1995 Buick Century - Overheating, ECT and Fan Issues (very confused)

Hey guys. I have a 1995 Buick Century V6 3.1L 150k miles.

Lately I’ve noticed my temperature gauge needle has been creeping way above the halfway-point, and the fans won’t come on to mitigate the problem. I’m worried it’ll overheat and blow my head gasket, but I’m also worried that I’m paranoid and this is normal car behavior. Please let me know what you think:

I’ve replaced the Primary Fan relay, checked the fuses, replaced the ECT sensor, replaced the ECT plug itself (soldered a new one), and replaced the thermostat recently, but the car temp needle still climbs above the halfway-point, even during idle, and the fans still won’t turn on.

I know the fans are only supposed to come on once my car is at 223-230°F, but I don’t know where that temp reading is on the temp sensor dashboard. Is the car at 230°F when the needle reaches the halfway-point? Or when the needle is at 3/4? Or when the needle is on the red? I have no clue.

Primary and secondary fans DO TURN ON when I unplug the ECT wire from the sensor. Hoses leading into and out of the radiator were super hot before I unplugged the ECT wire. When the fans came on after unplugging, they cooled down the coolant in the hoses and the temp gauge needle dropped.

I don’t know if I have to wait for the needle to hit the red in order to trigger the fans to turn on. I’m scared to test that though, cause I don’t wanna blow my head gasket. I think it’s weird how both fans only turn on when I unplug the ECT wire.

I’m worried my car will overheat if I don’t solve this problem or get some clarity. Technically speaking, I can just keep that ECT wire unplugged to guarantee my fans will always stay on, but I still want to solve this problem completely.

Any help would be greatly appreciated! Thank you for reading.

Have you tried bleeding the air out of the system?

Are you using Dexcool coolant?

This is what it can do to a radiator and other components if air gets into the cooling system.



Yes, the water outlet assembly has a screw that lets air out when loosened. I did that a few times today to make sure air was out of the system when I ran the car on idle.

I’ve been using Prestone, not Dexcool. I’ll have to buy the recommended brand for my car at some point though.

I would never use Deathcool.

It’s supposed to last 5 years or 150,000 miles, but there have been problems with this coolant. Cooling systems that use Dexcool exhibit more acid buildup and rust in the system when the coolant level gets low and oxygen is allowed to enter the system. The acid eats away at head gaskets and intake gaskets.Mar 6, 2020


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Wow thank you, didn’t know that at all. I’ll stay away from it then.

My Corolla (slightly earlier vintage that your Century) develops very similar symptoms, has happened 3 or 4 times over the years. Cooling system works good when driving on freeway, but overheats when idling, slow speed driving, and engine compartment fan never turns on. Caused by faulty engine coolant temperature switch, screws into coolant jacket, and is part of the circuit which turns on the fan relay. Check to see if your Buick uses a similar gadget. The design changed in that era on most cars from a separate switch (like my car uses) to the computer reading the the coolant temp directly from coolant temp sensor, and turning on the fan if temp reading is too high. If the computer turns on the fan for your car, suspect that part of the circuit, or the part that reads the coolant temp sensor.

Removing wire from coolant temp switch turns on the fan full-time automatically in my car too. Done that way as a fail-safe. Cooling system is pretty much guaranteed to overheat if engine compartment fan doesn’t turn on. Uusally that fan will turn on if you turn the A/C on. If that part works you could try turning A/C on when engine shows signs of overheating as a temporary fix. Also turning on heater to full blast will produce some engine cooling as a temporary aid.

The ECU controls the operation of the fans by grounding the circuits to turn the fans on.



You’re right, it seems that when I’m on the highway, the wind blowing through the front grill acts as a natural fan and cools the engine, but once I’m going 25mph the temp creeps up again.

For my Buick, the PCM reads the temp from the ECT sensor (which sits close to the thermostat housing) and operates the fans. It now makes sense that the circuit is being broken or disrupted when the ECT plug is in, since the PCM will turn the fans on when the ECT is unplugged.

Yes the A/C usually activates the primary fan, but I learned a while ago that my A/C doesn’t work, so it won’t activate the fans, even on full blast. Since winter is coming, putting the heater on blast, as well as keeping the ECT unplugged, is a decent temporary fix.

Yes, this makes a lot more sense now. I’ll have to do some electrical tests then (to the best of my ability), or try to trace the wire and see how it’s breaking the circuit. I hope I won’t have to replace a large portion of the wiring harness.

I already cut off the original plug and soldered a new one, but that obviously wasn’t enough. The problem could lie deeper in the wire, an area I haven’t found yet.