Coolant temp sensor

The two most common causes are: fuel pressure leakdown after sitting for a while and an engine coolant temp sensor. The culprit is a leaking check valve in the fuel pump. To test, turn the key to ON for 2-secs, then OFF. Repeat 3 times. The try starting. If it fires right up, have the shop do an overnight leakdown test. If the fuel system loses more the 5-8-psi overnight, then the pump must be replaced. Test 2 is to check the engine coolant temp sensor. A sensor that's off usually reports a warmer engine than actual so the computer provides too lean a startup mixture. Try depressing the gas pedal halfway. If it starts right up. have the coolant sensor tested.

Cooling fan switch ?????? Has a engine coolant temp. sensor , That inputs info to the PCM/ECM - engine computer ! The PCM / ECM interpits the voltage signal from software programmed into it as to what temp . of the coolant is . When the engine coolant temp. reaches
The PCM commands the low-speed fans ON under the following conditions:
• The engine coolant temperature exceeds approximately 106°C (223°F).
• The transmission fluid temperature exceeds 150°C (302°F).
• The A/C pressure reaches 199 psi (1372 kPa).
The cooling fans will switch from low to OFF when the coolant temperature drops below 102°C (216°F).
The PCM commands the high-speed fans ON under the following conditions:
• The engine coolant temperature reaches 112°C (234°F).
• The transmission fluid temperature is more than 151°C (304°F).
• The A/C pressure reaches 260 psi (1792 kPa).
• Certain DTCs are set.
The cooling fans will switch from high to low, except when DTCs are set, when the coolant temperature drops below 106°C (223°F).

The PCM turns on the cooling fans by energizing three different relay's .
Watch this video
Relay Driven Dual Cooling Fan Diagnostics

it could be you have an air lock in the cooling system
remove the rad cap from the rad, start the engine and let it run until the coolant drops (add to fill it back up) or until the coolant boils out of the rad spout.

maybe pull the heat sensor. -replace it OR clean it off, reinstall it.

From what I just read about your vehicle,it has a history problem with the gauge,may be the coolant temperature sender or the gauge,it says the whole cluster will have to be changed to be fixed if the gauge is bad,found some forums and all had gauge problems.Right above the coolant temperature sensor is the coolant temperature sender,this is what sends the temperature to your gauge,the sensor sends a signal to the computer,they may have replaced the wrong one,here's a link to the sensor,the hole above it is where the sender goes,it's a one prong part,the sensor is a 2 prong part.A lot of people don't know that there are 2 types of coolant temperature gauge sending unit and sensor.

Have you checked the coolant level in the overflow tank? Make sure it's to the proper level. The coolant warning system can lose its little brain on the VW's. Sorry I can't be more helpful.

P0710 TRANSMISSION FLUID TEMP. SENSOR MALFUNCTION
P0711 TRANSMISSION FLUID TEMP. CIRCUIT RANGE/PERFORMANCE PROBLEM

P0710 DTC Detection Condition:
Either (a) or (b) is detected for 0.5 sec. or more: (1 trip detection logic)
(c) Temp. sensor resistance is less than 79 W
(d) After engine has been operating for 15 min. or more, resistance at temp. sensor is more than 156 kW

Trouble Area:
1. Open or short in ATF temp. sensor
2. ATF temp. sensor
3. ECM

P0711 DTC Detection Condition:
Both (a) and (b) are detected: (2 trip detection logic)
(a) After 12 sec. of engine start, temp. of atmosphere and that of engine coolant is more than -10°C
(b) After normal driving for over 20 min. and 10 km, ATF temp. is less than 10°C

Trouble Area:
1. Open in ATF temp. sensor
2. ATF temp. sensor
3. ECM

I’m happy to help further over the phone at https://www.6ya.com/expert/jeremy_69f3cc28d95bf514

Wow these are hard to find!!

Diagnostic codes for 1987 Pontiac

13 Open Oxygen Sensor Circuit
14 Coolant Temperature Sensor (High Temp Indicated)
15 Coolant Temperature Sensor (Low Temp Indicated)
21 Throttle Position Sensor (Signal Voltage High)
22 Throttle Position Sensor (Signal Voltage Low)
23 Manifold Air Temperature (MAT) Low Temp Indicated
24 Vehicle Speed Sensor (VSS) Circuit
25 Manifold Air Temperature (MAT) High Temp Indicated
32 Exhaust Gas Recirculation (EGR) Vacuum Control
33 Mass Air Flow Sensor (MAF) (GM/SEC High)
34 Mass Air Flow Sensor (MAF) (GM/SEC Low)
41 Cam Sensor Circuit
42 C3-I EST or Bypass Circuit Failure
43 Electronic Spark Control (ESC) Circuit
44 Oxygen Sensor Circuit (Lean Exhaust Indicated)
45 Oxygen Sensor Circuit (Rich Exhaust Indicated)
51 PROM Error
52 CALPAK Error
55 ECM Error
If you have your owners manual they should be in there. I couldn't find one online but did find this.

Future reference
http://autorepair.about.com/library/ts/obd-i/bl-dtcs-28.htm?once=true&

I hope these helped you

on your car are two sensors one is an engine temp sensor the other is a coolant temp sensor
change the coolant temp sensor that is what goes bad most of the time when the water gets to 210 degrees the coolant temp turns on of that is working change the radiator fan relay

This all sounds like a leaking head gasket, the fast guage swing is caused by air pockets around the temp sensor.

Trouble Code 33 and 34 are the MAF/MAP Sensor Circuit. It sounds like you've replaced that sensor already. Did you also have the code 34 before putting in the new sensor?
Or did this code start after it's replacement? Any other codes appearing?

Troubleshooting it would depend if it has the Speed Density Update. The dealer should be able to provide that info.

Here's your fault code list:

13 oxygen sensor circuit 14 coolant temp sensor circuit (high temp) 15 coolant temp sensor circuit (low temp) 21 throttle position sensor (TPS) circuit (high signal) 22 throttle position sensor (TPS) circuit (low signal) 23 manifold air temp (MAT) circuit (low temp) 24 vehicle speed sensor (VSS) circuit 25 manifold air temp (MAT) circuit (high signal) 31 wastegate solenoid electrical circuit 32 exhaust gas recirculation (EGR) circuit 33 mass air flow (MAF) circuit (gm/sec high) 34 mass air flow (MAF) circuit (gm/sec low) 41 cam sensor circuit 42 C3I EST or bypass circuit failure 43 electronic spark control (ESC) circuit 44 oxygen sensor circuit (lean exhaust) 45 oxygen sensor circuit (rich exhaust) 51 PROM error 52 calpak error 55 ECM error