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Conflicting Oil Temp Vs Oil Pressure Readings

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#1 cjreyn

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Posted 25 October 2020 - 01:05 PM

Hi,

This is my first mini, so apologies in advance for any daft questions!

 

I have a 1989 mini Thirty 998cc running 20w50 and a vmaxcart supercharger. Mileage is ~125K miles... my guess is that it has had engine maintenance work done to get there as it doesn't smoke or rattle, and is a vey well maintained car. It has a slight blow in the exhaust (on my list to fix), some slight milky residue on the oil cap (the oil is new, but looks fine and not like milky coffee). It appears to have a couple of mods to the sensors as follows:

 

  • An oil temperature sensor that is located on a spacer between the block and the oil filter mount housing.
  • An oil pressure sensor where the oil pressure switch is normally located.

 

The switches connect back to some extra gauges in the cabin. The oil temperature is a circular gauge embedded into the dash, and has markers at 50C-120C-170C (min-med-max), whilst the pressure sensor is mounted on top of the dash next to a boost gauge.

 

At cold idle the oil pressure reads 60psi (ambient air is 16C). After a short 15-20 min drive, the pressure drops to a steady 40psi, but temperature reads all the way up to 170C (into the red section of the gauge). At this point I pull it over, open the bonnet and wait 5 mins for it to cool. Resuming driving, the temperature is still reading 170C. Coolant temps look fine.

 

A little research suggests the pressure readings are about right, but the temp readings seem high to me. I'm not driving it for more that 20mins for fear of inflicting bearing damage. 

 

Should I be worried by the temperature readings, or should I just monitor the oil pressure? I am open to fitting an oil cooler (the setrab ones look pretty good), but only if there is actually an issue.

 

 



#2 mini13

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Posted 25 October 2020 - 01:38 PM

I'd bet that the Oil temp gauge is lieing, die to bad calibration, fed fromt he wrong voltage, the wong sensor being used, or bad earthing. or of couse its heat soaking from the block rather thn measuring the oil.

 

what brank guages are they?

 

I would say that the oil would be black pretty quick, as usually 130 deg is the upper limit for a lot of oils.



#3 cjreyn

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Posted 25 October 2020 - 01:59 PM

I don't know the brand, but I've attached a photo:

Attached File  gauge (2).jpg   30.81K   5 downloads

It looks to me like there is a hole drilled in the spacer for the sensor which would see the oil flow, but I can see how temp from the block might skew the reading. 

 

The sensor itself has a single wire, although I've not checked the back of the gauge to see how it connects (earths etc).

 

Does the oil blacken if it overheats?

Is there a better location for the temp gauge?

How are they calibrated? 

What oil pressure level (low threshold) should I be concerned by?

 



#4 mini-mad-mark

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Posted 25 October 2020 - 06:08 PM

Looks like the centre gauges/wooden dash are an add on (as not standard for a 998?) so I agree with mini13, possibly installed by someone that didn't know that the temp gauge needs 10v from a voltage regulator (usually - looks like a smiths gauge) but I don't know what reading you would get at 14.4v (assuming that's what you have?) could be much higher than actual but I would say only 44% higher (14.4 vs 10v?)

 

I would check that the sensor wire isn't snagged and shorting direct to to earth as I think that might give high readings? sure someone else will be along to confirm this one



#5 Moke Spider

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Posted 25 October 2020 - 08:13 PM

I think those Temperature readings are in Fahrenheit (which kinda makes sense given the Pressure is in PSI) and that sounds about right or a little on the cool side.

 

You'll usually find the Oil Temp closely follows, though with a little time delay, the Coolant Temp.



#6 cal844

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Posted 25 October 2020 - 08:16 PM

I think those Temperature readings are in Fahrenheit (which kinda makes sense given the Pressure is in PSI) and that sounds about right or a little on the cool side.

You'll usually find the Oil Temp closely follows, though with a little time delay, the Coolant Temp.


It's printed on the guage °c, however I do agree with the scale on oil temperature and coolant temperatures

#7 Magneto

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Posted 26 October 2020 - 02:02 AM

I think you need to check that temp gauge for accuracy, one thing you could do is shoot the engine in several places on the oil sump when it's reading hot with one of those infra-red laser thermometers, just to compare.

 

Did you say the sensor is or isn't in contact with the oil itself? I don't know how you can get an accurate reading if it's not.....


Edited by Magneto, 26 October 2020 - 02:02 AM.


#8 Johnmar

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Posted 26 October 2020 - 09:00 AM

Take the sender out off the block, and place the back end in boiling water (while keeping the wiring above the water level offcourse. Then you know how accurate the the current sensor/gauge/volts combination is.

That would be my starting point.

#9 tmsmini

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Posted 26 October 2020 - 02:51 PM

That looks like the gauge used by the Rover MPi cars.

After 20-25 years the sensor could have gone bad. The sensors were NLA for awhile but a replacement has been developed.

 

https://www.minispar...YCB100320.aspx?

 

As mentioned I would check the ground. On the MPi, they are mounted directly in a block port.



#10 cjreyn

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Posted 26 October 2020 - 08:12 PM

Thanks all for your suggestions. @mini-mad-mark you might be on to something. I disconnected the sensor and measured the voltage between the sensor end of the wire and the chassis ground. It's a steady 12.4V so no grounding issues. This contrasts to the coolant temp wire that hops around suggesting its connected to the 'I' output of the voltage reg at 10v. I read that it oscillates in a way that makes it tough to read! So my guess is that the oil temp gauge has been connected at the 'B' connector of the regulator as you said.

 

I will pull the dash out and check the wiring and report back.







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