Cooling System

General Information

General Information

Info - all V6, 84-86 L4 models

The Almost Complete Explaination of Fiero Cooling Sensors, Switches, and Other Mysterious Things

There appears that an explaination of the Fiero cooling system electricals is in order; lets look at the engine first. There are four (4) sensor/switches on the Fiero, 4 cyl or 6. On some of the 4 cyl A/C cars, there's a two speed fan; it has two switches in one shell to activate the fan. These cars are identified by a large white ceramic resistor on the fan frame rail. So when you ask the guy at the parts counter for a coolant sensor, you better know which one! We'll do the two temp sensors first. The others are switches.. Temp sensors are nothing but variable resistors that change resistance with temperature; as the temp goes up, the resistance goes down. One sensor (2 wire) is for the ECM. This is the ONLY coolant sensor the ECM sees. It tells the ECM, obviosly, how hot the motor is so it can go into closed loop and aids in determining proper timing & fuel mix. The other sensor is combined with a switch in one assy. Both this switch and sensor operate the dash lamp and temp gauge respectively.

As for switches, they close at predetermined temperatures and make something happen. The previously mentioned switch in the same shell with the gauge sensor, lights the temp lite on the dash when it closes.

I have found that the connector shell on the V6 gauge/lamp sensor is melted or gone due to the proximity to the exhaust manifold. The wires can be simply slipped on the terminals without the shell. How do you tell which wire and/or terminal is which? Easy. Install either wire on either terminal. Run the car. If the gauge doesn't come off cold, reverse the wires. To check the gauge and lamp, short either wire to ground with the ignition on. The gauge should go full scale, and the lamp will illuminate, respectively.

A stand alone switch is for the fan. It closes at 235f. This switch, along with one built into the A/C controls, completes a relay coil circuit to ground. The relay contacts actually supply the large current needed by the fan. This relay is located in the LH front corner of the front compartment. To check the fan, short the Dk GRN/WHT wire on the fan switch on the engine to ground with the ignition on. The fan should activate. This fan switch is notorious for not operating. If you find your car (this goes for other GM's as well) overheating when not in motion, but cools down at speed, its a good chance this switch is bad. Turning on the A/C automatically activates the fan.

Speaking of A/C, there's another switch on the compressor that turns on the fan when pressure reaches 280#. Why this is there, I don't know, because, as mentioned above, the fan runs anytime A/C is dialed up. The replacement compressors I've been getting lately don't have this switch so I leave the lead off. (they also don't have the clutch switch, but that's another story).

As mentioned, some A/C'd 4 cyl cars had a two speed fan. The switch is in the same postion, but now it contains two switches in one assy. and has two leads. The low speed comes on at 221f and the other closes at 248f.


coolant fan switch            dk grn/wht
 84-86 two speed   3050223    lt grn/blk    L4- LH front of head
 84-86 non A/C     3040674
 85-88 V6 all      3040674    dk grn/wht    V6- RH top of engine
                                                 near thermostat

gauge/lamp sensor             dk grn
  84-88 all        25036628                 L4- LH front of head
  85-88 all        25036809   dk grn/wht (V6)
                              tan (L4)      V6- LH rear head next to
                                                edge of exhaust manifold

ECM coolant sensor
 84-88 all         10045847   blk & yel     L4- thermostat housing
 85-88 all         25036979                 V6- RH top of engine near

From: Tin Man

Info - 87-88 L4 models

The coolant fan of 87 - 88 4-cyl. Fieros IS controlled by the ECM, and at least according to the Helms manual, its operation is speed dependent.

From: Jukka Alve

Here is the paragraph from the Helms manual on Electric Cooling Fan Control (pg 6E-11 in the 87 manual):

Under certain conditions, the ECM may control the electric cooling fan to cool the engine and A/C condenser. At cruising speed, the ECM may turn the fan off for better fuel economy.

From: Lee Brown

Bleeding the cooling system

Here's what I do to purge air from the coolant system.

The trick is to take the thermostat out and leave it out until you are sure the _all_ of the air is purged from the system.

Park on level ground.

  1. Remove both the radiator cap, the thermostat cap, and the thermostat.
  2. Slowly begin pouring your 50/50 mix of coolant into the thermostat opening, carefully watching the radiator opening.
  3. When coolant begins comming out the radiator opening, re-install and tighten the radiator cap, top off the coolant in the thermostat opening, re-install and tighten the thermostat cap.
  4. Start the engine for ~30 seconds then kill the engine.
  5. Remove the thermoststat cap and top the coolant off. Loosen the radiator cap and wiggle it until coolant begins to come out, retighten the cap, top off the coolant in the thermostat opening and re-install the thermostat cap.
  6. Repeat steps 4 & 5 until you are satisfied that _all_ of the air is out of the system. You might want to make a short run to the beer store with the thermostat out, LET THE ENGINE COOL, then repeat steps 4 & 5 again.
  7. Re-install the thermostat.

NOTE: Fiero owners should bleed the coolant system _anytime_ the coolant reservoir runs dry. i.e. system has sucked air into the radiator.

From: Terry Loveless


There is a drain plug at the bottom of the radiator, under the coolant fan, on the passenger side. You won't be able to see it from above, but you should be able to reach it. It is just a thumb-screw plug that you should be able to remove with your fingers. If it is too tight, you might need pliers to loosen it.

There is a drain plug towards the rear of the coolant pipe on each side of the car. They are Torx, I believe they are T25 or T30.

From: Sketch


Temp never rises
Temp rises when not moving
Fan never comes on
Fan always on
Coolant overflows
Overheating, but no leak

"My temperature gauge never goes above the first tick mark."

This is usually caused by a thermostat which never closes. Check your thermostat. If the "handle" is bent or the O-ring is missing or misshapen, replace the thermostat. They are only $5-$10 from any auto parts store so if you suspect it, you might as well replace it.

"When I sit in traffic, the temperature on my gauge will slowly rise. As soon as I start to move again, it goes back down to normal."

This is usually caused by an inoperative radiator fan. When the car is moving at 30 MPH or more, there is usually enough airflow to keep the coolant cool enough, but when you slow down or stop, the radiator needs help from the fan. See next question.

"My radiator fan never comes on."

The most common cause of this is a bad coolant fan temperature switch. Note that there are 3 different temperature sensors/switches on the engine, so if you are replacing it, make sure you get the right one. The other likely culprit is the coolant fan relay which is next to the fan, on the driver's side of the car (put the headlights up, and look in front of the headlight). To test: Pull off the coolant fan relay and open it up (be careful, the red wire ALWAYS has power, ignition on or off). Ground the Dark Green/White wire. If the relay clicks and the fan comes on, the temperature switch on the engine is bad. If the relay clicks but the fan doesn't come on, then the fan motor is bad. If the relay does not click, the relay is bad.

"My radiator fan runs all of the time."

This is not a very common problem, but it can lead to your radiator fan motor wearing out prematurely. Either your cooling fan relay or temperature sensor is shorted, or the A/C high pressure switch on the A/C compressor is closed (indicating high pressure in the A/C system) or shorted.

"Coolant overflows out of the recovery bottle (next to radiator) and my car overheats."

The Fiero's cooling system is pressurized. The pressure cap on the radiator is designed to only allow water into the recovery bottle when the pressure in the system is too high, and it needs to vent out some of the water. If the pressure cap is bad, it may allow all of the water to leak out of the recovery bottle.

"My car warms up, and just keeps getting hotter, but it doesn't leak any coolant."

There are three likely suspects. Your water pump may have failed, in which case it should drip out of a small hole on the bottom (not a major leak, but signifies water pump failure). A blocked radiator may not be allowing enough water to pass through it. The third (and worst) is a bad head gasket, or cracked head or block. Check your oil dipstick. If the liquid on it is very watery or does not look like it should, you may have a bad head gasket. The easiest way to check (if you can get your car that far without major overheating) for the last two is to find a radiator shop which does free checks/estimates. They can check for a blocked radiator or a blown head gasket or cracked block or head.

From: Sketch

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