was: burning oil? now: thermodynamics (applied to rocco cooling systems)

[amalventano@sc.rr.com (Allyn)]

ok, here is how a minor issue can become catastrophic (from my own personal
experience):
- old radiator gets crapped up with gunk (nice and technical, eh?)
- coolant rate of flow through the system drops (due to obstruction)
(this is most pronounced at idle, where flow rate of pump is slowest)
- since the coolant is flowing slower than normal, coolant spends more time
in radiator, resulting in coolant getting nice and cool as it goes across,
unfortunately this nice and cool water can only be sipped on by the engine
since it is flowing so slowly.
- meanwhile the engine does just the opposite of the radiator, it rapidly
heats up the cool water, still flowing slowly, so slowly that the water is
extremely hot (read: past boiling) when exiting the engine.
- the hot outlet gets hotter while the cold inlet gets colder, this
continues until you have room temp water coming in and >212 water coming
out.

now for the good stuff,
* deficiencies in scirocco design that make this problem worse *
1. radiator fan temp switch is located HALF WAY across the radiator coolant
path:
-- in the situation above, the water gets cool so fast that it is too cold
to trip the fan switch by the time it reaches it. in the case that it does
actually hit teh temp sender while its hot enough to trip it, the fan kicks
on, almost instantly cooling the slow-moving water, resulting in the fan
being turned back off within ~10 seconds.
2. radiator fan temp switch is mounted at BOTTOM of radiator:
-- in a low flow area which is most likely filled with crud, resulting in a
less responsive temp sender.
3. thermostat is in path FROM the radiator (the place where the really nice
and cool water is coming out), this nice cool water can actually make the
thermostat start closing while the engine sits there overheating! this is
regardless of what temp thermostat you might have in there.

- bottom line:
crap-filled radiator will bring a scirocco to its knees, in more ways that
one
- way to diagnose:
1. at idle: if your engine temp is screaming up while your radiator fan
kicks on for only short bursts. also, the air coming out of the fan will
quickly become cool each time the fan turns on.
2. if you keep engine at 2000 rpm and the fan stays on longer and the engine
temp goes down from where it was at idle, your coolant system is filled with
crap
- recommended treatment:
remove radiator, fill 3/4 way with purple power (50/50 mix with water),
shake vigarously, rince, repeat until what comes out doesnt look
rust-filled. gunk engine clean and carbeurator cleaner make some nice
additives to the cocktail as well.
*note - DO NOT run purple power through coolant system, ONLY use it to flush
a disconnected radiator, and flush it VERY WELL with water before
reinstalling.
- secondary treatment:
a very low idle speed makes this problem much worse. idle MUST be at least
900 rpm for our coolant systems to work properly at idle.

hth
Al

Allyn Malventano, ETC(SS), USN
87 Rieger GTO Scirocco 16v (daily driver, 180k, rocco #6)
86 Kamei Twin 16V Turbo Scirocco GTX ('it has begun', rocco #7)
86.5 Occo 16v Trailer (dotting 'i's across the country, rocco #8)
87 Jetta 8v Wolfsburg 2dr (wifes daily driver, 285k, 0 rattles)