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

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.

Not all of them, but some do.

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

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?
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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
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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 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.

Yes.

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
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On 22 Jan 2010 11:31:30 -0500, Scott Dorsey wrote:

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/store/screen/ctgy/store_code/6134/category_code/CCS.htm
Is that a recommended solution to this, the most severe E39 design problem?
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On Sat, 23 Jan 2010 08:04:25 +0000 (UTC), Brent wrote:

Besides VANOS?
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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.

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

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

Sacrificing a radiator is a better tradeoff than an engine. The system pressure is developed to make sure all of the engine has adequate cooling. The hottest place in most engines is the combustion chamber where the temps cannot be easily monitored. If the system pressure is not high enough, the coolant will boil away from the combustion chamber area allowing localized over heating which may have catastrophic consequence if continued. There will be no indication this is occuring unless you crack a head or blow the head gasket. Proper pressure with the correct coolant mix ensures this does not happen assuming the rest of the system is functioning properly. The radiator, being a problem in and of itself, is still a more desireable failure in your case. 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. Someone may have already addressd this problem. Lowering the pressure is definitely not the solution unless you also reduce the thermostat rating which has it's own problems.
Lugnut
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concern for this failure,

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

Fillet the necks with JB Weld.
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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 Fri, 22 Jan 2010 14:30:33 +0000 (UTC), Brent wrote:

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

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

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.

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

If this is the case, a pressure test with a normal cap tester would indicate the PSIG operating pressure of the cap. Thus, he would know if the cap is a 14-15 PSIG or 30 PSIA. If it is a 30 PSIA cap, it should be no problem. It seems to me that BMW simply has a poorly designed/spec'd radiator. As another poster pointed out, it is possible that weak, failed or defectively designed engine mounts that allow too much movement may also contribute especially when the system is normally heated and pressurized. Sometimes, design does not account or compensate for other manufacturing deficiency tolerances.
Lugnut
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