This article is about most common BMW cooling system faults and how to diagnose and repair it.
Well, to start we need to know what perfect temperature specs for this V8 V12engine.
When car is fully warmed up and idling for 10 minutes you should see these numbers:
Upper Hose temperature-103-105C
note: Lower hose should be within 60-70C, this is very important because it indicates proper cooling system function. Lower hose or coolant output readings should be within 60-70C with open tstat.
very common mistake when mechanic thinks he bled the system right, but he doesn’t know the heater control valve is clogged and there is no way to bleed the air out and lower hose stays COLD, even auxiliary water pump fails because of this.
This screenshot shows proper transmission temperatures, when cooling system is fully functional and sealed:
NOTE: Some cars have cold firmware which opens up thermostat earlier therefore engine running temps are lower.
ALWAYS REPLACE RADIATOR CAP EVERY 5 YEARS, IT DOESNT LAST FOREVER!
If temps stay at 105-110C, it definitively means you have an airlock, which causing overheating and lower hose running cold.
This engine has self bleeding system, but it only works if cooling system is in great shape!
So most common problem you cannot bleed the air out is HEATER CONTROL VALVE, which always goes bad after 5-7 years or even earlier. Its very easy to replace and costs around 140$. Usually rubber valves crack and deteriorate clogging the valve so you cannot bleed the air out, moreover it clogs heater core, which is a real pain to replace. So remove the valve and at least inspect it:
-it should be clean inside and valves are open without 12V applied and easy to blow air through, in other words it should not have any restriction, but word of advice-REPLACE it, don’t wait, and it will also improve heat cabin.
-Front seal aka weep hole problem can be easily diagnosed checking weep hole on the timing cover
Can be easily fixed with aftermarket solutions or with simple aluminum+RTV solution, which actually works pretty good.
-Valley pan gasket leak. You need to remove intake to replace it. Be careful not to damage knock sensor which is very common.
-Rear cover plate. Hardest point here-you need to remove transmission for that
Other leaks include lower hose sensor leak and hoses also get old and is a good idea to replace them every 5 years. Also Vent hose connected to the heads near throttle body goes bad every 5years or 80-100kmls
Always pressure test your cooling system- you cannot rely on visible findings saying there is no leaks.
You cannot see 30% cooling hoses located in the rear of your engine.
MAKE SURE YOU ALWAYS BLEED AIR OUT FROM COOLING SYSTEM EVERY TIME YOU WORKED ON IT:
-FILL COOLANT TANK TO FULL WITH BMW COOLANT, LEAVE CAP OPEN.
-TURN ON IGNITION (DONT START ENGINE YET) AND SET HEATER TO 90F WITH LOW FAN SPEED. MAKE SURE AUXILLARY PUMP IS RUNNING-YOU SHOULD HEAR IT ON THE DRIVER SIDE, LET IT RUN FOR 5 MINS
-START THE ENGINE AND LET IT RUN FOR 5 MINS, CLOSE THE RADIATOR CAP
-DRIVE YOUR CAR FOR AT LEAST 30 MINS
-NEXT MORNING CHECK THE LEVEL, ADD COOLANT IF NEEDED
Always take care of any leaks and check running temps! BMW cooling system is very important not only for the engine but for transmission as well- tranny cooler is connected to the same main radiator.
Poor condition cooling system will ruin your engine and transmission very quickly. Running with airlock will eventually deteriorate all rubber gaskets including valve seals and stuck piston rings.