Your first stop will be to check cylinder compression and leak down. If those numbers are good then you maybe lucky. All I can say is best of luck.

 

Your first stop will be to check cylinder compression and leak down

These test are not so helpful in this case. Even if the gasket is leaking, it's so small that the tests will not be able to detect it.

 

see the temperature needle fully in the red

Did the small red LED light came on?
In this case, the engine overheated first, created a lot of steam/gas and the expanded volume led to the tank rupturing.

I'm not sure what caused the overheat; could be low coolant. Don't depend on the coolant light unless you had tested that it worked with low coolant in the tank. When is the last time the coolant level in the tank was checked?

The main thing right now is to check if the head or gasket has been damage by the overheat. So replace the tank, fill water, bleed the air properly and drive with an eye on the temp gauge, and check the coolant level couple time per week to see if the level stable or lowering. But I would plan for the worst that the head might need to be out.

 

I don't believe the red led came on - the needle was just deep in the red. I checked the coolant the previous day as I was driving a long distance and everything was looking good. The floater was lined up with the top of the expansion tank.

Appreciate your response a lot. I'll get the tank and other damaged components replaced and send an update once it's back running again.

 

So you didn't see the yellow coolant level light before the overheating?
With the tank intact, the only way to cause overheat is either low coolant, or no coolant flow, or electric fan not running.

 

I'll go through the steps you mentioned above once I get it back to running condition, but yeah I have never had the yellow coolant light come on whilst I've owned this car. I regularly check my fluids ever week and have never noticed a drop in coolant level.

Is there a possibility that the thermostat could have seized closed? I know their supposed to be a fail safe that they will stay open, but I'm starting to wonder if that might be the case. Anyways, I'll update you as the repair goes on.

Thank you Sapote!!

 

Thermostats are designed to fail open. What’s the maintenance history for the cooling system?

 

Thermostats are designed to fail open.

By what mechanism? To me, if the wax motor leaking wax, then it won't open.

 

Yes, the expansion tank can loudly pop.
Did you see if you can see the crack in the expansion tank?

Even with no coolant, you can start the car cold for a minute or two and see if it idles correctly.
This may tell you if there is overheat damage

 

I am reading an overheat at highway speeds followed by an expansion tank failure.

-Not a fan problem
-Not likely a leak or a low coolant problem - you checked levels
-Doesn't sound like an air pocket problem either- would have happened sooner

I am betting a flow problem.. most likely failed water pump or some kind of clog in the system.. I wonder if anybody ever used stop leak in your car's coolant system

These cars don't take overheats well, and by the time the gauge goes red, its too damn hot (the gauge is buffered from factory so that it stays in the middle over a wide range of temps..like 75c - 110c or something). As Sapote said you need to see if your head is warped or head gasket is compromised. Not the end of the world...These engines do take head gasket replacements well.

 

The expansion tanks are known to split & crack along that seam, though usually not quite so spectacularly. I had a Behr Hella ET that lasted only 88k miles & barely 5 years; it cracked on that same corner but the split was only a couple inches long.

Use distilled water when you put it back together until you know if the HG is good or not, and that other components are good or are replaced once HG is confirmed good.

• Radiators have plastic end caps that with age also tend to leak at the seams or split.
• Coolant hoses with oil on them or ATF fluid from power steering leak dripping on them are also suspect.
• Plus if original, the coolant hose orings might not reseal well upon reassembly.

One rough test for combustion gas leakage into coolant is feeling the upper rad hose on a cold start to see if it "inflates" like a balloon before the coolant has warmed up and thermally expanded, like within a minute or so of start up.

• On cold engine, open expansion tank cap & close it. Upper rad hose should be pliable. Start engine and idle, if upper hose gets hard/inflated before coolant/water has warmed up that indicates a possible HG issue.

Water pump flow you can test, again on a cold engine, by removing ET cap and revving engine up to 2k or 2.5k rpm or so while looking down into ET to see if a stream of coolant is shooting from the side when it revs up. Use a helper or set up a camera to observe, and only run engine briefly with ET cap removed.

As noted above, temp gauge is near useless due to being buffered at 12 o'clock from 75c to 115c. You can use the hidden test menu on instrument cluster to display the actual coolant temps. Select test 7.0. See link in my signature below.

 

there was not a drop of coolant left in that engine.

bummer.

t
would pressurize things and see if they leaked into other things.

 

Dear Pal,

If it means anything apparently the red means too late. I’m constantly peaking at my dash, I know I’m not paranoid. I have read of other folks hitting the overheating mark and being there for minutes and still not having a destroyed engine. So hopefully the Lords have blessed you.

In my experience though I Haven’t ever ran out of coolant in the entire engine. Even when I had cracked or exploded hoses and tanks. The system itself still had coolant. I believe to completely empty the coolant there is another area other than the radiator.

Hope that’s reassuring. I do remember after a coolant refresh I had what seemed like a bad bleed job and the temp gauge spiked while I drove the bloc home wasn’t far but the light really scared me and I shut off asap. Just a bad air pocket is what I summarized, I have a steady idle now and no engine issues that I can tell and it drives fine so maybe these engines aren’t as brittle as the warning is. Still no more overheating.

Cheers.

 

Mine popped on me about 10 years back. I was idling waiting to pick someone up.

Then I happen to glance up Oo would you look at that needle is climbing. Oh wait a minute is that smoke??? Poof .

 

I am extremely sorry for the extremely delayed response. I was finishing school and just recently remembered I never posted the end result. Thank you to everyone who posted a response, it really helped me along the way and built my confidence as it seemed like a daunting task at first. While the car was out of commission I ended up doing quite a bit more work (P/S system replacement, Intake Manifold Gasket, CCV system refresh, Intake boots, Dipstick O-Ring, OFHG, Valve Cover Gasket, fuel injector O-rings, and new silicone vacuum lines)

I ended up replacing the majority of the cooling system - All the soft radiator hoses, new expansion tank, new water pump, new thermostat, new ATF cooler thermostat, new o-rings, all new sensors, and of course fresh BMW coolant. (Kicking myself now that I didn't replace the hard lines under the intake manifold while I had it off)

Since the car overheated pretty badly I performed one of those chemical head gasket tests where it tests for combustion gases in the cooling system, and a compression test to reinforce my results. Both turned out great, no combustion gases in the cooling system and around 165-175psi across all cylinders on an engine that was sitting off for around an hour. So far so good, I've done around 5000-7500 miles since then and have had no issues.......until the other day.

Now a few days ago I got in the car and my low coolant light was on, so I checked the reservoir and I would say it was almost empty. Took about 1-1.5L of coolant to get the red bobber back to its happy spot. Looked underneath, no visible leaks, I can't find any white residue near connections, and has been at a steady level since I filled it up. I am genuinely confused as to where that coolant went, although the cap was not as tight as I usually have it when I checked it after the low coolant light came on. I topped it up, tightened the cap, and the car has been driving fine since. No overheating issues, although it does get up to temperature pretty quickly some days but never deviates from the middle of the temp. gauge. Is it possible the coolant in the expansion tank evaporated after I shut the engine off?

I am planning to do a cooling system pressure test in the coming days (more likely weeks) but I will post those results then. Thank you again to everyone!

 

I am extremely sorry for the extremely delayed response. I was finishing school and just recently remembered I never posted the end result. Thank you to everyone who posted a response, it really helped me along the way and built my confidence as it seemed like a daunting task at first. While the car was out of commission I ended up doing quite a bit more work (P/S system replacement, Intake Manifold Gasket, CCV system refresh, Intake boots, Dipstick O-Ring, OFHG, Valve Cover Gasket, fuel injector O-rings, and new silicone vacuum lines)

With the intake out and you didn’t replace the old hard pipes?

Forgot to give us the update until new problem?

Check spark plugs in the cold morning for condensation.

 

I am hearing that you are losing coolant, not obvious from where and that you have an overheat in your history.

I would check the cylinders for coolant to rule out a head gasket leak.

Run the car to full op temp one day shut it off for the night. The next morning, before starting, remove each spark plug and place clean white rags over the spark plug holes and turn it over a few revolutions. Check the rags for coolant.

 

I wonder if one of the coolant hard lines are leaking and the coolant that didn't spill out evaporated. Residue underneath the intake manifold would be difficult to spot. At any rate, a coolant pressure tester should solve this mystery for you pretty quickly.

 

subscribed.

 

I had same thing happen to me after a partial cooling system refresh. everything not under the intake was replaced.

Started losing coolant after every drive and nothing visible either.
Got a $45 pressure kit from Amazon and turned out the ONLY non-BMW hose used was leaking. As soon as the system pressurized to 2ATM, coolant started dribbling out of the hose. You will not see this driving or on idle. The aftermarket top hose was $23 and was basically money wasted. Get the tool to pressurize the system.

 

AutoZone rents out for free the coolant system pressure testers with the correct adaptor:

https://www.autozone.com/loan-a-tools/loaner-radiator-pressure-tester/p/powerbuilt-cooling-system-pressure-testing-kit-22-piece/449951_0_0

 

What brand of aftermarket radiator hoses did you use? Did you bleed the system correctly with heater on?

I'd recommend doing a pressure test which you planned on doing anyways.

If you find the upper or lower radiator hoses are leaking, I'd highly recommend replacing only the O-Rings - the best and quite cheap way is to replace it with Genuine Audi coolant O-ring (4E0121666). It'll cost you about $20 for 4pcs and being genuine audi part they will seal much better and withstand higher pressure. Even if they aren't the source, I'd recommend replacing them anyways as the aftermarket seals tend to leak pretty quickly.

There's a hose that attaches to the expansion tank half way up of the back of it. Replace this one with Genuine BMW 11517642402 or ELRING 473.920. Again, this is a relatively cheap but extremely effective solution. IIRC the other end of the hose connects to one of the hard coolant hoses so I doubt you would have replaced this hose as a whole. I'd replace this regardless of it leaking or not, as there's a good chance it'll leak sooner or later especially since you have removed and reinstalled it.