Temperature gauge not reading right

[Eagle]

Car is a 89 325i (while not a 5 series is a m20b25)

When i brought this car the temp gauge was always just over the 1\4 mark so naturally i thought thermostat was the issue so replaced that with no change so starting going thru the list. New coolant temp sensor, another instrument cluster, brown dme coolant sensor, also replaced water pump and viscous fan. Only thing i havent done is the radiator.

If the car is left to idle for awhile it warms to dead on the halfway mark and never moves until you start to drive(then it returns to the normal middle of 1\4 & 1\2 position).

Normal driving its between the 1\4 and 1\2 marks, thrashing it does tend make the temp increase a few notches but never goes beyond half.

Open to any ideas, im at a dead end and no clue if the gauge is correct or what.

[bsell]

Do you have an instant read thermometer that will goe to 200 degrees F? If so, use it to measure the temp of the pipe near the coolant temp sensors when the car is warm. Sure, reading 'through' the pipe may not be the best thing, but these engines are hard to get access to the hot coolant without making a mess of everything. So do the best you can with what you have.

I figure if your engine is really running cool, without flogging it, then the independent thermometer should show the area around the temp sensor as much cooler than the 185-195 your thermostat should be keeping. If you see something in the neighborhood of the upper 170's, then I figure you have either a guage problem or possibly a poor grounding situation between the engine and chassis causing the gauge to read low.

If the thermometer reads lower than you would expect, then a partially open (read bad) thermostat could be the culprit. The symptoms you describe sound like a poorly performing thermostat, i.e. too cool during low demand, and too hot when the heat is on.

Brian

[Eagle]

thx see if i can get hold of one. Old thermostat did the exactly the same thing, id be surprised if 2 thermostats were bad

[Ross]

It will be interesting to see what the actual temp is. There is a post here that someone put up a while back showing what actual temps various gauge positions correlate to.
If indeed you are running too cool I would suspect an overactive fan clutch that is always or too often engaged.
If temps are normal then all I can think of is perhaps a poor connection somewhere limiting current flow to the gauge.

[genphreak]

I haven't tried it yet, but I believe you can swap the two once the car is warmed up to check if it si the sensor that is reading incorrectly. Or simply read the voltage directly at a known temperature (engine cold at 20 degree ambient is a normal metric used in the Benlty Manual) to know.

If it is reading correctly and your coolant is not staying at the right temp during normal operating conditions (ie half way) then you have probably got problems with;

1. Thermostat
2. Air in the system
3. a slow leak that is actually letting air in/pressure out
4. a faulty viscour coupling on the fan
5. faulty thermo fan (switch/resistor are most liekly culprits)

Good luck hunting.... :) nick