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BMW Radiator problem

The radiator in my 2003 BMW 540i burst just 5 days after I had a major service. The only warning I got was a “check coolant level” message, at which time the damage had already occurred. Dealer’s explanation was that the thermostat got

stuck in the closed position, preventing coolant from flowing to the engine, and the resultant pressure build up caused the radiator to burst. is this a plausible explanation, and could it have been caused by some mistake made in the service? Why was there no sign that the engine was overheating?

A stuck thermostat will cause engine overheating, but I’ve never heard of “pressure build-up” in a radiator because of a stuck thermostat. A stuck thermostat would keep all the “pressure” in the engine, not the radiator.

The radiator leaked coolant and the loss of coolant meant the engine temperature sensor was trying to read air temperature, not coolant temperature. The sensor is not very good at reading air temperature, that’s why there was no high temperature warning.

I don’t believe a stuck thermostat caused the radiator to burst. Have you seen the damage yourself, or are you taking the service adviser’s word for it?

It is odd that the temp gauge did not rise. Also that the radiator cap did not limit the pressure, unless the radiator was corroded and fragile. Did they replace the coolant at the major service?

So maybe new coolant disturbed the grunge on an old thermostat. I certainly would have replaced the thermostat and rad cap at the same time as coolant if they were 4 or more years old.

Did the radiator upper hose outlet break off the top radiator tank? BMW has had radiator quality problems for 20+ years but this is the first for a 540i(for me).

The thermostat on your car is not the normal garden variety thermostat,it has a monitor that will set a code if it is not operating properly. Never have I replaced a BMW thermostat for “sticking”. We replaced plenty on the 3-series on a warranty campaign basis (for reporting false data but not for sticking)

Codes P1112 or Perhaps a P1624 M52 engine or P1115 all could,I say could have been set. This is not some $5.00 Stant Thermostat
If your radiator broke where I described someone probably stressed it by leaning on the outlet. I know this sounds like a pretty lame way for a radiator to break but the “just in service” and the radiator uper outlet breaking off was very common on earlier 7 and 5 series cars.

Not the first broken BMW radiator.

Your dealer’s full of it. A stuck thermostat can’t cause a pressure buildup, the other radiator outlet is not blocked. (edit-it can cause engine overheating, of course) Also, all (repeat, all) engine cooling systems have pressure relief radiator caps. Oldschool’s right, I bet someone leaned on it, cracked something.

“Stuck thermostat” sounds fishy to me as well. The water pump is not a very “high pressure” pump. Bursting pressure would have to be from expanding coolant which means no coolant flowing, which means an overheating motor. A stuck thermostat in the closed position would mean coolant stays in the engine block and overheating will occur rapidly if that happens.

Old age of the radiator, a defect in the radiator, or some external injury to the radiator are the likely culprits. Examine the radiator carefully for some evidence. Where did it burst? In the cooling fin area, the expansion tanks, or near where the hoses are attached? You need Sherlock Holmes looking for clues to figure out your mystery. Stuck thermostat is not answer.

A stuck thermostat will indeed cause the pressure in the whole cooling system to rise (including the radiator). A stuck thermostat leads to higher temperatures, which leads to higher pressure.

However, the radiator cap’s pressure relief valve should open to relieve the pressure long before the radiator bursts open.

Don’t BMWs have the pressure cap on the coolant reservoir, instead of the radiator? If the cap was bad, it could cause an over-pressure, I guess. But the OPs reported lack of an overheat situation still points to the radiator being weak or damaged.

I never saw the damage myself, I took the dealer’s word for it. The only evidence I saw was that coolant had splashed all over the engine, hoses, etc., and when I added water to coolant tank, it disappeared immediately. I have attached a picture.

Coolant was replaced after 4 years in 2007, during the recent service it was only topped off. The radiator does not have a cap, it is on the coolant reservoir. I have sent the details to BMW customer service, asking for an explanation. The radiator was in good condition. I am a layman in these matters, I took a second opinion from another BMW dealer, but they were not very helpful since there was
no incentive for them. I paid around 2K for the repair, and this is the first expense I have incurred on the car, as I have a 6 year free maintenance warranty.

But isn’t the coolant reservoir subject to the same pressures as the radiator and the rest of the cooling system?

I would be open to the possibility of either:
[list]The radiator cap not releasing pressure when it should have, or [/list]
[list]A weak radiator that just happened to burst at a pressure which was below the cap’s release pressure.[/list]


Since I never personaly saw the problem I phoned up my BMW Master Tech friend (Lead Tech at a Auto Nation BMW Dealership). He says they see the problem quite commonly,not so much as I described (upper outlet breaking off which was more common in the earlier cars but still happens on the e-39)but with the upper radiator tank splitting.

When I related the suspected cause of sticking thermostat he immeditatly asked “did the customer see the engine temp go up”? seemed like a reasonable question.

This problem occured due to a design or manufacturing/material defect with the radiator.

These are pictures I took when I worked at the BMW Dealer in Milwaukee. The burned car is a 2001 e-46 that had its electric fan catch fire (we had a massive fan replacement campaign after this, BMW gave us all a very nice BMW watch as a extra “thank you”) The e-38 (7 series) was a pretty well totaled car that I brought back to life.It took me about a month of heavy work,don’t know why it was fixed and not totaled,whould you want this much work done on your 7 series?
The car with no wheels was the victim of thieves (Its a 540i). The police station was next door but we were always getting hit by thieves. In Milwaukee it is common for the Dealers to leave their lots open so you can come and stroll around after hours to check out what they have, is this a good idea? The 540i cost about $2000.00 to get back up (I had to bust up all the bearings to put new backing plates on).

Agreed. Mercedes Benz and BMW alike both had problems with the plastic radiator failure at the upper neck. When it fails you do get a sudden burst similar to splitting a hose, only more so as the whole upper hose separates from the radiator.

Suspected causes of failure for the upper hose neck (Beside poor design) are using the thin factory German hose clamps with too much much torque. I like to switch the upper hose clamp to an American made wide Gates clamp, and then torque to spec.
The other known cause of failure are bad engine mounts, which transfer severe vibration through the upper hose to the radiator neck.

The new “improved” radiator design includes steel reinforcing in the neck. This seems to have substantially helped.

If you have the old style radiator change and properly torque the upper clamp, make sure the engine mounts are good and never lean on the upper hose.

I’ve never heard the burst by over pressure theory before…

I too agree with oldschool’s reply.
Thanks for providing it.