E12 Cooling System

by Wietse Wullink, Peter Florance, Robert Bondi

Updated August 25, 2003

Back to Frequently asked questions

Figure 1.  Diagram of E12 Cooling System (Robert Bondi)
(click picture for larger image)


1.  T gauge accuracy.  Verify the accuracy of the sensor from electrical specs and make sure electrical connections and grounds are clean.  The temperature sensor is in the thermostat housing.   Erratic readings may also indicate a need to bleed the system.  Try to verify the system temperature with another instrument such as an IR gun.  I have a permanent mechanical T gauge in an otherwise unused thermostat housing port.

2.  Pressure check the system – The system should be around 15 psig with the engine running.  If the system is not under pressure, it will boil over more easily.

3.  Radiator – Make sure air flow through your radiator isn’t blocked by leaves and bugs.  More likely, your old radiator will have significant scale build-up inside degrading heat transfer; in this case,  recoring or a new radiator will be necessary.

4.  Coolant – Change your coolant at regular intervals.  Convert from conventional green anti-freeze to a long-life anti-freeze.  You must completely flush your system to convert; don’t mix the green anti-freeze with a long-life anti-freeze.  I use Prestone 5/150, which is orange..  Havoline Dexcool is a similar product.  The long-life anti-freezes use a carboxylic acid corrosion package which is exceptional in terms of reducing scale deposits.  Remember to use distilled water for the water component when possible; this helps reduce scale deposits.

5. Coolant ratio – This ultimately depends on your climate.  50/50 is common, but I use about 30% anti-freeze in Texas.  A common misconception is that anti-freeze helps cool your system; however, the advantages provided by ethylene glycol anti-freezes are expanded liquid range over water and a corrosion inhibitor package.  Anti-freeze lowers the coolant freezing point much more than it raises the boiling point.  Anti-freeze is actually inferior to water in terms of heat transfer properties:
Table 1.  Selected Thermal Properties of Water and Ethylene Glycol at 100C  (Robert Bondi)
Thermal Conductivity, k (cal/sec/cm/C)
Heat Capacity, Cp (cal/g/C)
Water 0.0016 1.007
Ethylene Glycol 0.0006* 0.67
*property at 80C

Water is more than 2x effective as a heat transfer medium than ethylene glycol.  Also, ethylene glycol coolants will heat up faster.

6.  Bleeding the system -  Bleed screw is on thermostat cover.  This is a necessity after replacing the coolant.  Open the bleed screw at idle after the engine has warmed to let air escape.  Run the heater.  Not bleeding the system will likely cause no damage, but you will probably get artificially high temperature readings.

7.  Thermostat – Make sure the t-stat is neither stuck open or stuck closed.  It should begin to open at the specified temperature.  Always use a t-stat.  The absence of the t-stat can allow hot water coming to the housing to bypass the radiator and go straight to the head!!  I think the 4 cylinders have the same issue.

8.  Fan clutch – Spin the fan blade.  Some resistance should be present so that the blade only rotates 90 to 180 deg.  The clutch action is provided by a special viscous fluid that has properties that vary with RPM and possibly temperature?

9.  Auxiliary fan – Make sure this turns on at the appropriate coolant temperature and during the A/C duty cycle.  While this probably only helps a little, this fan sitting in front of the radiator and not running probably impedes air flow through the radiator a little.

10.  Ignition timing -  There isn’t a huge amount of freedom here, but retarded timing will make the engine run hotter.  Many cars are timed incorrectly to TDC instead of the 22 degree BTDC mark.

11.  Fuel mixture -  There isn’t necessarily much freedom here, especially on lambda FI systems, but engines that run a bit rich will run a little cooler.

12.  Oil -  Run a 20W50 or 15W50 in most climates.

13.  Check for coolant in oil - If your oil looks like a chocolate milkshake, forget the cooling system and get the head or head gasket repaired before you ruin the bottom end of the engine. Coolant is a lousy lubricant.

14.  Check for exhaust in coolant - If you have mysterious overheating problems while driving, consider getting the coolant checked for the presence of exhaust gas. Again, if true, it will lead to head R&R, but at least you'll know.

15.  Fan shroud -  There is some debate in my mind which way to go on this.  On one hand, using the shroud helps the fan pull more air in; on the other hand, the shroud itself must also reduce air flow in the perimeter regions of the radiator somewhat.  There is some curvature to the shroud which I think helps funnel air from the perimeter areas in toward the fan.  I happen to use mine. (Robert Bondi)

16.  Turn off the A/C and run the heater in an emergency situation.  The A/C makes the air reaching your radiator warmer.  The heater core removes heat from the coolant.

17.   Note that the coolant temperature will rise after the car is shut-off.  Heat from the hotter block and head will conduct radially outward from the source while the engine is off, so the temperature at the sensor will rise.    Restart the warm engine and watch it drop quickly.

18.  Remove thermal reactors – This only applies to the older E12s, such as the US 530i.  The thermal reactors were notorious for running the engine hot and cracking heads.


1. E28 radiator – This is a significant improvement over the stock E12 radiator.  The E28 radiator is aluminum, while the E12 radiator is a brass alloy.  Aluminum is fundamentally a better heat conductor than brass alloys by a factor of 2.  Compare thermal conductivities:  0.49 cal/cm/sec/C for Al versus 0.25 cal/cm/sec/C for brass.  See Radiator FAQ   for installation details.

2. Redline Water Wetter – This product helps lower the high surface tension of water.  This helps prevent the formation of insulating bubbles on heat transfer surfaces.  The 12 oz bottle claims to lower coolant temperature up to 20F if running pure water.

3. Low T thermostat – Lower spec t-stats are available from aftermarket suppliers, like BMP.  The t-stat spec temperature divides the rapid warm-up period after cold start from the gradual warming period near steady-state operation.  The t-stat spec is just the temperature at which the t-stat starts to open and allow coolant to visit the radiator.  The thermostat does not provide on/off switching.  It starts to open at the spec temp and continues to open further as the coolant becomes warmer.
Do not expect the t-stat spec to set the operating temperature of the coolant.  The stock spec is 80C for the E12, but 75C and 71C t-stats are available.  If you choose a spec too low, your engine may stay in warm-up mode and consume more fuel.

4. Cool down pulley – Metric Mechanic is one provider of these.  A smaller diameter fan/water pump pulley increases air flow through the radiator at the expense of some HP.

5. High flow water pump -  Metric Mechanic is one provider of M30 high flow pumps.  MM shows some variation in specs even across stock pumps, but their unit has the beefiest figures in terms of impeller diameter, impeller blades, impeller height, and suction inlet diameter.  The cost is roughly twice that of a stock unit.

6. Low spec temperature switches – Aftermarket suppliers offer a variety of temperature switches to control what temperature trips the aux fan.  Most 528i seem to have two switches to govern two fan speeds, while the 530i only has one.

7. Oil cooler – This is a standard item on many Motorsport cars.  This additional heat exchanger for your oil will increase your cooling capacity for your block and head.  BMW M oil coolers and various aftermarket places, like BMP and Korman, are good sources.

8.  Headers – This probably helps a little if each pipe bolted to the head is viewed as a fin for heat transfer.  Six pipes provide more heat transfer area than 1 or 2 downpipes.

9. Cylinder head – Only an issue for older E12s, but cylinder heads manufactured in 1980 or later came with larger coolant passages to help solve head cracking issues.


The design of the E12 cooling system is such that air can get trapped in front of the thermostat. This prevents hot coolant from getting to the thermostat and telling it to open. It's almost impossible for the trapped air to naturally escape.

There's a bleed screw on the top of the thermostat cover (where lower radiator hose joins engine). This screw is opened to allow trapped air to escape out the horizontal slot in the cover in front of the screw. This screw can seize if it hasn't been used in a while. The old style slotted screw can't handle any real torque before one side of the screw breaks off. Make sure you have the new style hex head bleed screw

Here's a picture of an thermostat cover with the new style hex head bleed screw. Click on the picture for larger version. Your screw should look like this. Note the crusty buildup on the hose fitting. This should be removed to allow the hose to seal well. I usually use a brass scrubbing pad (Chore Boy, etc) and/or a little lime removal jelly.

To bleed the system:

  1. Be sure coolant overflow tank has proper fluid level.
  2. Set left side heater control to full warm (clockwise).
  3. Run engine until normal at operating temperature.
  4. Loosen the bleed screw.
  5. Run engine at high idle until clear coolant with no bubbles runs out of slot in front of bleed screw.
  6. Tighten the bleed screw gently (4.7 to 7.0 ft-lbs).
  7. You can drill an 1/8" hole in the thermostat and orient the hole towards the top of the housing to help the system 'self bleed'.
Frozen/Seized bleed screw:

I purchased a 528i recently with a frozen and broken bleed screw. I removed the thermostat cover and clamped it in a vise with wood to protect the sealing surfaces and exit pipe from the vise jaws. I ground the rust off the outside of the screw. Then I drilled a hole in a piece of 1/2" x 1/8" steel flat stock. I then placed the steel over the screw, with the screw inside and extending through the hole I drilled. Then I MIG welded the the steel to the screw. Now I had something solid to tap gently on until I loosened it up. I put plenty of penetrating oil (BG Blast) in around the screw and in the slot. Then tapping back and forth with a hammer it loosened up and I was able to unscrew it. Be careful not to bind it, but work in slow small angles until it's really loose; like tapping threads in metal, the screw threads fill with debris that will bind it.

Here's a picture of the screw welded to the piece of steel flat stock. Click on the pic for larger version.

Once you've got the screw off, chase the bleed screw hole threads with a 8mm x 1.25 tap. Clean and degrease the thermostat cover to remove all the oil. Bead blast the outside if you want it to look pretty. Eastwood company makes a nice clear coat (Diamond Coat) that I intend to try.

I usually plane my covers flat on 220 (then 400) grit wet/dry sandpaper taped to flat surface (my marble table - I'm not married) to make them seal well and keep them from leaking and seizing the bolts to the thermostat housing.

Use the new style bleed screw and bleed as above.

Good luck

Back to Frequently asked questions

© 2002,™