's supposed to maintain while hot) to avoid a negative pressure situation which further stresses every component of the cooling system that's in contact with the coolant. The main reason I'm considering this is my 175k mile 2004 has developed a coolant smell and I really do not want to do the radiator retrofit and fix the leak (wherever the hell it is).

To partially answer my own question, I realize the coolant doesn't have to "boil over" to be in an engine-threatening situation; there are hot spots within the heads where the coolant could vaporize without the pressure Art et al designed into it, thereby not providing proper cooling, leading to all sorts of fun. However, I wasn't sure if anyone had any ACTUAL EXPERIENCE with this (4.2L) engine. I ran a pressureless Chevy 307 for 150k miles with no ill effects I could see, and this includes 4-wheeling (therefore moving slowly with the engine working hard, so not much air through the radiator - especially since the fan was 2 feet away from it and there was no shroud).

Just something to think about... thanks
Tom

Shark_V8

What theory?

Cooling system is closed to facilitate:

- higher heat capacity (ie boil over)
- no loss of water through evap
- presumably better flow and heat xfer (no pump cavitation or perhaps pockets of steam generated - could happen in the heads.)

Use a liquid Prestone sealant. I did.

richard-tx

I don't worry about it. The team of engineers that designed that engine have more education and knowledge and make more money than I do. On top of that, their track record is pretty good. As a result, I trust them. They have the tools, dyno, and more to make sure the engine lasts a long time. No need to second guess them at this stage. The basic 4.2 engine been around since 1988. It is now 29 years old and apart from direct injection, timing chains, etc, it is still the same basic engine. Why second guess it now?

MP4.2+6.0

First, look for the leak. If underhood, often not so bad--hose, o-ring seal, etc. If heater core on the other hand, painful. If water pump blow out port (leading to bearing area...), gotta fix it very soon before it gets worse and leaves you at side of road, if you are lucky. Obviously that backs into timing belt replace questions. If radiator, gotta face music but fair amount of work.

When I ran my 2000 C5 A6 4.2 w/out cap--only to fill it or while working on post drain type stuff basically, it would get to where it was going to start to boil over through the reservoir. These ain't the 165F type thermostats you could dynamite in an old Detroit iron ride--or even run w/out here in coastal CA. I don't think they are 180F IIRC either like many older motors were stock. Efficiency also drops as you lower running temp--why modern engines have it pushed much closer to (non-pressurized) 212F/100C.

richard-tx

If you are not developing pressure when at normal operating temp, then you have a problem and you need to get it fixed. If you have a vacuum when cold then you likely do not have a major leak. If it were mine, I would get the system pressure tested. If it can hold pressure for an hour, I doubt you have a serious leak. When pressure testing, it is best to have as little air in the system as possible.

jlbell

My best advice to you is if you want no pressure do not swap the radiator or whatever you have in mind, but instead swap the coolant to Evans Waterless. The only reason you have pressure build up in the system in the first place is due to water, 50/50 coolant mixture, that vaporizes upon heating, then condenses upon cooling. Evans waterless does not use water. I swapped both my fathers and mine, when I had it, 2001 BMW 7 Series coolant with Evans waterless due to the fact that everything in the car was designed by an engineer except the coolant system. The common saying amongst E38 7 series owners is that the cooling system was designed by an artist. I am talking around 10 times less reliable than my A8 cooling system. During every college break, before swapping to Evans, I would always be fixing a component of the coolant system, that is at least 2-3 times a year. Finally besides Evans not being pressurized the coolant also captures and removes more heat from the engine due to the lack of, if I am remembering right, "hot spots" or in other words parts of the head that is so hot during running conditions the water vaporizes thus not taking heat from that part of the head. I have proof to support this claim to. BMW has a hidden menu in their instrument cluster, one of the functions is to read the outflow coolant temp. Whenever I swapped to Evans the outflow coolant temp increased by a few degrees, on both our E38s. So hopefully that may shed some light on what you might want to do. Ever since swapping to Evans on my father's E38, right after we purchased it, we haven't had to replace a single component, in around 2.5 years of ownership.

richard-tx

As near as I can tell Evans is just 70% ethylene glycol, propylene glycol, water, and a corrosion inhibitor. The MSDS sheet at https://www.google.com/url?sa=t&rct=...,d.amc&cad=rja says so. In it's pure form ethylene glycol boils at 387 degrees F. THe problem is that pure ethylene glycol has a specific heat capacity about one half that of water. A 50/50 mix by mass with water has a specific heat capacity of about 3140 J/kg C (0.75 BTU/lb F) three quarters that of pure water, thus requiring increased flow rates in same system in comparison with plain water.

I will let those interested to research propylene glycol and why running pure antifreeze is a bad idea.

There is no free lunch. If it were mine I would either fix the leak or sell the car.