Engine having trouble keeping steady coolant temp

I have a 1993 SC-2 with 105K on it. Up until now, the temperature gauge has
always been quite steady in all types of weather and driving conditions,
typically staying just below the mid-point, or just above it. I just noticed
that the engine runs hot when I have the AC on (it goes over 3/4 of the way,
but not quite to the red danger zone) and, without the AC, I've noticed a
more than usual fluctuation in the engine temp. Going up hills will send the
temp close to 3/4, and going down hills can send the temp to below 1/4. Even
during steady driving, the temperature needle seems to wander around much
more than I remember.

Is this a case of a broken thermostat, or perhaps a faulty coolant temp
sensor? If so, how hard are they to replace?

 

Possibly. Could also be signs of a defective cooling fan, defective AC fan,
a clogged radiator, a bad water pump, or a leaking/blown head gasket too.
What preventive maintenance has been done on the cooling system in this
vehicle over the years? If it has been more than two years since the last
complete radiator flush and fill, I'd start there with a good radiator
flush.

Bob

 

I had a complete radiator flush at 80K, which was probably just under a
couple of years ago.

One other thought: about a week ago, I got a low coolant light...I
immediately pulled over and filled up with 100% antifreeze (about a quart)
as I didn't have water to mix in with it. I've put about 250 miles on the
car after that with no problems before this problem showed up. Could that be
related?

 

when the coolant was changed did you change the thermostat also??

 

No. Just did a radiator flush and coolant change.

 

Assuming your fans are running correctly and there is no debris blocking the
front of the radiator, I'd suggest you flush again now since it is nearly 2
years and due for this anyway. If possible, I also suggest use of a good
radiator flush and follow all bottle directions. Make sure you get as much
antifreeze out as possible before using it and then make sure you drain it a
few times using fresh rinse water afterward, then fill to 50/50 minimum and
65/35 maximum (depends on where you live and coolest temp seen during
winter). I also recommend the garden hose flush systems available from
Prestone, etc. They do a nice job and you will see a lot of rust come out
when you reverse flush the system. Also change the thermostat and make sure
you remove and completely clean the overflow reservoir. I use my [pressure
washer to get all that gunk off the plastic container. When refilling, use
the Saturn approved correct fluid available from the dealer since Saturn is
funny about what they recommend and most off the shelf coolants contain
phosphates which is a no no for Saturn. Also, make sure you get the pellets
from the dealer and crush and put them into the radiator as well since this
will help prevent a head gasket problem.

If this still doesn't correct the problem, I'd replace the Coolant
Temperature Sender next.

Good luck and post what you do and if it helps for others benefit.

Bob

 

Check to see that the plastic air dam is still under the car and hasn't been
torn off. It has a measurable effect on cooling.

Lane [ lane (at) evilplastic.com ]

 

I lost both the air dam and the splash panel in a flood a couple of years ago.
There's been no effect on cooling as far as I can see. Of course, I'm in San
Antonio...

 

Maybe the cooling fan has packed it in. I recently lost the fan on my 91
SL2 and had cooling problems at lights and in slow traffic.

 

I can hear my cooling fan kick in when I pull up to my garage at night, just
before I shut the engine off. I initially thought it was the cooling fan,
but that wouldn't explain why the car actually runs cool at times.

 

The fan motor could have a bad winding. If so, it will not run reliably.
The fan motor is cheap and easy to replace.

 

That still doesn't explain why the car actually runs cool at times, though.

 

I found that when I was moving in traffic, my car stayed cool. If I stopped
at a light and the fan motor was not getting good RPM's, the enginre would
get hot and the A/C would not cool well.

It turned out that the internals of the power connector on my fan motor was
messed up. If I wiggled it, the fan would get full speed. So I would hit a
bump and the connector got good contact it worked good... next bump, poor
connections, poor cooling....

Anyway, 1 hour later (including the trip to the part store) and $25, the
problem was solved.

 

Turns out the problem was a bad coolant temp sender. Replaced both
(apparently, Saturns have 2), and the car's running fine again.