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E34 (1989 - 1995)

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  #1  
Old 09-28-2012, 01:03 AM
robertobaggio20 robertobaggio20 is offline
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Thermostat or temp gauge question

Hi everyone,

If the engine is performing normally, and the upper radiator hose is hot, and the temp gauge in the dashboard sometimes shows 1/2 and sometimes shows cold, and there appears to be no coolant leaks or abnormal losses and no air bubbles in the cooling system, can we safely :

1. Rule out a buggy thermostat ?
2. Rule out a buggy water pump ? (no leaks)
3. Suspect a faulty dashboard temp gauge ?
4. Suspect a faulty cylinder temperature probe ?
5. Suspect loose wiring somewhere ?

Thanks.


Roberto
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  #2  
Old 09-28-2012, 02:40 AM
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_Ethrty-Andy_ _Ethrty-Andy_ is offline
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typically the temp sender dies it dies, it wont move at all or only a few degrees (pun intended) around the dial. thats not to say this isn't at fault here. It sounds to me like the thermostat is on its way out as is responding very slowly to temp changes
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  #3  
Old 09-28-2012, 03:46 AM
robertobaggio20 robertobaggio20 is offline
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Hi Andy,

Thank you for your response. If the thermostat was buggy, would the upper radiator hose be nice and hot after a 20 minute drive on the highway, while the temp gauge showed cold ? Is that a possibility ?

Stuff like this is happening more frequently over the past few days. It used to happen only once every 2 months or so.
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Old 09-28-2012, 07:03 AM
robertobaggio20 robertobaggio20 is offline
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In fact, yesterday, I brought it up to operating temperature, then shut the engine down, and restarted it like 10 minutes later and the gauge was showing cold.
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Old 09-28-2012, 03:51 PM
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_Ethrty-Andy_ _Ethrty-Andy_ is offline
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Quote:
Originally Posted by robertobaggio20 View Post
Hi Andy,

Thank you for your response. If the thermostat was buggy, would the upper radiator hose be nice and hot after a 20 minute drive on the highway, while the temp gauge showed cold ? Is that a possibility ?

Stuff like this is happening more frequently over the past few days. It used to happen only once every 2 months or so.
okay, the temp gauge probably is the issue in that case

your car is M50 right? pretty simple to change. if its the M20 version its even easier. its unlikely that its the gauge itself, they are pretty robust, its the earlier E28 and E21 versions that were temperamental as electric gauges were a very new thing at the time.
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Last edited by _Ethrty-Andy_; 09-28-2012 at 03:52 PM.
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  #6  
Old 09-28-2012, 10:52 PM
robertobaggio20 robertobaggio20 is offline
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Im sorry Andy but your comment is unclear. Would you please clarify ? Yes i do have the m50. Thanks.
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  #7  
Old 09-30-2012, 05:47 PM
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1995i540 1995i540 is offline
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You should replace the cheapest part first if you can't perform proper tests. I would start with the sensor.

Your thermostat could fail in either open or closed state depending on what year the part was manufactured in, not sure how many hoses you have but the bottom hose that runs from radiator to engine should be cooler than top hose that runs from expansion tank to radiator.

If it's very hot that means your thermostat is stuck closed. You should also make sure you check your coolant level and don't just assume that it's not leaking, if your mechanic used water instead of a coolant (i heard stories like this) you might never see it on the ground because it will evaporate.
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  #8  
Old 10-05-2012, 11:00 PM
Photojunky Photojunky is offline
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Mine (1991 525i) was doing the exact thing and it was air in the cooling system. I had to bleed it 5 or 6 times. Now it is fine.
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Old 10-06-2012, 01:15 AM
robertobaggio20 robertobaggio20 is offline
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Quote:
Originally Posted by Photojunky View Post
Mine (1991 525i) was doing the exact thing and it was air in the cooling system. I had to bleed it 5 or 6 times. Now it is fine.
That is unusual. You only need to bleed it once, and perhaps one more time the following day after you've had a day of driving (not necessary but its fun).

Make sure you've brought the engine to operating temperature before you start bleeding it. Make sure your cabin heater is on at full blast (if the cabin heater works at all) while you do this. I've never had any trouble with bleeding and never had any overheating issues related to bubbles in the system.
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Old 10-06-2012, 01:20 AM
robertobaggio20 robertobaggio20 is offline
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Quote:
Originally Posted by 1995i540 View Post
You should replace the cheapest part first if you can't perform proper tests. I would start with the sensor.

Your thermostat could fail in either open or closed state depending on what year the part was manufactured in, not sure how many hoses you have but the bottom hose that runs from radiator to engine should be cooler than top hose that runs from expansion tank to radiator.

If it's very hot that means your thermostat is stuck closed. You should also make sure you check your coolant level and don't just assume that it's not leaking, if your mechanic used water instead of a coolant (i heard stories like this) you might never see it on the ground because it will evaporate.
Well, I changed the temp gauge in the instrument cluster to a spare one and everything seems to be working fine now.

Yes i checked the coolant and topped it up it was due for a top up anyway. I top it up once every 2 months.

Yes the top hose was very hot but that's how its always been during normal operation. I did not check the bottom hose.

I used a laser temperature gun to check temperatures. The part of the radiator that's nearest to top hose (left side from the driver's chair) was 20 degrees hotter than the opposite side. I guess that means that its working as it should.

I've been driving with the clutch fan restrained without any problems. I will remove it very soon. Thanks for the tips my friend.
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