Would it save my BMW to lower the radiator pressure cap from 30psi to 15psi

In article >,
Brent > wrote:

> From reading what goes wrong with BMWs, one scary well known flaw is the
> lousy leaking plastic radiators.
>
> Since BMW apparently puts 30psi pressure caps (marked 200, or 2 Bar) on
> these plastic radiators, I wonder if simply changing the pressure cap to
> something more akin to what the US manufacturers use (I think that's 14psi,
> right?) might work.
>
> The boiling point tradeoffs may or may not be worth it, so that's why I
> ask.
>
> It seems, almost every BMW eventually (generally by 100k miles) cracks at
> the badly designed joint where the upper and lower radiator hose connects
> to the plastic radiator section.
>
> Do you think lowering the radiator pressure limit by changing the $15 cap
> is a viable tradeoff to save your BMW engine?

You might get pockets of boiling near hot spots of the engine if you
lower the pressure. It can blow your coolant out at high throttle or
cause stress in the engine. For how much trouble old caps have given
me, I'd never voluntarily switch to one set to a lower pressure.
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On 22 Jan 2010 11:31:30 -0500, Scott Dorsey wrote:

> Just change the radiator when you see cracks beginning to form.
> There are aftermarket radiators out there which are more reliable, too.

Hi Scott,
Damn good advice.

I'll keep an eye on the radiator (but I'm not sure exactly where to look).
If someone can explain WHERE the BMW E39 radiator fails, that would be
helpful.

I see Zionville Auto has an alloy E39 replacement radiator for just $654.oo
http://www.zionsvilleautosport.com/s...y_code/CCS.htm

Is that a recommended solution to this, the most severe E39 design problem?

On Fri, 22 Jan 2010 14:30:33 +0000 (UTC), Brent wrote:

> Do you think lowering the radiator pressure limit by changing the $15 cap
> is a viable tradeoff to save your BMW engine?

If you must, I've heard you can retrofit the OEM pressure cap.

Just disassemble the plastic cap (make sure to have a spare one, in case
you break it). The top of the cap that spins around on the cap assembly is
only snapped on, use a screw driver to snap it off, underneath you will see
another "plastic piece" which holds the pressure relief spring (be careful
when disassembling this part, aim away from other people and your face as
the spring might jump out if you don't hold it in place).

If you shorten this spring, than you reduce the pressure it puts on the
pressure relief valve therefore reduce the overall cooling system pressure.

You now have a 140 1.4Bar or 20 PSI cap modified to 15PSI (just like
American cars), you'll have to cut off the spring little by little and than
verify the pressure using an old overflow tank, a tire pressure gauge, an
air compressor, and some brass tubing.

It has been reported that the stock 200 cap ((30PSI = 2 Bar = 200) actually
begins to open at about 35PSI (that my friends is a lot of freaking
pressure).

On Sat, 23 Jan 2010 08:04:25 +0000 (UTC), Brent wrote:

> Is that a recommended solution to this, the most severe E39 design problem?

Besides VANOS?

On Fri, 22 Jan 2010 14:30:33 +0000 (UTC), Brent
> wrote:

>From reading what goes wrong with BMWs, one scary well known flaw is the
>lousy leaking plastic radiators.
>
>Since BMW apparently puts 30psi pressure caps (marked 200, or 2 Bar) on
>these plastic radiators, I wonder if simply changing the pressure cap to
>something more akin to what the US manufacturers use (I think that's 14psi,
>right?) might work.
>
>The boiling point tradeoffs may or may not be worth it, so that's why I
>ask.
>
>It seems, almost every BMW eventually (generally by 100k miles) cracks at
>the badly designed joint where the upper and lower radiator hose connects
>to the plastic radiator section.
>
>Do you think lowering the radiator pressure limit by changing the $15 cap
>is a viable tradeoff to save your BMW engine?


This 30 psi sounds fishy to me. Normal water temp should be just
under 200 degrees. You do not need a 30 pound cap for that. A 15
pound cap raises the boiling point to 260 degrees, a 30 pound cap
would raise it to 302 degrees, way beyond what's needed and would
greatly increase the likelihood of blowing a hose, heater core, or
radiator.

What I suspect is really the case here is that BMW is rating their
radiator caps in Absolute Pressure rather then gauge pressure. At
sea level, a 2 bar absolute pressure is the same as a 1 bar gauge
pressure, which is 15 pounds, just like any other car. Their 1.4 bar
cap would be 6 pound cap, which a few cars do, or did use, at one
time, and which would be adequate for most vehicles during the winter.

On Sat, 23 Jan 2010 14:26:16 -0700, Ashton Crusher wrote:

> A 15 pound cap raises the boiling point to 260 degrees, a 30 pound cap
> would raise it to 302 degrees, way beyond what's needed and would
> greatly increase the likelihood of blowing a hose, heater core, or
> radiator.

That's the whole point of the OP.

BMW is infamous for radiators that "blow up", especially at the neck of the
plastic where it's badly designed and cheaply made.

> What I suspect is really the case here is that BMW is rating their
> radiator caps in Absolute Pressure rather then gauge pressure.

I have no idea. Maybe this is the case. If so, a lot of enthusiasts are
plain wrong. It wouldnt' be the first time, but, someone would need to
cross check.

Brent > wrote:
>On 22 Jan 2010 11:31:30 -0500, Scott Dorsey wrote:
>
>> Just change the radiator when you see cracks beginning to form.
>> There are aftermarket radiators out there which are more reliable, too.
>
>Hi Scott,
>Damn good advice.
>
>I'll keep an eye on the radiator (but I'm not sure exactly where to look).
>If someone can explain WHERE the BMW E39 radiator fails, that would be
>helpful.

If it's an E39, look at the plastic nipple on the top where the supply
hose attaches to the radiator. That nipple will craze and crack, and then
a few months later it will come loose.

>I see Zionville Auto has an alloy E39 replacement radiator for just $654.oo
>http://www.zionsvilleautosport.com/s...y_code/CCS.htm
>
>Is that a recommended solution to this, the most severe E39 design problem?

That seems a reasonable one. Frankly, though, on an E39 I'd worry more
about the water pump than the radiator.

For a little more money you can get a copper radiator that will outlast all
of us. That's probably overkill.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."