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Elementary Question about Coolant Change Procedure

Christian Williams

E381 - Los Angeles
Senior Moderator
Blogs Author
My review of the usual steps for coolant change in the 5432 suggests:

Either run engine till thermostat open or remove thermostat.

Drain coolant from two petcocks, under alternator and at inaccessible rear of manifold.

Drain coolant at heat exchanger.

Run distilled water through block. Refill with coolant and troubleshoot for air locks. Expect to hand-fill thermostat hoses and maybe water heater hoses.

Here's the question:

Why not just run the engine with a petcock open, and as the coolant level drains keep refilling the old with new until you figure the old coolant is displaced and mostly replaced?

There must be something wrong with this idea, which would presumably not introduce air and change most of the coolant (while wasting a few gallons).
 

Frank Langer

1984 Ericson 30+, Nanaimo, BC
Christian,

Changing the coolant is a good idea to keep it reasonably fresh, but wouldn't it be better to do it in combination with servicing the heat exchanger, replacing coolant hoses or fresh water pump, all of which likely require draining the coolant? Once the repair is done, filling with new coolant will achieve a coolant change. I service the heat exchanger about every three years to keep it from getting blocked with buildup, and that's a pretty good interval for replacing coolant. If the coolant still looks really good with no impurities, one could save the old coolant as it drains into a pail and reuse it, though coolant isn't all that expensive.

Also, I learned a few years ago that there is a coolant made specifically for diesel engines, which I now use. It's available at many auto outlets and is the same price as the more common coolant.

As I don't have your engine, I don't have any answers to your questions on where to drain your engine, but I'm sure others will.;)

Frank
 

Joliba

1988 E38-200 Contributing Member
Christian,
I do not have an answer to your question either as I also only exchange the coolant when I am performing other work on that system. In addition to the drain sites that you mentioned, I have used the drain (which may either be a petcock or a threaded plug) located on the starboard side of the engine about 5” aft of and slightly higher than the oil filter. I have never found refilling and bleeding to be difficult. Your proposed drain and dilute method sounds more difficult than the usual method to me. Trying to add water incrementally as the coolant drains remaining confident that no air was introduced sound iffy. But I guess that’s a matter of preference. On mine there is bleed valve in a hose to my water heater that is about the highest point in the system. I also have a bleed valve on the top of my thermostat. After running the engine for brief periods I repeat the bleed at both points. Before my next use of the engine, I look under the pressure cap to verify that there is coolant all the way to the brim. After starting the engine on the next occasion, I verify that there is no more air at the 2 bleed points. Then I add coolant as needed in the reservoir to an index line with the engine cool so I can quickly visually inspect for coolant loss before routinely starting the engine.
Mike Jacker
 

gabriel

Live free or die hard
Maybe risk letting in cold coolant into an empty hot passage when the thermostat finally opens?
 
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