With the engine cold unplug the coolant temperature
sensor and start the engine. Both radiator cooling fans
should come on immediately at high speed. If they do,
the coolant temperature sensor is the most likely
suspect. If they don't, check the three relays in the
underwood fuse box. The two that have four pins are
for each fan, low speed. The one with five pins is for
both fans, high speed. The fans should also come on
when you select recirculation on the air conditioning
control panel with the air conditioning on. These
engines are also prone to intake manifold gasket
failure. Both upper and lower. If you need to replace
these, replace the upper intake manifold, don't reuse it!
These engines rarely blow head gaskets. So if the oil
and coolant are mixing, it's not the head gasket, it's the
intake manifold gaskets. You could also have air
trapped in the cooling system. These engines are great
for that too! There's a bleeder screw near the
thermostat housing that must be open when refilling
the cooling system. If your heater is blowing cold air I'd
suspect air in the cooling system. If it's overheating
while driving it's a coolant circulation problem, such as
air in the cooling system, a bad water pump, clogged
cooling system passages, or a stuck closed
thermostat. On my 2000 Grand Prix the impeller in the
water pump was rusted away to the point I had poor
coolant circulation. No other symptoms. No noise,
leaking, or anything. If you're overheating at idle then
it's an airflow problem. Such as the fans not coming on,
a radiator or air conditioning condenser, that's in front
of the radiator, plugged with leaves, dirt, etc., or missing
air deflectors under the vehicle, or a broken or missing
fan shroud, if originally equipped. The Buick Dealer,
unfortunately has no interest in working on a 19 year
old vehicle. If you want to take it to the Dealer, try your
local Chevrolet Dealer. They're less snooty. Hope that
helps! Jim