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Jim Grant's Tech Tips
90 Volkswagen
Jetta, Overheating in the Rain
Q: Our '90 Jetta overheats in the rain. The mechanics I have spoken
with want to replace the head. I am reluctant however, since this problem
only occurs when it is raining heavily. The car runs fine under dry conditions,
shows no signs of coolant loss, or steam in the tail pipe. The problem
has persisted for 8 months without complications other than overheating
under rainy conditions despite getting lots of use. The electric fan appears
to work. In any case, the problem occurs at highway speeds when the radiator
should be getting enough air. Any suggestions on how to diagnose this
problem and resolve it are greatly appreciated. Puzzled and frustrated.
A: Why
can't your Jetta get rid of this heat on a rainy day? Did you know that
moisture will reduce the heat transfer ability of a radiator? It's true;
it's harder for a vehicle's radiator to remove heat from the coolant on
a humid or rainy day versus a dry day.
Coolant condition is critical. Your Volkswagen "must" have a
50/50 mix of anti-freeze/water and "must" not be more than two
years old. Anti-freeze contains additives such as borates, nitrates, silicates
and other corrosion inhibitors to protect the metal surfaces inside the
engine. Over time the additives are used, consumed while protecting the
engine. The result is a form of aluminum salts that circulate in the cooling
system. The salts will collect on the cooler radiator tubes, reducing
the radiator's heat transfer efficiency. Even a thin coating of these
salts can reduce radiator cooling capacity by as much as 20-30%. A radiator
testing to show a coolant flow capacity of 90% may still be functioning
at only 60-70% of its original cooling efficiency due to coating of aluminum
salts.
Be watchful for air flow restrictions. Radiators with bent cooling fins,
or blockage because of insects, will reduce air flow. If your car has
A/C then the A/C condenser which is located in front of the radiator can
have the same problems and restrict air flow to the radiator.
You shouldn't wait to have your overheating problem diagnosed. Repeated
overheating will damage your vehicle's engine and could leave you stranded
on a less than desirable stretch of highway.
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