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Old 11-15-2012, 04:08 PM
robertobaggio20 robertobaggio20 is offline
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Originally Posted by dc_wright View Post
Unfortunately steps 4 and 5 of your bleeding process are incorrect. You do not run the engine and bring it up to temperature. If you re-read your Bentley manual where it directs to start the engine is AFTER you have run sufficient coolant to complete the bleeding process (no air bubbles coming out), replaced the bleed screw, and replaced the reservoir cap.
When you are adding coolant to do the bleeding process you do not "fill the reservoir up quickly", it must be done deliberately and very slowly otherwise you get more air entrapment.

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 quickly 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.

Hi DC,

I took another look at the Bentley manual. You are correct, it says differently. You're supposed to bleed it once with the engine cold, then fix the bleed screw and rad cap, then start the engine with cabin heat onto high, wait till the car reaches optemp, then merely loosen the bleed screw, more coolant and bubbles will run out, then replace the bleed screw when only coolant runs out, then shut the engine down, and top up the radiator if the water level has fallen.

Well, I'm sure this method works fine if its in the manual, but the method I use eliminates the need and the time taken to stop the engine, let it cool down (which can take up to 4 hours), and then top up coolant. That would essentially be the same as doing 2 bleed cycles on 2 separate days (for practicality), which I've found to be unnecessary, but I still sometimes do so for the fun of it.

That being said, I'm going to do what the manual suggests on my next flush to see if there's any significant difference.

Secondly, while you are correct that pouring in water quickly may introduce air into the system in theory, in practice I do not believe it happens :

1. Expansion tanks always have some water in them usually, even if they are quite low in terms of the volume mark. When you pour water, air is not pushed in through the system, as the connection between the expansion tank and the radiator is at the bottom and the pre-existing coolant serves as a blockage to that opening.

2. Even if air goes in, it gets trapped in the vertical space right under the bleed screw and does not circulate around as the engine is still off and the wp is not functioning. So when you do the cold bleed, this air gets evacuated.

3. When the engine is started and 10 minutes is allowed to pass, any additional air that you somehow introduced into the system will circulate and accumulate under the bleed screw without entering the engine as the thermostat would have been closed for the first 3-5 minutes of the engine's operations and the coolant coming from the radiator circulates past the thermostat and returns to the radiator without entering the block. And of course, it can be easily bleeded out when you do the hot bleed.

4. When you do major cooling system stuff like replacing engines, or radiators, no matter how carefully you pour coolant in, air will remain trapped in the system. This is not dangerous to the engine. All you'll have to do is to top up coolant more than once if the water levels appear to fall quickly over the subsequent days. Remember, the dropping coolant level is actually air in the system accumulating itself out into the expansion tank (when there is alot of air still left). So in such situations especially it would be nice to do a second bleed cycle a few days later. Under normal situations such as radiator flushes, only one bleed cycle is needed.

Regardless, your points are well taken and I will ammend my OP accordingly. Thank you.


p.s. Padre, my op has been ammended.

Last edited by robertobaggio20; 11-15-2012 at 04:18 PM.
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