VW Cooling System

vw beetles do not have a temp gauge so instead when the car is cold the light is blue at normal temp no light when over heating or low coolant red light and it beeps, you need the right coolant for your vw it is pink in color and some napas have it (Not DEX Cool) and you probably have a coolant leak, usually a plastic flang at rear of cylinder head goes bad

In addition to the 4 primary glow plugs, the cooling system also has 3 Coolant Glow Plugs to help provide extra heat to the cooling system in order to warm-up the vehicle's interior more quickly, and enhance overall engine warm-up time. (Example - Some Mercedes diesel engines use electrical heaters in the cabin because their engine is so thermally efficient.)
Likewise, this auxiliary source of heat from the 3 Coolant Glow Plugs is necessary because of the VW 1.9 TDI engines outstanding efficiency, which wastes very little heat.

The 3 Auxiliary Coolant Glow Plugs are located at the end of the aluminum cylinder head (in a protruding flange) directly below the vacuum (brake booster) pump, which also connects to a coolant hose.


The 3 Auxiliary Coolant Glow Plugs use two (underhood) relays which are monitored and controlled by the ECU. During a cold start, the vehicle's interior temperature selector switch helps the ECU choose one of three modes depending on the amount of heat required to warm-up the coolant.

For example, Coolant Glow Plug #1 can be selected to operate all by itself, or #2 and #3 can operate together as a two-some, or lastly, all three glow plugs can form a triple source of heat for maximum enhancement of engine warm-up time.

After the engine is completely warmed up and the thermostat is open, the Coolant Glow Plugs normally shut-off and remain off unless the (thermally efficent) engine cools down sufficiently to have the computer signal them to come back on to keep the engine and cabin air warm. Extensive idling times at stop lights or in the driveway are scenarios where these glow plugs are most likely to recycle on and off.

In conclusion, the period of Preglow and Afterglow is determined by the engine's coolant temperature (during a cold or hot start), which helps the ECU select a predetermined time-cycle for the combustion chamber and / or Auxiliary Coolant Glow Plugs to follow.

Finally, one myth that needs to be immediately debunked, is the belief that the TDI's advanced Glow Plug System is linked to the opening of the driver's side door, which is false! It is also noteworthy to mention that some earlier VW diesel glow plug systems were operated in this manner, but not so with the advanced VW Jetta 1.9 TDI engine.

In addition to the 4 primary glow plugs, the cooling system also has 3 Coolant Glow Plugs to help provide extra heat to the cooling system in order to warm-up the vehicle's interior more quickly, and enhance overall engine warm-up time. (Example - Some Mercedes diesel engines use electrical heaters in the cabin because their engine is so thermally efficient.)
Likewise, this auxiliary source of heat from the 3 Coolant Glow Plugs is necessary because of the VW 1.9 TDI engines outstanding efficiency, which wastes very little heat.

The 3 Auxiliary Coolant Glow Plugs are located at the end of the aluminum cylinder head (in a protruding flange) directly below the vacuum (brake booster) pump, which also connects to a coolant hose.



The 3 Auxiliary Coolant Glow Plugs use two (underhood) relays which are monitored and controlled by the ECU. During a cold start, the vehicle's interior temperature selector switch helps the ECU choose one of three modes depending on the amount of heat required to warm-up the coolant.

For example, Coolant Glow Plug #1 can be selected to operate all by itself, or #2 and #3 can operate together as a two-some, or lastly, all three glow plugs can form a triple source of heat for maximum enhancement of engine warm-up time.

After the engine is completely warmed up and the thermostat is open, the Coolant Glow Plugs normally shut-off and remain off unless the (thermally efficent) engine cools down sufficiently to have the computer signal them to come back on to keep the engine and cabin air warm. Extensive idling times at stop lights or in the driveway are scenarios where these glow plugs are most likely to recycle on and off.

In conclusion, the period of Preglow and Afterglow is determined by the engine's coolant temperature (during a cold or hot start), which helps the ECU select a predetermined time-cycle for the combustion chamber and / or Auxiliary Coolant Glow Plugs to follow.

Finally, one myth that needs to be immediately debunked, is the belief that the TDI's advanced Glow Plug System is linked to the opening of the driver's side door, which is false! It is also noteworthy to mention that some earlier VW diesel glow plug systems were operated in this manner, but not so with the advanced VW Jetta 1.9 TDI engine.



Please do rate my response. Thanks!

In addition to the 4 primary glow plugs, the cooling system also has 3 Coolant Glow Plugs to help provide extra heat to the cooling system in order to warm-up the vehicle's interior more quickly, and enhance overall engine warm-up time. (Example - Some Mercedes diesel engines use electrical heaters in the cabin because their engine is so thermally efficient.)
Likewise, this auxiliary source of heat from the 3 Coolant Glow Plugs is necessary because of the VW 1.9 TDI engines outstanding efficiency, which wastes very little heat.

The 3 Auxiliary Coolant Glow Plugs are located at the end of the aluminum cylinder head (in a protruding flange) directly below the vacuum (brake booster) pump, which also connects to a coolant hose.


The 3 Auxiliary Coolant Glow Plugs use two (underhood) relays which are monitored and controlled by the ECU. During a cold start, the vehicle's interior temperature selector switch helps the ECU choose one of three modes depending on the amount of heat required to warm-up the coolant.

For example, Coolant Glow Plug #1 can be selected to operate all by itself, or #2 and #3 can operate together as a two-some, or lastly, all three glow plugs can form a triple source of heat for maximum enhancement of engine warm-up time.

After the engine is completely warmed up and the thermostat is open, the Coolant Glow Plugs normally shut-off and remain off unless the (thermally efficent) engine cools down sufficiently to have the computer signal them to come back on to keep the engine and cabin air warm. Extensive idling times at stop lights or in the driveway are scenarios where these glow plugs are most likely to recycle on and off.

In conclusion, the period of Preglow and Afterglow is determined by the engine's coolant temperature (during a cold or hot start), which helps the ECU select a predetermined time-cycle for the combustion chamber and / or Auxiliary Coolant Glow Plugs to follow.

Finally, one myth that needs to be immediately debunked, is the belief that the TDI's advanced Glow Plug System is linked to the opening of the driver's side door, which is false! It is also noteworthy to mention that some earlier VW diesel glow plug systems were operated in this manner, but not so with the advanced VW Jetta 1.9 TDI engine.

There are devices in the Bosch CIS-E system not functioning that may need replaced. Most commonly it would be the thermo-time switch. This switch is also a sensor telling the ECU whether your engine is cold or hot so it would compensate fuel delivery depending on temperature. An auxiliary air regulator also opens (more air) or closes (less air) dependent from the thermo-time switch signals. It's basically telling the ECU when it's a cold morning to supply a richer air/fuel mixture or not to when it's at operating temperature. There is also a cold start valve (fifth fuel injector) that could be sticking or leaking or simply not supplying fuel during cold start up. Again, this has a higher level of understanding and special VW tools to diagnose your fuel injection problem that should be left to a VW tech to work on. It could be dangerous because fuel is involved, a single spark from static electricity could ruin your day.

16501/P0117 - Engine Coolant Temperature Sensor (G62): Signal too Low Possible Symptoms

• Difficulties with starting a cold engine.
• Starting the engine when engine is warmish or warm, idle rpm immediately is 1200 - 3000 rpm.
• RPM at idle is irregular.
• With all engine temperatures seldom idle rpm will vary between 900 - 1000 rpm.
• With engine warm (90 °C) the engine temperature suddenly drops (70 °C) and slowly comes back to 90 °C.
• Bad acceleration during warming up.
  • Diesel: an indication is extreme long time burning time of the glow indication light.

Possible Causes

• The electrical signal coming from G62 is too low
• Wiring and/or connections faulty (short circuit?)
• Engine Coolant Temperature Sensor (G62) faulty

Possible Solutions

• Check wiring and connections
• Check / Replace Engine Coolant Temperature Sensor (G62)

Special Notes

• In MY 1995-2002 (?) in Europe at VW-cars the G2/G62 had the bad reputation of breaking down often. The default G2/G62 is black and all replacements are green.

check the plastic connectors in the cooling system.vw is notorious for the plastic stuff cracking causing a slow leak

Not certain, but you may be referring to the blue light that indicates when the water temperature in the cooling system is "cold". It's typically blue when the car first starts, then changes color when the water temperature rises. If the light remains blue the water temperature isn't heating up, and you may want to have the cooling system flushed and checked. Depending upon the cars mileage, it may be time to flush the cooling system anyway.

First off need to know if it is a 2.0 or 1.8T engine. Most likely you have a 2.0L engine. The coolant light on the dash will stay blue untill the temp reaches a set temp by ECM, then goes out. Since the Beetles do not have a temp gauge you dont know what the actual temperature is. I replace thermostats on these all the time for this problem on these cars. To test this, start car and let run at idle and set heater fan on low. Let run untill you feel warm to hot air coming out the vents. Once its as warm as it is going to get, go drive the car at a steady cruise and see if the temp drops. If so then you need a thermostat replacement. I only use OE VW thermostats 050121113C. Hope that helps you out. Kevin