How to read the temp gauge

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rodrigo96
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How to read the temp gauge

Post by rodrigo96 »

Hello everyone, first sorry for my english because i speak spanish, i bought an e21 320/6 and i think that i am having problems with the temperature or problems reading the gauge because if i push it( something like 5000 rpm) on the highway it pass the middle line between 1/2 and 3/4 marks, is that normal? To be sure I already changed the water pump, thermostat (80c°) and cleaning the radiator (by my self) and nothing changed .i found some values of the resistance of the temp gauge on the internet and others values in the shop manual and here start my problem, somepeople says that the 1/2 mark the temp should be 80C° (186f), in that point the resistance that the gauge read should be 55 ohms (i found that information becuase there is a guy that test the gauge and shows the values of resistance), is something like 1/4 on the gauge = 66 ohms , 1/2 =55 ohms. But in the shop manual it says that 65 ohms = 82.4 C°, 45 ohms=94.5C°, so when the gauge mark 1/4 is 80°C or that temp is in the middle of the gauge ? i have to worry if it marks near 3/4 or is normal when you push it hard? driving near 4000 rpm in 5th gear the temp is on the middle. I hope that what I wrote can be understood, sorry again for my bad English. Thank you very much


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BertjeConti
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Re: How to read the temp gauge

Post by BertjeConti »

Your englisch is clear, very readable.

I don't know relation about sensor resistance and temperature.
What i do know about the temp gauge and engine temp is:

A little before middle is 84 degrees. Thats always my engine temp when engine is warmed up and it doesn't matter if cruising or pushing hard.
Engine temp is stable since i put in a new 80 degree thermostat.

Before, with the old thermostat, engine temp was stable at 88 degrees, needle in the middle, but when pushing hard the temp went up upto 3/4 temp gauge and read about 94 degrees.
The old faulty thermostat didn't fully open anymore, therefore engine temp went up when pushed hard.

Recap gauge and engine temp:

A little before half is 84 degree
In the middle is 88 degrees
3/4 is 94 degrees

How do i know?
I have a second temp sensor in the thermostat housing for the ecu with a digital readout on the dash, also a M20B20 engine.
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Megasquirted '77 E12 520-6

Aspen Silver '96 E39 523i
rodrigo96
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Re: How to read the temp gauge

Post by rodrigo96 »

Hello, thanks for the answer, that is what i was looking for. Reading that i assume that i have a problem with my car, i already changed 2 times the termostat, the first i used was a topram and the one it have now is coolXpert both 80° (when i bought the car it didnt have one). maybe i have a problem with head gasket
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BertjeConti
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Re: How to read the temp gauge

Post by BertjeConti »

No thermostat in the housing is a absolute NoGo, because there are two valves involved, one opens the radiator cirquit and the second valve closes the bypass cirquit. Without a thermostat both valve passages are fully opened, half of the pump capacity coolant runs through the bypass, and doesn't help cooling.

Buy a decent brand like Mahle Behr with authenticity qr code on the box.

I don't say that a good quality thermostat will fix your problem, you might also have a clogged radiator.
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Megasquirted '77 E12 520-6

Aspen Silver '96 E39 523i
rodrigo96
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Re: How to read the temp gauge

Post by rodrigo96 »

yes, at the moment i saw it didn't have a thermostat i put one, when i buy the last one they said to me that was a good brand (made in germany) also they told me that is better that the other one(topran). the last thing before taking out the head gasket is check the radiator
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BertjeConti
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Re: How to read the temp gauge

Post by BertjeConti »

Oh wait with taking the head off, do it only if there is evidence for a leaky headgasket like:
Exhaust fumes in the cooling system
Cooling system pressure keeps going up.
Coolant in the engine oil, discoloured oil, mayo on dipstick and oil filler cap.

If you have a old radiator, it's always a wise idea changing it for a fresh/new one.

Look for a hot climate radiator!
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Jeroen
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Re: How to read the temp gauge

Post by Jeroen »

A problem is that these gauges aren't all that accurate and neither are the temp senders, the only way to be sure is to measure with a proper temp probe.

But ok, back to basics so I also understand haha. So usually temperatures are fine, but the gauge goes up to somewhere in between the 1/2 and 1/4 mark if you push the car? Even though the temperature difference between 1/2 and 1/4 is only about 9 degrees centigrade so the temperature only goes up slightly, this is not behavior you want, especially living with a warmer climate throughout the year like in Spain. My 323i does the same, but then it really should be pushed hard, long times of high revs or driving > 180 km/h for a longer time, on a warm day.

Important factor is flow, so a clogged radiator or old water pump may limit capacity, but I must also say that the cooling capacity of the M20 E21's is limited from the start. Since I mainly drive my car in the summertime or when going on summer vacation, I fitted a thermostat for warmer climates, one that opens at 71 degrees instead of 80. Not sure BMW still supplies those, but OEM supplier Behr Mahle still makes them. In fact, I ordered a few last week! Here is a link:
https://www.autodoc.es/mahle-original/7815252

In this way, you achieve that when you really put your car to work, the temperature is on an ideal level. A slightly lower temperature when the engine is not really performing seems no issue to me. I believe that my oil temperature (as the most reliable measurement I have at hand) went from 82 degrees to 77-78 degrees with the 71 degree thermostat, and you really have to push hard to get the gauge over the 1/2 mark. I'm very happy with this solution.

Note 1: if the engine always runs warm, or if the temperature goes up in traffic, then the cooling system requires further attention.
Note 2: in many cases it can be a good idea to switch to an electrical cooling fan instead of the viscous coupled mechanical one, but it won't make much difference in your situation, as the flow of any fan will not have much impact anymore at high driving speeds imo.
Regards/groeten, Jeroen

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rodrigo96
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Re: How to read the temp gauge

Post by rodrigo96 »

BertjeConti wrote: Tue Aug 16, 2022 7:59 am Oh wait with taking the head off, do it only if there is evidence for a leaky headgasket like:
Exhaust fumes in the cooling system
Cooling system pressure keeps going up.
Coolant in the engine oil, discoloured oil, mayo on dipstick and oil filler cap.

If you have a old radiator, it's always a wise idea changing it for a fresh/new one.

Look for a hot climate radiator!

the last week, before changing all that i mentioned i start the car and let it just idling for 20 minutes and it didn't seem to built pressure on the system just the normal i think (when i push it hard it build pressure ). Here is a video of what i did, the car was running for 20 minutes just idiling ( yes a bit high idling but that is another problem i have to fix ), you can turn off the sound of the video because i speak spanish
rodrigo96
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Joined: Mon Aug 15, 2022 4:46 pm
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Re: How to read the temp gauge

Post by rodrigo96 »

Jeroen wrote: Tue Aug 16, 2022 9:38 am A problem is that these gauges aren't all that accurate and neither are the temp senders, the only way to be sure is to measure with a proper temp probe.

But ok, back to basics so I also understand haha. So usually temperatures are fine, but the gauge goes up to somewhere in between the 1/2 and 1/4 mark if you push the car? Even though the temperature difference between 1/2 and 1/4 is only about 9 degrees centigrade so the temperature only goes up slightly, this is not behavior you want, especially living with a warmer climate throughout the year like in Spain. My 323i does the same, but then it really should be pushed hard, long times of high revs or driving > 180 km/h for a longer time, on a warm day.

Important factor is flow, so a clogged radiator or old water pump may limit capacity, but I must also say that the cooling capacity of the M20 E21's is limited from the start. Since I mainly drive my car in the summertime or when going on summer vacation, I fitted a thermostat for warmer climates, one that opens at 71 degrees instead of 80. Not sure BMW still supplies those, but OEM supplier Behr Mahle still makes them. In fact, I ordered a few last week! Here is a link:
https://www.autodoc.es/mahle-original/7815252

In this way, you achieve that when you really put your car to work, the temperature is on an ideal level. A slightly lower temperature when the engine is not really performing seems no issue to me. I believe that my oil temperature (as the most reliable measurement I have at hand) went from 82 degrees to 77-78 degrees with the 71 degree thermostat, and you really have to push hard to get the gauge over the 1/2 mark. I'm very happy with this solution.

Note 1: if the engine always runs warm, or if the temperature goes up in traffic, then the cooling system requires further attention.
Note 2: in many cases it can be a good idea to switch to an electrical cooling fan instead of the viscous coupled mechanical one, but it won't make much difference in your situation, as the flow of any fan will not have much impact anymore at high driving speeds imo.
Hello and thanks for the answer, in normal driving condition the gauge stay bit near to 1/2 (like the video i post previously) it only goes up if i push it, if i drive it near 5000rpm it pass the middle between 1/2 and 3/4, if i let off the gas to 4000rpm it start going down to 1/2. I didnt know that there was a 71 degree thermostat i will try to buy one and see if they ship to uruguay. Another thing that maybe can impact the temp, the previous owner take off the heater of the car, so the coolant never goes to the heater and other thing is that my car didnt have a viscous fan, it is fixed to the water pump like a 316
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