Here's hoping it's a broken fan clutch...and not a cracked head.

Dear Car Talk

Dear Car Talk | Feb 01, 1998

Dear Tom and Ray:

I own a 1978 Pontiac Grand Am with a 301 V8 engine. This engine was rebuilt
professionally several years and 50,000 miles ago. Last month, the water pump
came apart. Apparently, the ball bearings just started rolling out along with a
big gush of coolant. I replaced the water pump with a new one I bought at NAPA.
But ever since this episode, the car has been running hot. My initial response
was to change the temperature-sending unit. Next, I installed another
temperature gauge. Then, because it's cheap, I changed the thermostat. It still
runs hot. I tried a different brand of water pump and tried flushing out the
whole cooling system. Still no improvement.
I keep a 50-50 mixture of glycol and water in the radiator. The radiator cap is
new. I can knock 20 degrees off the temperature by turning on the heater. And
interestingly, while driving, hard acceleration will make the temperature return
to normal for several minutes. I'm at my wits' end. What else could it be? --

RAY: Well, I can think of a couple of things, Allen. One I know you're not going
to like, so I'll save that for later.

TOM: Since the problem occurs at low speed and disappears at high speed, I'm
going to guess that it's an air problem and not a water problem. And in this
case, I have to suspect a malfunctioning fan clutch.

RAY: The fan clutch has a thermostatically controlled mechanism that allows the
fan to "freewheel" when the engine is cool. But when the engine gets hot, it
locks up and turns the fan at the same speed as the engine. And when the fan is
turning fast, it pulls lots of air through the radiator, which is what removes
heat from the engine.

TOM: When you "accelerate hard" (i.e. drive fast), your speed forces lots of air
through the radiator, so for those few minutes, the fan (and fan clutch) becomes
unnecessary. That's why your temperature temporarily returns to normal.

RAY: If it's not the fan clutch, then we have to explore more nefarious
territory -- .like the possibility that the engine overheated when the water
pump blew and cracked a head or head gasket. But let's save that theory for a
dark, stormy night. In the meantime, replace the fan clutch for 60 bucks. And if
you've led a good, clean life, that's going to do the trick.

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