Re: Overheating ! (tech guys MUST read - LONG)

["Allyn" <amalventano1@home.com>]

ok, here we go;
- That is normal... for a dirty radiator. putting the idle at 1050-1100 and cleaning the radiator
with purple power will result in oil temp not breaking 105 (ever, even idle on hot day), and water
temp very rarely hitting 5/8.
- the big thing here is that our radiators consist of horizontal slots that are only 3/32"x1/2". the
narrow width easily collects deposits over the years. also, since there are lots of parallel paths,
coolant simply stops flowing through slightly clogged channels. in the end you have a radiator with
very few channels working at full flow.
- now for the other part: our water pumps, being centrifugal type, have a very low flow at idle
speed (i.e. stall speed, yes, these pumps have a stall characteristic). at a given speed, a
centrifugal pump will generate a constant d/p across it. with a radiator having alot of flow
restriction, the radiator nearly matches the d/p of the pump, resulting in extremely low flow. in my
case the flow was so low that the water velocity was so slow through the radiator that it was
actually cool before hitting the fan temp switch (so it didnt turn on at all at idle!).
- now the reason most sciroccos run at the same temp when moving is that the water pump is capable
of generating very high d/p at a higher speed, overpowering the radiators restriction and cramming
water through there regardless (once past the stall condition, flow goes up proportional to speed,
and d/p goes up roughly squared as speed goes up.).
- so when you've got a crapped up radiator, the higher d/p stalls the pump at idle speed, resulting
in lower flow, resulting in higher temp drop before water reaches the fan temp switch, resulting in
fan coming on later than it should (and for a shorter time, and blowing semi-hot air for only the
first few seconds, then goes to blowing cold air). now even though you (all) may think "hey, if the
water is colder, why is it so f&#(^@ hot!!!"??? What must be considered is that the water is
literally crawling through the radiator. Sure its cold, but its only pissing this cold water back
into the block/head, doing effectively nothing for cooling.
sorry if i threw some nuclear theory in there (yes, reactors work kinda the same way), and i hope
this helps.
al

Allyn Malventano, ET1(SS), USN (Electronics Technician, Reactor Operator)
87 Rieger GTO Scirocco 16v (daily driver, 160k)
87 Jetta 8v Wolfsburg 2dr (daily driver, 260k, 0 rattles :)

----- Original Message -----
From: "Dan Kraus" <dirtbike@warwick.net>
To: <Captnbr@aol.com>
Cc: <scirocco-L@scirocco.org>
Sent: Sunday, July 08, 2001 12:34 AM
Subject: Re: Overheating !


> At normal driving my gauge reads 100C to just over 105C. As soon as I hit a
> little bit of traffic say 5 mins, the gauge starts to read 120C-125C. The
> coolant gauge reads a little better than 3/4. Everything cools down as soon
> as I start moving.
>
> This is normal?
>
> TIA,
> Dan Kraus :-)
>
>
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