This problem has been a bit of a mystery and I could not find any information online while attempting to diagnose it. I have a Saturn L300 V6 with 178k which has had this weird creeping overheating problem that would only occur occasionally. This car was well cared for and the only real problem I've had is with the piece of crap transmission pan seal not sealing. I got the car last fall and also noticed slight overheating occasionally but it didn't seem to be a problem until it gassed me this spring.

Problem-
Car overheating slowly during stop and go traffic. It would also overheat when stopping and starting the engine multiple times. I only let it go to just above the halfway mark because my car starts spewing fuel out of the leaking fuel return line when it runs hot or rich(A/C on in hot weather at idle/stop and go). Fuel pump was replaced right before I got the car so bad job, faulty parts, or just misdiagnosis. Car runs fine cruising above 35mph.

Diagnosis-
Check fans-Both come on with the A/C and both stay running after the A/C is shut off.
Check fan speeds-Got car hot with A/C and fans kick on high speeds.
Check thermostat-OK
Leaks-None
Cooling system bleed-Done
Coolant-Clean
Flow-Normal, the car warms up normally and even maintains normal temp during cold weather. Heater warms as expected.
Check upper hose temp and pressure-Hot and good pressure
Check lower hose temp-good

Here's where the problem lies:
Shut down the vehicle at operating temperature and restart.
Let the vehicle cycle coolant.
Check upper hose-Good
Check lower hose temp-Getting hotter and hotter and no fan kicking on.

So problem solved, The fan is not working like it should, and if you diagnose it incorrectly you wouldn't find the problem. I have yet to find out if its the fan control module or just a fuse/relay but I will post that when I do. One step closer to defusing the Saturn L300 bomb. Just have to fix the fuel return leak problem now.

 

Addl food for thought as to why fans may not be turning on in that situation:

1)Engine Coolant Temp Sensor (ECTS) -- the resistance of the ECtS below a certain value (or an AC Request) trigger the fan relay Request -- at least on S cars -- I doubt that would differ on an L car. Main difference is that this functionality is likely controlled by the Body Control Module (BCM) on the L car vs PCM on the S car.

I would consider replacing this straight out of the chute--it is inexpensive Replace the connector/pigtail w AC DELCO PARTS (aftermarket do NOT have quite the correct R vs T behavior) if there is any corrosion on the contacts. Alternatively you can measure the resistance of the ECTS as a function of time but will need to find the R vs T chart for the L car ECTS. (it's out there...)

2) Water pump/Insufficient Coolant flow. Mostly corroded off "fins" (not really) reduce the rate of flow through the cooling system. Worst problems with cooling would theoretically be at slowest flow through system with no external (air) cooling. That condition is at or near idle -- stop and go traffic

3) Mostly clogged radiator/Insufficient Coolant Flow. Coolant leaves the engine into the lower radiator hose. If this hose continues to climb in temperature while the top stays a more or less constant temperature, this would lead me to CONSIDER that in fact there may NOT be sufficient flow through the cooling system, since the super hot coolant should be pushed through the rad and cooled somewhat, but not as much as you state in your earlier test.

It may be an evil case where the ECTS takes forever to see the coolant at that hotter temp due to insufficient flow and therefore never triggers the fans to turn on (never sends a FAN request to engage the fan relay).

When your fans are not operating when you think they should be, turn on the AC. As you already know, the BCM is programmed to turn them on w AC request (AC request triggers FAN request triggers FAN RELAY closure. I know you've already tried this test-and the fans turn on -- but you did not state under what conditions that check was run. Get the car to your problem state and repeat the test.

If the fans turn on, then the relay is fine.

BCM: Haven't heard of this issue w L cars but always a possibility that the internal circuitry controlling the actions which enable the fan relay are damaged.

IF funds allow, I would purchase a scan tool with the capability of reading coolant temp (from the ECTS) through the OBDII diag port. This will let you see exactly what the BCM is seeing for the temp as inferred from the ECTS signal. Otherwise, you are just guessing at why a temperature triggered event is not occurring -- is it because the FAN request is not being honored, the ECTS sensor is faulty, or that the ECTS never sees the superhot coolant due to insufficient coolant flow
___________

 

I replaced the fan control module and I still get the same situation. I know I have good flow and the instrument cluster would show the ects to be functioning properly . Whats weird is I also have a combined fuel leak that occurs when the vehicle a/c button is pressed. There are also some other odd things occurring. This car was a gift so I don't know but, here is where I'm at now. This car has been doing this since I got it.

2004 Saturn L300

-Replaced fan control module, no change.

Weird symptoms I noticed

-Turning on A/C causes fuel return leak to occur resulting in fumes. Source is above or in front of the fuel tank.

-Turning the dial to full heat and then turning the blower on results in the A/C activating. This actually causes the fuel smell to go away. Doing this also activates the radiator cooling fans.

-Turning the dial to full cool and then turning on the blower does nothing. It doesn't turn on the A/C or the fans.

-Getting a huge BBQ ignition style discharge when getting out of my car. (Thought it was because of a dry winter but no way, has to be a short. This doesn't happen with my other car which is parked right next to it.)

-Removed both A/C fuse and A/C relay. Fans don't work at all. Still getting massive discharges on the door. Heater no longer engages A/C.

-Fuel leak/smell does not occur if I don't turn the A/C on with the A/C switch. Then the fans never come on and the car overheats. Just to reaffirm, the vehicle also doesn't leak fuel if the heat is switched which also switches the A/C. It doesn't turn the light on the button, just activates the compressor and fans.

 

Is the service engine soon (check engine) light on?

Have you had any codes read?
Autozone or similar will read the P codes for free

However, it sounds more and more like a BCM issue to me based on the bizarre behaviors you're describing.
To read Body (B) codes, you need a more sophisticated obdII scanner that reads more than just the P codes.

Or you can eat $95 or so for a mechanic to read them and diagnose the issue.