BMW: OVER HEATING!!! HELP!!, bmw e30, temp guage

Question

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Followup To

Question -
MY BMW E30 318I 1988 HAS HAD NEW RADIATOR,THERMOSTAT,VISCOUS FAN,TEMP SENDER UNIT AND WATER PUMP, ITS STILL GOES UP TO 3QUARTER MARK.. WHY? THIS IS A HARD ONE 2 MECHANICS HAVE NO CLUE EVEN BMW ONES!! THEY JUST SAY THE GAUGE COULD BE FAULTY... BUT HOW? please mail me direct if poss!! thanks luke..

Answer -
Luke,
 This is very odd indeed.  The only possibility I can think of, is the thermostat that was replaced was a hightemp thermostat.  Some theremostats are rated for higher temps than others, and they are fully open at different temps.  Since changing the thermostat is very simple, I would pull the thermostat completely out, and run the car with no thermostat to see if it helps.  In the wintertime this would probably have your car running too cold, when the needle gets barely above the cold mark.
 I would try this first, to see if it is a thermostat issue.  It is also possible that you have an air bubble lodged in the engine, and it is creating steam, or a small hot spot.  You can elevate the front of the car on a jackstands, or on a steep hill, and start the car.  This will usually help with bleedig air bubbles out and into the radiator.  Sometimes bubbles are a real pain to get out, and can take a while.
 If either of these dont fix the problem, it could be that you have a blockage somewhere inside the engine.

 Good luck,
 Josh

Follow up...
Had the engine checked and there is no engine problem its been presser tested and head is fine,could it be right in saying the temp gauge is reading wrong? How would this be? Is there some thing apart from all the things you said? How and why can you get different thermostats? pleas answer in full i love my car and dont want to ruin the engine!!

Answer
Luke,
 I suppose the temp guage could be wrong, but usually it doesn't just randomly pick a higher temp.  These guages are known to fail, but they usually just stop working, or bounce around all the time.  I've never seen one just read 3/4 hot and stay there.  That's why I would not discredit the guage yet.  

Companies make different thermostats for different climates.  Like in Maine you would obviously need the engine to run a little warmer than you would in Florida.  There are other reasons for this too, but if you go into AutoZone, or some other parts store and ask for a thermostat, they will probably ask you what temp do you want.  This may or may not be an issue.

 Good luck,
 Josh