14 How do I flush my cooling system?
Description
This article is from the VW Technical FAQ, by with numerous contributions by Jan Vandenbrande others.
14 How do I flush my cooling system?
A: The basic principle is to drain the old fluid out, refill the car
with regular water and perhaps some coolant flush liquid, let the car
run for a bit (follow the instructions of the flush chemical), let it
cool, drain, fill it up with water again, run, cool, drain, repeat
until clear water comes out. Then you refill with premixed coolant
(40%-60% H2O for summer, 50%-50% or 60%-40% for colder climates).
Note that for 4 cyl cars you need to remove the thermostat housing
to drain (bottom of pump) and you refill or flush through the regular
radiator cap, though sometimes I have found it easier and faster to
just use one of the upper radiator hoses.
VR6 cars are actually easier as they have a drain plug (finally).
Following herewith are some additional details.
[Most of this is from ND with comments added]:
4 Cyl Cars
==========
The proper way to renew the coolant is to remove the thermostat
so as to drain the complete system.
Also don't forget to turn your interior heat to max as to drain that too,
and to open the radiator cap to let air in.
[ND] When we do a Coolant service at the shop we replace the thermostat and
o-ring with our special low temp 80 Celius unit from Germany.
[JAN] I am not sure whether that is advisable in colder climates
as the engine may never heat up.
[From Jens]:Correct! The German thermostat starts to open at 92 deg. C.
and is completely open at 108 deg. C.. In April 1979 VW changed the niveau
of the engine temperature by +5 deg. C. to make the engines run
better and longer!
If you still have the stock Fan switch we recommend
to replace it with the our low-temp unit which will allow the fan to
come on a little early to keep the temps down. If you replace the
thermostat we always drill a small air relief hole [1mm] in the main plate of
the thermostat so as to prevent a air pocket to develope at the
thermostatic bulb and cause the unit to stay closed. Just had a call
from a VW owner back east who had this problem. He called to thank me
for figuring out why his VW overheated after changing his thermostat.
[Jan: I usually don't do it that way, instead I loosen the upper
radiator hose and poor the coolant through several channels and then
squeeze the tubes to let the bubbles out. However, do carry the rest
of the coolant with you on your next drive just in case.]
Another trick to get around the airbubble problem came from Donald Borowski:
Heat up the thermostat on a pan of water until it opens, and then insert an
asprin pill as it closes. This will keep it open long enough to
fill the system and get the air out, and then will disolve.
I don't think that acetosalicylic (sp?) acid is very strong, and
the amount is rather small.
[Written by Jan]
6 Cyl Cars
==========
The VR6's have a drain plug located right next to the dip stick.
So the whole process is much easier than the 4 cyl cars. You reach
the drain plug with a very long screwdriver (from under the car),
and when the plug pops off, make sure you dont swallow any fluid as
it is deadly if ingested.
The flushing procedure is very similar as the 4 cyl cars except that
you dont have to mess with the thermostat, nor does there seem to
be the problem of the bubble (but I am not 100% sure).
To put that plug back, I just use my "crows claw" emergency pick up
tool (comes with a flex handle, Sears and other hardware stores sell them).
Whatever car you have, always take some premixed coolant fluid with
you on the next drive, in case a bubble did move out and you need to
top off the coolant.
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