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Old 11-15-2012, 08:18 AM
robertobaggio20 robertobaggio20 is offline
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Join Date: Oct 2009
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Trapped air in the cooling system woo woo !!!

Hi Everyone,

Recently, I had to change my radiator as my bleed screw socket's thread got screwed and it was too tempting not to try to change the rad as my fan clutch was not in the way.

So I changed the rad. Needed to fill up 10 quarts of water and coolant concentrate or nearly that, including bleeding.

Watched for a few days and noticed that the water level was dropping fast. Topped up to full again 3 days later and did bleeding for the heck of it. Not much bubbles showed up.

Water levels kept dropping, much faster than my old radiator. I feared the worse...i do have a leaky HG for nearly a year, and there is some coolant and oil loss but its tolerable so I've left it. Suddenly it seemed to be worse, and this is a new radiator !

I feared the worse, particularly because a discussion on steel seal, blown HGs etc was going on over here at the same time. My teeth started a-chattering.

Now, when you have a blown HG or a cracked head and you're losing coolant, combustion gases will get into the cooling system, and coolant would go out the tailpipe. Open your rad cap and you'll see tiny bubbles showing up. I topped up water for the 3rd time, and decided to man up and check for bubbles. I also shot a video. The Padre has kindly uploaded it for me :

Its a 90 second video. The engine is pretty loud so please turn down the volume a bit.

Scary right ? I braced myself to do a compression test etc etc. Not happy at all.

So I decided to poll my team. Spoke to my regular mechanic. Finally, a sigh of relief. Here's why.

When you change your radiator, alot of air gets into the system. This makes its way into the expansion tank and looks like falling coolant levels. You may require between 2-3 topups in short order for the coolant levels to stabilise and fall at normal rates (M50 engines, the coolant level drops from max to under 1/2 over 6-8 weeks, and you'll need to top up.) This is normal. This is not dangerous to the engine in any way.

After I shot that video, I monitored the situation twice a day. Water dropped again to the 3/4 mark fairly quickly, and then dropped more slowly..about a mark a day. Yesterday, 20 days after the rad was changed and after 3 prior full topups and bleeds, it was at the cold mark (1/2) when cold, and after a long drive, it was 1 mark higher. While the engine was still very hot, I opened the rad cap. The water level immediately rose up to slightly above the full mark. There was no shooting out. This is a normal response to depressurisation on a normal radiator that is initially pressurised.

Here's the thing :

There was NO HINT of overheating through this entire saga, and I drive my car every day.

My MkII eyeballs were practically glued to the temp gauge throughout this whole period. The car's performance was perfectly normal under all kinds of driving conditions. Each time I topped up water, I bled the car, and apart from the first time when I did this after changing the radiator, there were hardly any bubbles seen during the other bleedings. Yet, the fast drop in coolant levels indicated (in this case) the presence of air pocket accumulating itself out at the expansion tank.

So these are my real world observations. From this, I make the following conclusions.

1. Bleeding is easy.
2. There will always be air in the cooling system.
3. The car is PERFECTLY FINE with LOTS of air in the cooling system. It is very very unlikely for these to accumulate into a sufficiently large air pocket to actually cause problems, if you bled the car according to the Bentley manual's instructions correctly. The cooling system is designed to evacuate all this air out to the expansion tank during driving. I'm assuming that nothing is wrong with your cooling system, particularly the radiator, water pump and thermostat.
4. If you have trouble bleeding, you are not doing it right.
5. If you have bled correctly, and your car is running hot or overheating, you have a different problem in either your cooling system or the engine.

I hope this post and that video in particular helps us put to bed the overwhelming fear that some of us feel about air in the cooling system. Even I freaked out when I saw that stuff. Writing this post, I recall the emotions of that period, and now genuinely feel the sense of relief that all is well, all over again.

How to bleed your radiator (m20/m50)

1. Start when the car is cold.
2. Open the rad cap and bleed screw (m20 engines have an extra one on the thermostat housing).
3. Fill up water into the expansion tank at an even pace until it runs out of both bleed holes. At that point fill up slowly (just to save water) until all the bubbles seem to have run out from both holes, then stop pouring.
4. Start the engine and set cabin heat to high. Blower does not need to be activated.
5. Let the car reach operating temperature ...might take you up to 10 minutes.
6. Pour water into the expansion tank and watch it run out of the bleed hole. There should be a few more bubbles (but its ok if there are none).
7. At some point, only coolant will run out. While pouring water into the expansion tank with one hand, quickly tighten the bleed screw down over the water pouring out of the bleed hole.
8. When the water flow has stopped, stop pouring water and tighten the bleed screw with a screwdriver or a coin. Do not overtighten, you will strip the threads. As long as it feels kinda tight, its fine. Once again, if you keep tightening, it will keep not do this. I must have done this somewhere along the line which was how I screwed my old radiator.
9. M20 engines will have to repeat step #8 with both bleed screws alternately.
10. After this, fill up water in the expansion tank right to the brim, then fix the rad cap back and tighten it good. Don't worry about jamming the water down, compressing the water, etc. It does not happen.
11. Don't rush through this. That being said, its not a delicate task either. Each bleeding either side of a cold/hot engine would only take about 3 minutes. Add 10 minutes to heat up your engine and you've got a total of 16 minutes for bleeding.

12. If you like, and if you feel you have not reached the Tibetan E34 owner's league's state of nirvanic Utmost Peace of Mind, then drive the car for a day or a couple of days, then repeat the bleeding process once again from scratch. And, even if you notice alot of bubbles this time, do not bleed a third time. NO NEED. There will always be air bubbles. It is designed into the system.

13. Please note that the Bently manual suggests a slightly different method, that involves buttoning up the system after the cold bleed, then starting the engine and only opening the bleed screw after it heats up, then closing it after all the bubbles have left, then shutting down the car and topping up coolant if needed. I am sure this works fine, but I believe the process I use above is faster overall as the E34 can take up to 4 hours to cool all the way down.

Enhanced bleeding methods used successfully by festers but entirely unnecessary in my opinion:

1. Pouring hot water into the top radiator hose.
2. Blowing into the expansion tank during cold bleeding (although this is a good method to flush out more dirt during a rad flush than merely by running water, I've been told on good authority).
3. Attaching an air compressor to the expansion tank's mouth to pressurise the system during a cold bleed.
4. Creating a long tubelike funnel afixed to the expansion tank, standing on a chair and pouring water into it.
5. Parking the car on an incline or elevating the front of the car with a jack and jack stands, before bleeding.

I invite comment to this thread, but especially anyone who chooses to criticise, please, view that video first. It is the pivot for this post, essentially.


Last edited by robertobaggio20; 11-15-2012 at 04:12 PM.
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