Early Miatas had limited coolant flow around #4 cylinder, due to the
water flow being out the front of the head for inline mounting rather
than originally intended as a transverse mounted engine with coolant out
the rear of the head.

That isn't a problem by itself but if you have some other factor
limiting coolant flow it makes it worse.

Heater cools cyl #4 by taking that rear path. Temp sensor is also at the
back of the head as I recall.

All cylinders working properly otherwise? Spark plug deposits all look
similar?

Bob


On Fri, 17 Jun 2011 16:30 -0600, "YES NOPE9"  wrote:
> circa 1990   same engine , replaced water pump 5 years ago.
> gradually overheats on long drive
> no coolant is missing
> fan belt is quiet ... does not appear to be slipping
> turning on interior heater with fan brings temp down
>      flow diagran seems to indicate that heater bypasses thermostat
> radiator is brand new ( 6 months )
>=20
> PROGRESS.....
> I have installed a new thermostat.  The old one seemed to have lost the
> ability to warm the car up rapidly.

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