Date: Wed Dec 1 12:18:36 1993   
From: John De Armond
Subject: Re: 535 overheating

emory!balltown.cma.com!bmw-request writes:

>I have carefully bled the system, and I still overheat!  Usually
>overheating does not occur when driving around town, but occurs on the
>expressway, just tooling along at 2500 rpm.  The engine never gets into
>the red, but its close! When I check the engine, it doesn't "look" like
>an overheating engine-the hoses are not rock hard, you don't smell
>coolant, and the engine isn't that hot.


It's pretty much a physical impossibility for a car to overheat with a
pressure-tight cooling system if it does not build any pressure.  Sounds like
something in the gauge circuit to me.  Easy enough to check.  Remove the
sender and with it still connected, immerse it in boiling water.  It should
indicate 212 degrees.  The water must be boiling for this to work.  A 
propane torch and a soup can full of water will do the trick.  You can also
check by inserting a therometer in the coolant tank.  If yours is like
my 635, the coolant tank has a decent flow of coolant when the engine 
is running.

>head gasket, which would set me back $1500-$2000, according to my local
>trusted mechanic.  This would cause serious damage to my Driver's School
>cash fund, so I'd like to avoid this if possible; does anyone know of a
>test I can do to determine if the gasket is okay, before I spend the big
>bucks essentially rebuilding the top half of my engine.

!GASP! Actually PAY someone to change a head gasket ?!? :-)  This is a 
real easy job that requires nothing more fancy than a torque wrench
and hand tools.  My 635 toasted a head gasket shortly after I got it.
In full accordance with Murphy, I was living in a garage-less house
at the time.  I did the job in the driveway.  About half a day to pull
the head and disassemble it, a couple of day at the machine shop to
have it trued and a long afternoon re-assembling.  Total cost:  $100
for the gasket set and $40 to have the head trued.

Anyway, if you have a leak serious enough to cause overheating, you will 
see/smell gas and/or oil in the coolant and/or hear the leak if it opens
to the outside world.  A leak-down test would confirm but I don't
think it is necessary, as your symptoms point toward a bad metering
circuit.

Oh, just as a check, what are you defining as overheating?  BMW 
apparently thinks the span of the temp. gauge should be narrow enough
that the normal range of operating temperatures covers most of the 
span.  My gauge just barely comes off the peg in cool weather.  In hot
weather it goes almost full scale.  This concerned me enough that I
instrumented the cooling system with thermocouples.  "Off the peg" is
about 160 degrees while full scale, just before the red is about 230
degrees.  In Bimmer-speak, the engine is not overheating until the needle
actually touches the red line at the top.

John