BMW cooling system


The thermostat of my 2001 BMW 325 (83,000 miles), is stuck in open. The part itself is only about $100. My (trusted) mechanic recommends replacing the entire module: radiator, water pump, reservoir, upper/lower hoses, sensors, thermostats, for about $1,300. His reasoning is that all of these will fail, probably very soon, and repairing only the water pump, or only the radiator, is very expensive in labor costs. Moreover, the module is mostly plastic, and messing with the hoses and the radiator is not a good idea.

I have two questions. (1) should I really fix all these things if they are not broken? It’s hard to believe that BMW’s radiators are designed such that you have to replace the entire thing. (2) My understanding is that the thermostat is a cold weather device (it delays the cooling function so that the engine can warm up more quickly). If this is correct, I live in San Diego and the car is parked in a cozy garage, and it’s never colder than 40 degrees outside. Could I leave the thermostat as is and wait until the other parts of the cooling system actually break, too? Would that put too much stress on the engine?

These thing are not all one module, they are separate items and can/should be changed as needed. The only thing that you loose when doing them one at a time is the coolant, and that needs to be changed periodically anyway. You might replace the attached radiator hose, but even that is not necessary.

You do need to get it fixed, you will not pass the smog check if you don’t. The engine has to reach proper temperature in order to go into closed loop operation. It will not reach that temperature if the thermostat is stuck open, unless you are crossing Death Valley in July.

Find a new mechanic, one who is not making boat payments, and have the thermostat replaced…

I have a BMW 325i 2002 with 215,000 miles on it. I’m not sure if you’re being sarcastic about the word “TRUSTED” or you really do trust your mechanic. But I wish I had him when I had mine fixed. I think your mechanic is coming from the point of view of I’ve-seen-this-happen-sooo-many-times-before because he’s right. It’s a good idea to ask other BMW mechanics as well or visit the e46fanatics forum, but if I were you and you have the money now, just go with his advice. This is a known issue with our car unfortunately, even Consumer Reports will tell you that, and you will see this several times in your car’s lifetime. One time when my thermostat failed, I took it to a dealer (Crevier) and nobody said anything about what your mechanic is telling you now. I think it lasted only for a week until the radiator failed too. Have a chat or call several BMW mechanics and I think they’ll tell you the same thing. I have asked several BMW specialist here in Orange County and I keep hearing the same thing about the e46 cooling system. Good luck!

Jtamba, The radiator is only connected to the engine by a pair of hoses, the thermostat is attached to the engine. There is no reason to replace one just because the other gave out. The labor doesn’t change, the only shared labor is opening the hood. You just had a coincidence.

I do think of the original radiator hoses are on this thing, then changing them is a good idea. I’d even go as far as changing the heater hoses too, but not the radiator or water pump.

With all due respect, the comment that “the only shared labor is opening the hood” must have come from someone who has never looked under the hood of an e46 BMW.

I own four three series BMWs, two e36s and two e46s. They range from 95k miles to 275k miles. I do all my own work. I have been doing all my own work since I worked my way through college as as an auto mechanic and nightschool auto tech teacher.

Take it from the voice of experience, your car is 11 years old. All of the plastic and rubber parts in the cooling are due to fail at any time. Your mechanic is absolutely correct, both from the overall cost of ownership and the reliability factor. I treat the cars that my wife and daughters drive a bit differently than I treat my own daily driver, but when it comes to the cooling system, I replace everything that I remove, or even get near, that is more than 6-8 years old. I find that I like myself better in the long run.

You could save over half that money if you did the work yourself with online parts, but I will warn you that it is not easy. Small hands and a colorful vocabulary come in very handy when working in the engine compartment of an e46.

Keep in mind that I and my loved ones drive my cars for a very long time, so I maintain them accordingly. If your situation is that you just want to try to get another year or so out of it before trading it off, that’s your call. A stuck open thermostat is hard on your engine and terrible for your gas mileage, but you probably won’t save enough on fuel in a year or so to pay for the repair.

Correction - I said that you could save over half that $1300 if you did it yourself. I checked some on-line prices, and for name-brand parts, you would spend over $700 to get all those parts yourself. I presume that you have an automatic, which affects the cost of the radiator on an e46.

Manolito, I have casually glanced under the hood of a few BMW’s, but I have not worked on one. Are you trying to tell me that the thermostat is in the radiator and not attached to the block? Is the radiator, waterpump and thermostat one single unit? Do you have to remove the radiator to get to the thermostat?

I am not questioning whether these tend to fail all at once and I am sure there is some cost savings in having them done all at once, but the cost savings will not be in either the parts or the labor, just in supplies like fresh antifreeze etc.

Now it might be true for the waterpump that the radiator has to be removed to get at it, I’ve seen that with some for/aft engine mounts with a tight engine compartment. But for a thermostat, I’d just do that for now.

My comment uncalled for. I apologize.

Though I have replaced every part in e36 cooling systems at least a couple of times, so far, all I have had to do so far at the front end of the two e46 engines was belts and tensioners. It is remarkably tight quarters up there.

I just walked out to the garage and examined the thermostat housings on the 325 and the 330, and I will concede that if one chooses to replace only the thermostat, that looks like it can be done without removing much else on the 325, just the air box and a few odd wires and the hoses.

This car has two thermostats, one is permanently fastened inside the thermostat housing that bolts to the front of the engine. The other is inside the expansion tank. My experience with plastic German radiators is that they crumble and fail after 8-10 years. My experience with German expansion tanks is that they crack and fail after 10-12 years. Rubber hoses can fail any time after 8 years, particularly if they are in a spot that gets a lot of engine heat. All these things are a lot easier to deal with if you pull the whole system out at once, replace everything at once, and rest easy for the next decade knowing that you don’t have to worry about your cooling system.

This is experience from four of the six cars that I have owned since 1990. The other two - my two '04s, have not yet had any major cooling system parts fail, but they both had O-rings on the water temperature sensor shrink and start dripping after 6 years.

BMW water pumps last about 150k miles, and I see his car has pretty low miles, so that may be a judgement call. That might be one of the “Is it my car or my wife’s car?” judgments. Or, it may come down to “I’m doing the radiator and heater hoses, my Saturday is shot, I am filthy and sweaty, I have antifreeze dripping into my eyes and armpits, I have two cut knuckles and a variety of muscle cramps from working in tight awkward spots. How soon do I want to do all this again to replace my water pump?” The alternative question is “How soon do I want to pay someone to do all this again to replace my water pump?”

BTW The water pump might come out with the radiator in place if the electric fan and shroud are removed, but it would be easier to take the fan and shroud out together with the radiator. The existence of a transmission cooler for an automatic transmission would be a factor in this decision. The water pump sealing surface is a lot easier to clean with the thermostat out of the way.

Manolito, I can appreciate your 3 series knowledge and your maintenance regimen. The e36 is one of my all time favorite cars.

Anecdotally, I would agree to err on the side of heavy preventative maintenance on the e46 cooling system. A family member had an '04 325i that overheated at 120k miles. The chain of events, as far as we could determine, was that at some previous point the electric fan failed. The owner prefers fresh air and rarely uses a/c, so the failed fan went undetected for however long…until the 98° day when she cranked on the a/c. The clutch driven fan couldn’t keep up, and the car started overheating in traffic. She caught it when the temp light came on, and before she could pull over the plastic expansion tank blew.

That story fascinates me because my '04 325 has no mechanical fan but my '04 330 has one. I presumed that this was the difference between a 325 and a 330, but since your 325 had a mechanical fan, perhaps it is the difference between a manual transmission and an automatic (automatic uses two fans, mechanical and electric) while the manual uses only an electric fan.

If that is the case, then replacing the thermostat on the OP’s car may be a bit harder than I described. The mechanical fan will have to be removed to get the thermostat out. Removing that fan is a bit of a pain, requiring a couple of special tools, and don’t forget that it is left-handed thread…

I’ve never heard something so ridiculous as this…that mechanic is trying to rob you…I mean Jesse James used a gun…what the hell is this guy trying to pull here…RIDICULOUS… Replace what you need when you need it…

You need that thermostat in the engine esp in San Diego…so just put one in and be done with it…its easy and can be done with basic tools…do it yourself and do NOT forget to burp the cooling system afterwards.


Seems unbelievable, but a stuck-open thermostat could well cause your engine to overheat. The thermostat has to be working properly for the coolant to get routed in the right proportion to the engine and the radiator to keep the engine properly cooled. This is especially important with performance cars. So it needs to be replaced.

If your mechanic has given you good advice before this, or he has good recommendations, my tendancy in general would be to follow his advice. Especially if you car had 283K on it, I’d say to go with what you mechanic suggests. He may have experience that when BMW’s becomes too old, it is nearly impossible to take the cooling system apart and get it back together without something else going wrong. If you’ve ever had to do a plumbing job at home, fixing a leaky toilet for example, you probably have a similar experience. Often you can save yourself time and grief if you simply replace everything while you have it all apart.

But with just 83K on the engine, if it were my car, I’d take a chance and just have him replace the thermostat.

By the way, not sure if it is recommended for BMW’s, but whenever I replace a thermostat, I always check the new one first. I put it in a pot of water on the stove on low heat, and put a temp guage in the water, and I watch it to make sure it opens at the correct temp. Usually they open as expected at the specified temp. but I’ve had a few that failed to open or opened at the wrong temp and this simple check saved me the time to take the whole thing apart again.

HB, I’ve heard from a number of current and former BMW owners that these cooling system parts are pretty much wear items, 10 years is typical life, and that this type of replacement is often needed. So I’d do it IF I had long-term plans for the car. If I was only going to own it a year or so, I might put it off.

Manolito, that’s interesting about the differences in the fan setup. This one was an auto, and it was probably a late 04–took delivery in September. She previously had an '01 325i (auto) that I vaguely recall having a clutch fan. Incidentally, a tree fell across that one and resulted in the purchase of the '04. She now has an '11 328i (6 speed manual) that I believe is only electric. I’ll have to look under the hood sometime to double check, but it’s missing so many other familiar things (e.g., dip stick) that the lack of a clutch fan didn’t stick out in my mind…

Dear all, thank you so much for the advice. It’s helpful the get detailed information about this.
I was not kidding when I reported that I have so far trusted this mechanic, and some of you have confirmed that perhaps I should just continue doing so. Yet I find it hard to get my head around fixing things that ain’t broke.

Not one of the several BMW mechanics I have since visited recommends replacing the thermostat only; they strongly advice to change out the upper and lower hoses. The tank is apparently the first thing to go in these BMWs, so they strongly recommend to replace that (plus its thermostat) while the coolant is out. And the challenge apparently is this: the hoses do not connect to the radiator with clamps, but it’s some sort of stick-it-in-a-plastic-opening setup. If you replace the hoses, you run the risk that they will not probably connect to the radiator, yet any leak is difficult to see with the naked eye. So the mechanic will not guarantee this part of his work and, as Jtamba suggested, has seen it fail too often before.

My car is a stick shift. But they tell me that the water pump is at the end of the engine and then some, and it takes 4 hours to replace. Hence the advice is to do it in 10 minutes while all other parts are out of the way, save a few hundred dollars and stop worrying about it for the next 8 years.

As it turns out, the thermostat just unstuck itself (the “check engine soon” light is off). I didn’t know this was possible, and was surprised to be told that I can trust the “check engine soon” light to come back on at the next episode. I guess I just won a few more days of time to ponder my options.