Coolant temp sensor or thermostat? Mk4 Golf

OhCdn · Nov 11, 2019

Today on the way to work which is a 45 minute drive and mtemp cluster just made it to 90°f, which usually only takes 10 mins usually so what I am wondering if it is sensor or the thermostat. A guy from work told me that if you unplug the sensor wires and put your car on accesoriy that it will go down past 0 and if it doesnt its your temp sensor would like some feedback thanks guys

Levi

Metal Man · Nov 11, 2019

After your next commute pop the hood and feel the rad hoses. As the motor warms the top hose should get hot with the bottom cold. Then as the t stat opens the bottom hose will warm. If the t stat is opening early the bottom hose will warm before it should and the top probably won't get hot.
Unplugging the temp sensor seems like an unlikely way to check it.

OhCdn · Nov 11, 2019

Metal Man said:
After your next commute pop the hood and feel the rad hoses. As the motor warms the top hose should get hot with the bottom cold. Then as the t stat opens the bottom hose will warm. If the t stat is opening early the bottom hose will warm before it should and the top probably won't get hot.
Unplugging the temp sensor seems like an unlikely way to check it.

I thought unplugging it wouldn't do anything either lol, thank you for the reply man will be checking that tonight awesome

fouillard13 · Nov 11, 2019

a new thermostat is $30 and extremely easy to change and is worth every penny. 10 minutes?? at -35, not plugged in, my 03 tdi is blowing warm (not quite hot) air within 1.2km of slow driving. about 2 minutes.

steve6 · Nov 11, 2019

Its your thermostat.. Change it

OhCdn · Nov 11, 2019

steve6 said:
Its your thermostat.. Change it

Thanks for the input man

OhCdn · Nov 11, 2019

fouillard13 said:
a new thermostat is $30 and extremely easy to change and is worth every penny. 10 minutes?? at -35, not plugged in, my 03 tdi is blowing warm (not quite hot) air within 1.2km of slow driving. about 2 minutes.

For sure man about to get the part tonight

OhCdn · Nov 11, 2019

Metal Man said:
After your next commute pop the hood and feel the rad hoses. As the motor warms the top hose should get hot with the bottom cold. Then as the t stat opens the bottom hose will warm. If the t stat is opening early the bottom hose will warm before it should and the top probably won't get hot.
Unplugging the temp sensor seems like an unlikely way to check it.

so on the way home it actually got up to temp pretty quick but i noticed it dropping off 90 while im driving down the highway mind you it is the cooldest day so far and it is sitting at -10 degrees Celsius and snowing like a *****, when i got home the top hose was warm and the bottom hose was cold like you said. its now almost acting like when i did my egr delete but it only did it when my car was idling for a couple of minutes in the cold weather now doing when driving 80-120 km/hr just a side note didnt know if it would help at all. Could it be working half ass? thanks

Levi

AndyBees · Nov 12, 2019

The coolant system only holds about 6 quarts (5.7 liters). On a very cold day, driving along at high-way speeds, the coolant returning via the bottom hose will be cold. See rationale below..

Keep in mind, the T-stat opens and closes slowly, it's not a pop-open or snap-shut process. Thus, as the hot coolant returning to the Water Pump housing from the oil cooler, EGR cooler, expansion tank and heater passes over the T-stat, it will open to allow some cooler coolant to flow in to maintain the normal full operating temperature.

I'd say, on a very cold day, the T-stat is fully closed more than it is slightly open. The coolant circulating thru the systems mentioned above are more than adequate to eliminate a lot of heat....... of course, ambient temp, vehicle speed, load, etc., all come into play.

OhCdn · Nov 12, 2019

AndyBees said:
The coolant system only holds about 6 quarts (5.7 liters). On a very cold day, driving along at high-way speeds, the coolant returning via the bottom hose will be cold. See rationale below..

Keep in mind, the T-stat opens and closes slowly, it's not a pop-open or snap-shut process. Thus, as the hot coolant returning to the Water Pump housing from the oil cooler, EGR cooler, expansion tank and heater passes over the T-stat, it will open to allow some cooler coolant to flow in to maintain the normal full operating temperature.

I'd say, on a very cold day, the T-stat is fully closed more than it is slightly open. The coolant circulating thru the systems mentioned above are more than adequate to eliminate a lot of heat....... of course, ambient temp, vehicle speed, load, etc., all come into play.

For sure man but if its more closed wouldnt that mean my car should be up to the right amount of operating temps and or not have drops like it was having its all back to normal now just still wondering why it did that, didnt even change the thermostat yet.

eddieleephd · Nov 13, 2019

Block half the radiator with waxed cardboard to stop some airflow through it and it'll warm faster. Put the highest temp thermostat in and all will be good.
Really this is extremely common and weather related, nothing to worry about whatsoever. Common maintenance.
I used to change thermostats with the season on my V-8's, helped quite a bit to keep it cooler in the summer and warmer quicker in the winter.

Sent from my moto g(7) power using Tapatalk

OhCdn · Nov 13, 2019

eddieleephd said:
Block half the radiator with waxed cardboard to stop some airflow through it and it'll warm faster. Put the highest temp thermostat in and all will be good.
Really this is extremely common and weather related, nothing to worry about whatsoever. Common maintenance.
I used to change thermostats with the season on my V-8's, helped quite a bit to keep it cooler in the summer and warmer quicker in the winter.

Sent from my moto g(7) power using Tapatalk

Alright man will do i will check back to see if that helps it

steve6 · Nov 13, 2019

Reading the temps with a ODBII device makes everything make more sense, the gauge will read straight up when its in a 'range' about 80c-95c. If the gauge is dropping from straight up (87c'ish) it actually means its dropped below about 75c, which if you are moving(highway or constant city driving) should not happen, even at -10/15 c outside temps. These thermostats have a tendency to open early as a safety device when they go bad(instead of over heating the engine by staying closed). New thermostat I believe will solve your issues.

OhCdn · Nov 13, 2019

steve6 said:
Reading the temps with a ODBII device makes everything make more sense, the gauge will read straight up when its in a 'range' about 80c-95c. If the gauge is dropping from straight up (87c'ish) it actually means its dropped below about 75c, which if you are moving(highway or constant city driving) should not happen, even at -10/15 c outside temps. These thermostats have a tendency to open early as a safety device when they go bad(instead of over heating the engine by staying closed). New thermostat I believe will solve your issues.

For sure man bought one of those will be doing it this weekend to see if it is the issue

Coolant temp sensor or thermostat? Mk4 Golf

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