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

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?

Brent > wrote:
>From reading what goes wrong with BMWs, one scary well known flaw is the
>lousy leaking plastic radiators.

This is on some models, not all. And there is warning before the thing
fails, too.

>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.

This would be a bad idea because the system is not designed for that pressure.
You will find the system never gets up to proper temperature.

>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.

Not all of them, but some do.

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

No, but inspecting the radiator every time you change your oil is a good
idea. Replace it BEFORE it breaks, not afterward. Also keep an eye on the
gauges. They are there for a reason and you should scan all of them briefly
about once a minute.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

On 22 Jan 2010 09:51:08 -0500, Scott Dorsey wrote:

> This would be a bad idea because the system is not designed for that pressure.
> You will find the system never gets up to proper temperature.

Hi Scott,
Thanks for the advice. I agree that the engine was not "designed" for the
lower pressure cap.

The problem is the radiator neck was never designed (properly anyway) for
that pressure either. The quest is to save the BMW engine from catastrophic
failure.

I'm thinking of replacing my 2001 E39 200 pressure cap to the one used in
my friend's 1988 325 (140 rated at 1.4Bar or 20PSI). I realize dropping
pressure from 30 psi to 20 psi lowers the boiling point of the engine
coolant by 30 degrees F.

But, isn't the cap only the high pressure limit? The actual pressure of the
coolant in the BMW would be something lower anyway, right?

"Brent" > wrote in
message ...
> On 22 Jan 2010 09:51:08 -0500, Scott Dorsey wrote:
>
>> This would be a bad idea because the system is not designed for
>> that pressure.
>> You will find the system never gets up to proper temperature.
>
> Hi Scott,
> Thanks for the advice. I agree that the engine was not "designed"
> for the
> lower pressure cap.
>
> The problem is the radiator neck was never designed (properly
> anyway) for
> that pressure either. The quest is to save the BMW engine from
> catastrophic
> failure.
>
> I'm thinking of replacing my 2001 E39 200 pressure cap to the one
> used in
> my friend's 1988 325 (140 rated at 1.4Bar or 20PSI). I realize
> dropping
> pressure from 30 psi to 20 psi lowers the boiling point of the
> engine
> coolant by 30 degrees F.
>
> But, isn't the cap only the high pressure limit? The actual pressure
> of the
> coolant in the BMW would be something lower anyway, right?

At least until you drove it on the Autobahn and then stopped for
gas....

If the pressure is usually lower anyway, why are you worried?

This made me think of something that happened to me many years
ago...In 1975 I was working in southern Arkansas. I had a new Datsun
280Z. One Saturday I drove down to Shereveport, LA for the day. I
headed back home very late. The road was empty and I was in a hurry. I
drove as close to flat out as I could for many miles without stopping.
I finally had to stop in Junction City LA/AR (it was on the border).
Just as I pulled off, the car erupted in steam...there was so much
steam I though I'd blown the engine. I opened the hood and the
radiator cap was laying on a support near the radiator. I suppose it
is possible I'd left it off.....at any rate, while I was driving at
high speed the air through the radiator was able to keep the water
cool enough that it stayed liquid. However, as soon as I stopped, and
the cool air flow was reduce to what the fan could pull through at an
idle and the water boiled off in seconds. The radiator was pratcially
dry. I let the car cool down and started wondering around looking for
water. This was a small town and it was essentially closed for the
night (it was 2 am). Eventually a guy drive up in a Chevy. It turned
out he was the night time law enforcement (the town was on the border,
one side handled day time enforcement, the other night time). He was
very helpful. He told me where to find a jug and where to get water. I
was able to refill the radiator and drive the rest of the way home
(only another 15 miles). To this day I don't know if I left the cap
off, or if the pressure forced it off. The later seems unlikely, but I
can't imagine the cap rode around under the hood for days without
falling off at some point.

Ed

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.

Dumping pressure is't a great idea. You'll risk damage to the engine just
as effectively as a leaking radiator. It is better for the radiator
to blow than to dump the cooling system first.

Maybe a radiator shop can make improvements.

Brent > wrote:
>On 22 Jan 2010 09:51:08 -0500, Scott Dorsey wrote:
>
>> This would be a bad idea because the system is not designed for that pressure.
>> You will find the system never gets up to proper temperature.
>
>Hi Scott,
>Thanks for the advice. I agree that the engine was not "designed" for the
>lower pressure cap.
>
>The problem is the radiator neck was never designed (properly anyway) for
>that pressure either.

The radiator neck on some of those cars is poorly designed. It will degrade
and crack no matter what. Reducing the pressure might buy you a little time
but it will still crack.

>The quest is to save the BMW engine from catastrophic
>failure.

The way to save your engine is to replace the radiator before it fails,
and to shut the engine off if it does fail.

The reason these engines suffer catastrophic failures is that drivers don't
watch the gauges. The thing fails, the needle pegs, and they keep driving
for several minutes before they realize anything is wrong. This is bad.

>I'm thinking of replacing my 2001 E39 200 pressure cap to the one used in
>my friend's 1988 325 (140 rated at 1.4Bar or 20PSI). I realize dropping
>pressure from 30 psi to 20 psi lowers the boiling point of the engine
>coolant by 30 degrees F.

Yes.

>But, isn't the cap only the high pressure limit? The actual pressure of the
>coolant in the BMW would be something lower anyway, right?

Yes, but not by much. Just change the damn radiator when you see cracks
beginning to form. There are aftermarket radiators out there which are
more reliable, too. Ask a good independant BMW shop.

On an E39, you should be able to do the whole job yourself in the driveway
in less than an hour. It'll cost you something like $400 which is a lot less
than the cost of a new engine.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

"lugnut" > wrote in message If there is a great
concern for this failure,
> I would watch it like a hawk with frequent inspections. If
> you are really paranoid about it, look around for an
> improved design replacement radiator.
> Lugnut

If it were mine, and I was really disappointed with the design or quality of
the
radiator, I would spend the time and money to find one that is robustly
built of
metal, I think.

If there are not aftermarket radiators already available, I am sure there
are
companies or people who will custom build one for the OP. I have
researched
this a few years ago, and found that it was indeed possible (for whatever
car
I was stewing about, anyway.).

I dont like aluminum and plastic in radiators.

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?

These things usually break due to hose movement stress fractures.
Figure out some way to stop the hose from moving at the radiator
when the engine tilts.

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?

Fillet the necks with JB Weld.

In article >, Paul > wrote:
>
>These things usually break due to hose movement stress fractures.
>Figure out some way to stop the hose from moving at the radiator
>when the engine tilts.

That's part of it, but part of it is the result of plastics that can't
handle the temperature.
--scott

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