1993 sc 400 overheats

You probably need to replace the Radiator ...they have a habit of scaling up
inside.
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I had the same prob and found that I had a very small heater hose leak at the back of the motor it was very hard to find. I found a holden hose that I could cut to fit and saved money. Maybe get it pressure tested to be safe. cheers

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Is it actually overheating or is the temp gauge inaccurate?

If it is overheating, check the fan clutch, make sure that the thermostat
was not installed upside down, and make sure that you use a 50/50 mix of
coolant and water.

If all those check out, then check for head gasket leaks and proper ignition
timing.
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Ray O
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