94 SL2 Overheating

[Andrew E.]

Hi,

I have a 94 saturn SL2 that's overheating. So far I've done the following:

1. The coolant doesn't leak
2. The radiator is new
3. Connected the radiator fan to the battery and the fan spins
4. Had a mechanic look at all the fuses and relays, all are fine.
5. Had three mechanics told me that it's the temperature sensor (one of them)

So my question is as follows:

My Hanes manual says that I should replace the thermostat. Is the thermostat the same as the temperature sensor? If not are there instructions on replacing it?

Where would I get the part numbers for what I need?

Thanks,
Andrew

 

[Andrew E.]

To clarify a bit,

the car overheats when it is sitting idling. If I'm driving over 30 or 40 mph the temperature comes back down.

Thanks,
Andrew

 

[Jim N2VX]

To clarify a bit,

the car overheats when it is sitting idling. If I'm driving over 30 or 40 mph the temperature comes back down.

Thanks,
Andrew

Sounds like the radiator fan isn't turning on. There is enough airflow over the radiator when driving and the fan isn't needed. Stop moving and it eventually has to come on to prevent overheating.

I always thought there should be some indicator that the fan is on.
It's a handy thing to know.

Does your car have A/C? Turn that on and see if the fan starts. Then you know the wiring to the fan is OK.

 

[Doug Miller]

Hi,

I have a 94 saturn SL2 that's overheating. So far I've done the following:

1. The coolant doesn't leak

How did you determine that? Just checking for drips underneath the car isn't enough -- for example, a leaking head gasket could leak coolant into the combustion chambers where it will be evaporated and blown out the tailpipe, and you'll never see any drips. Have you verified that the cooling system actually is, and remains, full?

2. The radiator is new
Why?

3. Connected the radiator fan to the battery and the fan spins
4. Had a mechanic look at all the fuses and relays, all are fine.
5. Had three mechanics told me that it's the temperature sensor (one of them)

I doubt it.

It's possible that a failed temperature sensor is causing overheating by preventing the electrically operated fan from coming on -- but as long as the vehicle is moving at any appreciable speed, there's enough air being forced through the radiator that the fan isn't really needed for cooling, so this is unlikely IMO unless you're doing a lot of driving in very heavy stop-and-go slow speed traffic. The more likely symptom of a failed sensor is a gauge that's *falsely* reporting the engine to be too hot, when the temperature is actually normal.

IS the engine actually overheating? (Noticeably hotter than normal when you lift the hood, smell of hot coolant, wisps of steam drifting out from under the hood, clouds of steam pouring out, etc.)

If so, I'm betting the problem is a failed thermostat. *Especially* if the thermostat wasn't replaced at the same time the radiator was. (It should have been. They're cheap, and they fail fairly regularly in older vehicles. As long as the cooling system is drained anyway, it's just silly not to replace it.)

So my question is as follows:

My Hanes manual says that I should replace the thermostat.

I agree.

Is the
thermostat the same as the temperature sensor?

It is not. The temperature sensor does just that: senses the temperature, and sends a signal to the temperature gauge. The thermostat controls how hot the engine gets; specifically, the thermostat is a valve that sits between the engine and the radiator. It stays closed until the coolant reaches the correct temperature (190F, I believe), and then opens. This prevents water from circulating through the radiator until the engine is warmed up.

If the thermostat is stuck open, the engine takes a very long time to warm up (which is bad in a variety of ways) -- and if it's stuck *closed* (or doesn't open fully), the engine overheats. Of course, that's also bad.

If not are there
instructions on replacing it?

If you have the same Haynes manual that I do, it's on pages 3-2 and 3-3. Or you could look in the index under "Thermostat, check and replacement".

The instructions for checking and replacing the temperature sensor are a few pages later, listed under "Coolant temperature sending unit - check and replacement".

Where would I get the part numbers for what I need?

You should be able to find a thermostat for your car at just about any auto parts store, or maybe even in the automotive department at Wal-Mart.

 

[Bob Shuman]

From what you state, it sounds like either a failed coolant temperature sensor or the fan relay.

Bob

 

[Andrew E.]

Hi

How did you determine that? Just checking for drips underneath the car isn't enough -- for example, a leaking head gasket could leak coolant into the combustion chambers where it will be evaporated and blown out the tailpipe, and you'll never see any drips. Have you verified that the cooling system actually is, and remains, full?

Yes it remains full over several months.

Why?

Because my other radiator cracked and I had to replace it.

I doubt it.

It's possible that a failed temperature sensor is causing overheating by preventing the electrically operated fan from coming on

This is exactly what is happening. The fan doesn't come on even when the temperature indicator on the dashboard is way over half. As I said, the fan is fine and the relays and fuses have all been tested (by a mechanic) and are fine. If the relay is shorted the fan turns on.

-- but as long as the

vehicle is moving at any appreciable speed, there's enough air being forced through the radiator that the fan isn't really needed for cooling, so this is unlikely IMO unless you're doing a lot of driving in very heavy stop-and-go slow speed traffic.

This is exactly when the overheating happens: during stop and go traffic. If I'm driving at an appreciable speed the engine stays at the 1/4 mark.

The more likely symptom of a failed sensor is a gauge

that's *falsely* reporting the engine to be too hot, when the temperature is actually normal.

IS the engine actually overheating? (Noticeably hotter than normal when you lift the hood, smell of hot coolant, wisps of steam drifting out from under the hood, clouds of steam pouring out, etc.)

Yes I've had it steam up before and it is *very* hot otherwise.

If so, I'm betting the problem is a failed thermostat. *Especially* if the thermostat wasn't replaced at the same time the radiator was. (It should have been. They're cheap, and they fail fairly regularly in older vehicles. As long as the cooling system is drained anyway, it's just silly not to replace it.)

I agree.

So given the above do you still agree?

If you have the same Haynes manual that I do, it's on pages 3-2 and 3-3. Or you could look in the index under "Thermostat, check and replacement".

The instructions for checking and replacing the temperature sensor are a few pages later, listed under "Coolant temperature sending unit - check and replacement".

So is this what I'm looking for? Does the coolant temperature sending unit cause the engine fan to come on?

You should be able to find a thermostat for your car at just about any auto parts store, or maybe even in the automotive department at Wal-Mart.

Does this apply to the coolant temperature sending unit?

Thanks,
Andrew

 

[Andrew E.]

Hi

I had the relay tested and it is fine (according to the mechanic). Does the coolant temperature sensor control when the fan comes on?

thanks,
Andrew

 

[Andrew E.]

Hi,

I tried turning on the A/C and the fan does not start.

Thanks,
Andrew

 

[Jim N2VX]

Andrew,

It's time to get a schematic diagram for the car. The official Saturn manual has one, don't know about other sources.

Figure out which relay pins are the coil.
Pop the relay and check for voltage to the coil. Have someone turn on the A/C or something like that.

Then take it from there.

 

[Andrew E.]

Hi,

Already done. That's what the mechanic did: he opened the relay and shorted the coil and the fan turned on.

Andrew

 

[Doug Miller]

Hi

Yes it remains full over several months.

Because my other radiator cracked and I had to replace it.

Why did it crack? Overheating, or some other cause?

This is exactly what is happening. The fan doesn't come on even when the temperature indicator on the dashboard is way over half. As I said, the fan is fine and the relays and fuses have all been tested (by a mechanic) and are fine. If the relay is shorted the fan turns on.

Depending on which terminals are shorted, that may or may not be a valid test.

-- but as long as the

This is exactly when the overheating happens: during stop and go traffic. If I'm driving at an appreciable speed the engine stays at the 1/4 mark.

Yes I've had it steam up before and it is *very* hot otherwise.

So given the above do you still agree?

Maybe not -- but I'd sure check it anyway. It's an easy test to perform.

So is this what I'm looking for? Does the coolant temperature sending unit cause the engine fan to come on?

Not directly, no. The coolant temperature sensor, and the fan relay, are both connected to the PCM (Powertrain Control Module). That's what causes the fan to come on. It's possible that a failure in the PCM, or in the wiring between the PCM and the fan relay, could be preventing the fan from coming on.

By the way -- the Haynes manual gives a detailed procedure for testing the coolant temperature sensor, using an ohmmeter. It's in Chapter 6, Engine & Emissions Control Systems, under "Engine coolant temperature sensor". Don't bother trying to find part numbers; there's no need -- just call up your friendly neighborhood auto parts retailer and say "I need _____ for a 94 Saturn SL2. You got one?"

Does this apply to the coolant temperature sending unit?

You won't find that at Wal-Mart, I don't think... but I'd be really surprised if the major auto parts chains wouldn't have it.

 

[SnoMan]

From what you state, it sounds like either a failed coolant temperature sensor or the fan relay.

Or the wiring between them. Also there is two temp sensors on that year engine. One for gage and one for ECM/PCM (engine control module or powertrain control module which ever you prefer to call it) The second one is used to control fan.

 

[Andrew E.]

Hi,

Yes this is exactly what two mechanics told me. But I don't see anything in the hanes manual about two sensors. Do you know where they are located?

Thanks,
Andrew

 

[Tom The Great]

Hi,

I have a 94 saturn SL2 that's overheating. So far I've done the following:

1. The coolant doesn't leak
2. The radiator is new
3. Connected the radiator fan to the battery and the fan spins
4. Had a mechanic look at all the fuses and relays, all are fine.
5. Had three mechanics told me that it's the temperature sensor (one of them)

So my question is as follows:

My Hanes manual says that I should replace the thermostat. Is the thermostat the same as the temperature sensor? If not are there instructions on replacing it?

Where would I get the part numbers for what I need?

Thanks,
Andrew

From your description, remember I am not there to do the work, so this is all guessing, and isn't a How-to:

Something with coolant flow must be happening. If you have a fan comeing on, and coolant level is sufficent, I would suspect coolant flow is the issue.

I would check the water pump for operation, the dumb way I would do it, is check if I get heat (strong heat) from the heater. Second, I would then figure what could be interupting flow, and the internal coolant thrmistat (a valve that opens and closes allowing flow to the radator). I would be suspect of that, since I've been told they fail over time.

I was once told how to check one, a valve thermistat, but figured that once I open the housing, it saves time to just install a new one.

If a person can safely work yourself, and follow directions from the Haynes book, I like them better than chiltons, since more pictures. A person could replace the item themself.

later,

tom @ www.NoCostAds.com

 

[Andrew E.]

Hi,

So now I have replaced the temp sensor. It had a crack in it and was misreporting the temperature. So now when the car's temp gauge gets to 3/4 the fan comes on and the engine cools down.

Here's the rub. I've had this car for 3 years and it's never gone beyond the 1/4 mark on the temp guage. So now I'm thinking that I might have misdiagnosed the problem. The temp guage was definitely cracked and broken, but I didn't realize that the car had to be at 220 degrees (3/4 of the guage) before the fan would come on.

Do you think this is a thermostat problem as you originally suggested? I read the thermostat "Check" portion in the hanes manual and it says if the upper radiator hose is hot then it's not a thermostat problem.

So I could just ignore the fact the engine gets to 3/4 of the temp guage because the fan will come on and cool it before it overheats.

Any suggestions?

Thanks,
Andrew

 

[Doug Miller]

Hi,

So now I have replaced the temp sensor. It had a crack in it and was misreporting the temperature. So now when the car's temp gauge gets to 3/4 the fan comes on and the engine cools down.

Here's the rub. I've had this car for 3 years and it's never gone beyond the 1/4 mark on the temp guage.

... probably because the gauge wasn't getting the correct input from the faulty sensor.

So now I'm thinking that I might have
misdiagnosed the problem. The temp guage was definitely cracked and broken,

You mean the sensor, right?

but I didn't realize that the car had to be at 220 degrees (3/4 of the guage) before the fan would come on.

Considering that (according to the Haynes manual) the thermostat isn't even fully open until 212, that's really not too surprising.

Do you think this is a thermostat problem as you originally suggested?

Not anymore, no.

I read the thermostat "Check" portion in the hanes manual and it says if the upper radiator hose is hot then it's not a thermostat problem.

Well, is it?

So I could just ignore the fact the engine gets to 3/4 of the temp guage because the fan will come on and cool it before it overheats.

Any suggestions?

Have you actually checked the upper radiator hose yet? If it's hot, then, like the book says, the 'stat is OK -- and I was wrong. If it's not hot, then replace the 'stat too.

 

[Andrew E.]

Yes the upper hose is very hot.

Andrew

.. probably because the gauge wasn't getting the correct input from the faulty sensor.

You mean the sensor, right?

Considering that (according to the Haynes manual) the thermostat isn't even fully open until 212, that's really not too surprising.

Not anymore, no.

Well, is it?

Have you actually checked the upper radiator hose yet? If it's hot, then, like the book says, the 'stat is OK -- and I was wrong. If it's not hot, then replace the 'stat too.

 

[BläBlä]

So I could just ignore the fact the engine gets to 3/4 of the temp guage because the fan will come on and cool it before it overheats.

3/4 is known to be normal.

 

[Andrew E.]

Ok, so that said, why would have the car never gone to that? The sensor that was cracked was the PCM sensor, not the guage... and the guage never showed the car overheating before...

Andrew

 

[Doug Miller]

Yes the upper hose is very hot.

Then your thermostat's probably good, and the problem was indeed the sensor.
Everything should be OK now.