I have an 86 GT with an issue I can't quite figure out. When the outside temp is in the upper 70's or above my temp gauge reads about 1 to 2 notches below 220, after awhile this will then pop the check engine light for being hot. My fan is installed on a manual switch that I leave on during the hot days, I have even removed the thermostat and still get the exact same issue. There is no air in the system, and when I check the overflow it is as if nothing has flowed in from the radiator being hot. My only guess is that the gauge is reading to high, it sits at 100 when cold and immediately jumps in between 100 and 220 upon start up. My last Fiero, the gauge was all the way to the temp symbol when shut off. My other guess, since the check engine light keeps coming on is that the sensor is bad.
Any help would be greatly appreciated

It could be just a faulty temp sensor or bad connector. If your sure thereos no air in the system you may want to look under the car and make sure that your one or both of your coolant pipes did not flatend by a car lift. A worn out but not yet leaking water pump can be part of the picture. Timing and or a lean fuel mix can eep the temps up. Oh, btw what does the gauge say when the engine dead cold?

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Sensor serves two circuits-the guage and the engine overheat light. If the two agree, as it seems from your description, you have an issue with your sensor. Unlike the ECM sensor you can't ohm this once to check. The sensor is easy to replace and not that expensive. I'd replace it.

BTW, you can check the accuracy of the guage by inserting a resistor into the circuit and check the guage readout. Check the trouble shooting on: http://www.fierosails.com/tempgage.html

Have a time.

BTW, never a good idea to run without a thermostat (IMHO). Cylinders farthest from the inlet will hotter than the rest. If you run without a thermostat you need a blanking sleeve

quote Tech-Tip - Coolant Sensors/Switches... by Elmer Schild HTML by Oliver Scholz The Fiero has numerous sensors & switches, all designed & engineered to make your Fiero start and run smoothly. In this tech-tip we will look at the sensors & switches that are coolant related. Coolant Temperature Sensor/Switch The sensor that operates the temperature gauge/overheat red light is located on the top of the head, directly behind the thermostat housing on the 4 cyl., and on the 6 cyl. is located on the right rear in the block, just below the ignition coil. (note: the right side of a Fiero engine is alongside the trunk - the "front" of the engine is on the passenger side - pointing sideways to the right side of the car). One of the wires (tan) to this 2 wire sensor/switch sends a signal to the gauge based on the sender resistance due to the coolant temperature. The other wire (dk.grn.) is "switched", i.e. when the sensor reaches 257°, the switch closes and your temperature light comes on. If your temperature gauge does not work, the sending unit is probably defective; usually the dash gauge is OK. Also, the spade terminals that plug into the sender unit can corrode, come off, or break off. Its best to check that connection first! The part # for this sender for the 85-88 V6 and the 85-88 4 cyl. is #25036809 (retail $34.50 ) and for the 84 4 cyl. #25036628 (retail $24.77). You can check the gauge by shorting either wire at the sender to ground with the ignition on. The gauge should go to full scale, or the lamp will illuminate, respectively. Fan Switch When your coolant reaches 235°, this one wire sensor/switch merely closes a ground circuit, and the radiator fan engages (on a 85-87 V6, the rear cooling fan located in the right rear trunk area also comes on at the same time). When you turn on your A/C, the same function occurs, regardless of coolant temperature, the ground is completed and the fan (or fans) is engaged. On the 4 cyl. (84-86) cars w/AC, there is a 2-speed fan. You can tell if your car (84) has this 2-speed by locating a large white ceramic resistor on the fan frame rail. On the 85-86 you can tell if it is a 2-speed by checking to see if 2 wires go into the fan motor. The low speed comes on @ 221°, the high speed @ 248°. On the 4 cyl. the location of the fan switch is on the side of the head, aiming forward toward the firewall & below the temperature gauge sender and has a single wire connector (single speed) or 2 wires (2-speed fan). The 87-88 4 cyl fan is controlled by the ECM and does not have this switch. On the V-6 it is located behind the thermostat housing, vertical position, with a single dk. green/white wire connector. You can check operation 2 ways: Turn on the AC, the fan (or fans on the 85-87 V6) will operate. At the temperature switch on the engine, ground the dk. gm/white wire with the ignition on (car not running!) The fan(s) should operate. This means everything from the sensor forward is OK. The only thing that the average owner can't check is the sensor itself. If you find your car overheating when not in motion, but cools down at speed, this switch is probably defective and should be replaced. The part # for the 4 cyl. 2-speed fan application is part #3050223 (retail $37.83). Part #3040674 (retail $14.83) is for the 1-speed 84-86 4-cyl., and for all the 85-88 V6`s. ECM Coolant Sensor This sensor is probably one of the most important sensors on your car. On the 4 cyl. this sensor is located on the thermostat housing, on the V6 it is located below the thermostat housing, just above the water pump, and screws into the intake manifold water jacket. This sensor sends coolant temperature data to the ECM, which uses the data to adjust fuel mixture, idle and other engine performance parameters. If defective, the engine could go into open loop, causing serious performance loss. Your car may be barely drivable at that point. You probably would get a service engine light and a code 13. Replacement ECM sensors are #10045847 (retail $3520) for the 84 4 cyl. and part #25036979 (retail $21.13) for all 85-88 engines. In many cases the reason for a trouble code relating to this sensor is a bad ground. On the 4 cyl., the grounds are above the trannsaxle on the left side of the engine and the top left hand front of the engine above the starter solenoid. Once you understand the operation of these sensors/switches, you will have an opportunity to check a few basic operations and possibly replace a part yourself before succumbing to a $60.00 per hour auto technician labor charge!

quote The Fiero Store Coolant Fan Switch Located next to the thermostat housing. This switch screws into the intake manifold vertically, next to the thermostat housing and the cold start injector switch. It has a one wire connector. Temperature Gauge Sending Unit Located on the top corner of the rear cylinder head. This sending unit screws into the corner of cylinder head, near the ignition coil and the valve cover. This also controls the dash mounted Temp light. Coolant Temperature Sensor (for the ECM) Located below the thermostat housing. This sensor screws into the intake manifold horizontally and has a two wire connector. Cold Start Injector Switch Located next to the thermostat housing. This switch screws into the intake manifold vertically, next to the thermostat housing and the fan switch.

http://www.fierostore.com/T....aspx?Part=v6sensors

Most likely your Coolant Temperature Sensor is bad. The Temp Gauge/Light sender unit is a totally seperate sensor on the engine. Both should indicate about the same. You are saying that you're getting the Service Engine Soon light coming on. I am thinking you mean also you're getting a Code 14 - CTS High. When reading the resistance you check from pin to pin. Both pins of the CTS should be open to ground.

Check the resistance of the Coolant Temperature Sensor against this chart. If the resistance is substancially off from the chart, then replace the CTS sensor. Make sure you get the CTS sensor, not the Gauge sendor.

Thanks for the help, I've been extremely busy lately and haven't had time to try any of these solutions. I was thinking along the lines of the sensor as well. Just wanted a little more info. I prefer to run my car with the thermostat installed, but removed it just to see what would happen. Since it stayed the same, I will put it back in when I get a chance. Also, the temp will read in the middle of 100 and 220 right off the bat when completely cold (not run in a few days).

I finally got time to check my sensor. It is way different than the chart provided, so I'll order up a new one. Thanks for all the help!

So which one? The gauge sender controls the gauge and is read with reference to ground. That is one of the pins. The other gauge sender pin is a switch to ground that closes at the overheat temp.

The Coolant Temperature Sensor is the one that could be causing the SES light to come on, and the CTS is read from pin to pin. The CTS is used by the ECM.

Both use the same chart for temp<->resistance

Actually no, the temp sender doesnot use the same resistance as the CTS. The air temp sensor (in the air filter housing) uses the same resistance schedule.The temp sender (on te guage side) does run + from the battery, through the guage, to the sender then to ground. The guage has internal resistance and therefore you cant ohm out the sending unit accurately. You can, though, put a resistor in line with the guage connector at the sender then to ground. By choosing the right resistor, you essentially "tell" the guage the coolant is either 100 or 220 (guage limits). Use the links above to get the right resistor values to check either extreme.

phonedawgz - again another amazing reply! Where are the (what I'm assuming to be) scans from a manual coming from? An official GM service manual or something of the like? Would like to get my hands on that puppy!

EDIT: Oops, forgot to ask. Would you by chance be able to point me in the same direction as the OP, only for my Iron Duke? I'm assuming that temp/resistance readings while checking the CTS will be considerably different..? Would like to check mine out to assist me with my abundance of issues. Thanks in advance!

[This message has been edited by ImperfectTruth (edited 08-10-2010).]

The coolant temperature sensor(CTS), the Manifold Air Temp (MAT) sensor and the temp gauge sender all use the same resistance table. That is on both engines also. And YES you can measure the resistance of the temp gauge sender, or the other two and see if the sender/sensor is accurate.

Manifold Air Sensor is called the Air Charge Sensor. The MAT/ACS sensor is located on the air cleaner housing. (yeah I know - wtf)

Sensor = used for the ECM engine control
Sender = used to control a gauge



Nice pics stolen from The Fiero Store
http://www.fierostore.com
http://www.fierostore.com/T...spx?Part=4cylsensors

The CTS and Air Temp sensors are read between the two pins.

The gauge sender has two pins however one of the pins is the switch for the temp light and the other is for the gauge. The Gauge sender is measured from the one pin to ground. You will know you have the right pin when you actually read resistance to ground. The other pin will be open untill 285 deg. Then it will close.

Manuals can be downloaded here. Under TECH is the Haynes. There is a service manual under 86 and 88

http://www.fieronews.net/fusion/downloads.php

Herk - if you still question it, throw a meter on it and see that I am right.

[This message has been edited by phonedawgz (edited 08-10-2010).]

The computer temp sensor is way different than the chart. With the engine completely cold, outside temps in the 70s the ohms read in the 200s.

Ok, you know from the chart the CTS should read 3400 ohms at 70* F? That should help with the SES light. However, the CTS has nothing to do with the gauge. The gauge/overheat sending unit is the one on the corner of the rear head by the coil.

Have you checked to see if the gauge is reading correctly?

After reading threw this post a few times. I think that I am just going to replace both the CTS and the temp sending unit. Since both are relatively inexpensive and easy to change, that may be my easiest solution.

I have not checked to see if the gauge is reading correctly, what is the best way to check that? If the gauge is not reading accuratly maybe I'll just get the one sensor then.

Get a few resistors that match the table, connect the resistor to the sender lead, the other to ground and see if the needle shows the temp it is supposed to. Some have gotten the needle to move on the shaft if it is off.

I just popped the needle off the shaft in the gauge and pressed it back in the correct position once I figured it out. While I had the instrument cluster apart to do that I did the pegged temp needle mod and was done with it.

Charlie