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      02-05-2014, 09:20 PM   #1
iZ4vel
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Exclamation Z4 Cooling System Issue... Need Help Please

My Z4 hadn't been run for about a year since it was involved in a minor front end accident.. Being that it sat for so long, I wanted to flush the all the fluids in the car (antifreeze was due for change too), I poured some coolant flush into the expansion tank, filled up the the rest of the system with tap water. Let the car run for about 15 minutes to get to temperature. Its usually 55*F out here in AZ right now so I knew it shouldn't have taken that long to warm up but it did. Before this flush I remember my car would heat to normal operating temp in about 5 minutes and my fan and thermostat would turn on when the needle went past half way, which it would do every so often. I allowed the car to stay on, hoping that it would go past the middle mark on the temp gauge but no matter what I tried, revving in park, a/c full blast, heater full blast, nothing made the needle move past half temp not the gauge, this meant the fan never turned on and the thermostat never opened. My fear here is that the engine temperature is actually wayyy hotter than the needle on the temp gauge is demonstrating.

Upper hose from expansion tank is always hot, I mean it gets extremely hot, but the lower hose the left side of the radiator is always cool to the touch.

1st thing I suspected was the coolant/oil temp sensor that's located on the cylinder head was bad and wasn't registering the temp correctly. Replaced it with a brand new part. Didn't help anything, needle still stuck at halfway.

2nd thing I suspected was that my thermostat was stuck open and therefore the car was taking so long to get to normal operating temp. Replaced that with brand new thermostat, again needle not moving past halfway, fan not turning on, and thermostat still not opening.

3rd thing I suspected was that the temp gauge needle was stuck and couldn't go past half way, I disconnected the coolant/oil temp sensor while the Z4 was running (cold engine) and that shot the needle into the boiling hot range where it actually turns on the red light, so I know the needle in the instrument cluster temp gauge isn't stuck and unable to go past half way.

UPDATE: I replaced lower hose coolant temp sensor with new part from autozone just now, again, top hose got extremely hot, lower hose cool to touch, cooling fan did not start, and needle did NOT go past exactly halfway.

All coolant/oil temperature sensors have been replaced, currently have pure distiller water in the system.


Could this be a computer issue? I have no check engine codes...

ANY ideas guys?? Thanks for the help in advance!
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Last edited by iZ4vel; 02-05-2014 at 10:20 PM.. Reason: Additional sensor replaced.
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      02-05-2014, 10:13 PM   #2
iZ4vel
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UPDATE: I replaced lower hose coolant temp sensor with new part from autozone just now, again, top hose got extremely hot, lower hose cool to touch, cooling fan did not start, and needle did NOT go past exactly halfway.
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      02-05-2014, 11:45 PM   #3
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Somebody with a 2.5i would have to back me up on this, but every car I've ever owned has had the gauges marked so that things where lower is generally better (like temp) read just below 1/2 way, and where more is generally better (like oil pressure and volts), read just above 1/2 way.

What you show looks normal to me. That's where the oil temp in my ///M sits.
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      02-06-2014, 05:31 AM   #4
iZ4vel
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Update: Fixed it!!!

Accessed "hidden menu" in CPU and ran test 7.0 (engine rpm/ coolant temp in degrees Celcius), Z4 thermostat is marked with 97* degrees Celcius opening temperature. As u can see from the picture, the _097 is showing that the coolant temp is 97*C, I got out of the car to check the lower radiator hose and it was warm, got back in the car, coolant temp went up to 98*C and the cooling fan FINALLY turned on.

After thought, after spending over $100 replacing and installing all the temperature sensors in my Z4, I realized that my previous antifreeze was really old and that the cooling system hadn't ever been properly flushed. This is what I think caused my Z4 to take longer to reach operating temperature, aka run cooler for longer, also the ambient temperature after the flush was about 40*F colder than last summer when the car was last run... The upside to this is that I don't have to worry about an overheating problem and now I don't have to worry about having to spend money taking it in to a mechanic.
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      02-08-2014, 02:35 PM   #5
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wow Im so lucky I have the exact same problem

2.5I 2003 also had a MINOR front accident.

Car smokes from the front and smells like the engine is burning but the temp needle stays in the middle! the car throws "thermostat value below threshold" code consistently after clearing it more than 5 times. also noticed the coolant fluid was almost empty

I was about to start replacing the thermostat, as the usual problem is that the thermostat is stuck open. guess not?

So help me understand how you fixed the problem? is the problem fixed by running that test ? or is it by flushing the coolant? or something else?

there are a lot of z4 overheating threads !

coincidentally no coolant flush has been done on this car in the last 4 years at least.
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      02-08-2014, 04:34 PM   #6
iZ4vel
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The test I ran was a diagnostic test to see exactly how hot my coolant temp was because I believed the engine temp was extremely past where it should be and I believed that either the computer, or a temperature sensor wasn't registering the engine coolant temp correctly so my fan wouldn't turn on and the thermostat wouldn't open because of that. No matter what I did to try to get the engine to "overheat" past the exact middle range on the temp gauge it just wouldn't. That's when I searched the Zpost forum online and found out the Z4 has "hidden menu's" that you can access to get diagnostic information. It's free to access those hidden menus and isn't difficult at all. I ran test 7.0, coolant temp in Celcuis degrees. Thermostat and fan on the Z4 usually kick on and open at 97* degrees Celcius, in fact it's marked on the thermostat at 97*C. So know that I knew exactly what the coolant temp was, I wasn't worried about letting my car run as long as necessary in park to get it to the 97*C, where before I would shut it off cuz I didn't know if the temp the car was registering was correct, here, I know it is because it's a direct digital read out from the brand new temp sensor on the cylinder head. My Z4 finally reached 97*C and the fan kicked on and the thermostat opened. Again I think the reason this happened is because I did flusht eh system so it no longer had a clogged system and also because the ambient temperature was a lot colder than it usually is, those are my explanations for the increased warm up time. I also checked my thermostat because I suspected it was stuck open, it takes about an hour to remove the thermostat to check and I know u said you did that already. Maybe try running the diagnostic test I ran. Search "access hidden mums Z4."
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      02-08-2014, 07:14 PM   #7
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make sure your cooling fan is working. turn the aircon on while in idle, and it should turn on within a minute or 2
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      02-11-2014, 09:51 PM   #8
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OP - I'm just sharing my story with you.... I'm running about 30% antifreeze and 70% distill water and that's because summer afternoons here in TX get reeeeally hot. However in December I drove my car when it was like 22 degress out and the needle never passed the 11 O'clock mark. The car took about 15 mins to reach that temp or mark, but now we know why. when I got my destination, I sat in the car with the engine running and the needle tried to creep to 11:30 :-) but it would slowly come back to the 11 mark.

Currently my boiling point is around 250 degrees compared to the 280+ that BMW recommends or likes to keep our cars.

Now - if you used tap water, that's bad news for you unless you are planning to sell the car soon. If not, I would flush the engine at least another 3 times with 50/50 to get all of those minerals out of your cooling system.

If you still want to run less antifreeze after few flushes then add distill water. Each gallon is like $2.oo. Remember the water pump needs some lubricants from the antifreeze other wise it will break soon.


Good luck!
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