Need Coolant Flush Tips

[David9999999]

I'm changing the coolant in my car (1986 325es) and would like to flush the system. Through some searches, it seems the general idea is to fill with water and flushing compound, run the engine for a bit, then drain and fill with the proper 50:50 mixture. I've got a couple questions. One, should I use a flush compound at all? Some people seem to think it can damage the rubber seals in the thermostat. Also, should the system be flushed several times, or just once? Thanks!

[dnguyen1963]

Is your coolant cruddy? Do you have a lot of back pressure? If you heater core and your radiator have little back pressure then I would not waste any time flushing the system.

[nebulous123]

there is a drain plug on the block. undo that, take off one of the hoses at the top of the thermo housing and shove a hose in. turn on the water and just let it run for a little while. watch the color of the water coming out, when it becomes clear turn the hose off and let it drain out from the block drain plug.

after that unhook the bottom rad hose and the top rad hose, plug in the hose and turn it on. again watch for the color of the water, once it comes out clear turn it off, let it drain. hook all the hoses back up and refill. works for me. if you are going to do a flush, I would also swap out all the hoses, especially if you don't know the last time you changed them. Its cheap and a must.

[David9999999]

The coolant coming out of the car looked fine, I just wanted to play it safe and flush the system anyway. How would I test for back pressure on the heater core/radiator? The car ran fine, no overheating.

nubulous123- Will that method also flush the heater core? Wouldn't that leave water behind in it?

[TurboJake]

If you had no issues with the cooling or heating system to begin with, I'd just drain it and backflush with the hose. Refill with coolant, bleed it all, and be on your merry way.

[dnguyen1963]

The coolant coming out of the car looked fine, I just wanted to play it safe and flush the system anyway. How would I test for back pressure on the heater core/radiator? The car ran fine, no overheating.

nubulous123- Will that method also flush the heater core? Wouldn't that leave water behind in it?

To measure the back pressure the right way would take too much time, i.e. a tee with a pressure gauge. Just use a garden hose and apply water to the heater core inlet and "feel" the back pressure. If your heater core is plugged up you would notice an increase in pressure right away. Your coolant system sounds fine. I would just drain it, fill it up with water and run for a few minutes with the heater on high, fan off. Repeat a few times then completely drain the system. Don't forget the engine block drain. Fill her up with 50:50 coolant, bleed, and off you go.

[jlevie]

1) Get the front of the car 1-2' in the air on jack-stands.

2) Drain the coolant, remove the thermostat, and set the heater controls for
full hot. O-ring sealed thermostats require you to make up a temporary
gasket, which can be made out of pasteboard.

3) Add flush compound and fill with plain water. Then run the engine at normal
temperature as specified by the flush manufacturer (usually about 15
minutes at operating temperature). In cooler weather it may be necessary to
block off some of the air into the radiator to decrease the warmup time.

4) Allow the engine to cool until you can place your hand on the cylinder head
w/o discomfort. Drain the system and refill with plain water.

5) Run the engine for 10-15 minutes or until temp is close to normal.

6) Repeat (4) and (5) until the water drained from the system is clear and
free of debris.

7) Fill with coolant. It is best to dilute anitfreeze with distilled water.
Tap water contains disolved oxygen and may contain minerals. Either of
which will reduce the lifetime of the coolant.

Notes:

You don't absolutely need to remove the thermostat, but doing so will allow
continuous flow through the radiator and will also make draining & filling
faster and easier.

Always allow the engine to cool to the point that you can hold your hand on
the cylinder head without discomfort. That will prevent scalds from hot
coolant and prevent thermally shocking the head when you dump the coolant.

Filling the system with hot water makes the process go faster and keeps from
having to run the engine a lot in a cold condition (rich mixture).