I thought I'd share a discovery I made that was revealed by some testing. I've had suspicions about this for a long time.
When the coolant is drained and replaced on a 2F, an air pocket gets created inside the sender's adapter fitting which prevents the sender from being fully immersed in the coolant.
When there's trapped air, only the very tip of the sender touches the coolant. That bubble interferes with the correct display of the coolant temperature on the gauge which gives the impression that the engine coolant is running hotter than it actually is.
This photoshop/composited image illustrates where the coolant touches (green) and where the air pocket exists after draining and replacing the coolant.
As long as that air pocket remains, the temperature sender gets heated more by conduction of the hot cast iron head through the sender fitting threads at the top of the sender than by the much cooler coolant flowing by at the very bottom.
When the coolant gets replaced and the temperature gauge then reads high (for possibly weeks or longer), the engine isn't running hotter than normal, only the sender is.
The Remedy—
After a coolant change, burp the cooling system using the regular tricks of a burping funnel, squeezing the radiator inlet hose, elevating the front of the car and most importantly driving the car for at least an hour. Ignore the temperature gauge. It's not valid and the engine isn't running hot.
Then the next day when the engine is stone cold (and most of the air in the cooling system has been purged) remove the radiator cap to release any vacuum and top off the radiator if needed. Then replace the cap. It's important to release any small vacuum that forms in the cooling system at the cap when the engine cools down so that when you remove the sender in the next step, you don't suck air back into the cooling system at the head.
Next remove the temperature sender and clean up the internal threads of the fitting that the sender screws into and the threads of the sender. You'll notice that the sender fitting "cup" is dry and so is the sender. Possibly even the tip of the sender is dry too.
Then fill the fitting "cup" (adapter/union) up to the brim with water.
Then wrap the sender threads with Teflon thread sealing tape, leaving the first thread bare so that it will make a good ground connection, and screw the sender back into the fitting. Don't use liquid thread sealer on the threads, you're screwing the fitting into wet coolant/water.
Start up the engine and check out the temperature gauge after it warms up and check to make sure the sender isn't leaking.
Your temperature gauge needle will now be back to where it normally was.
At any time in the next few weeks/month, if the temperature gauge needle seems to ride a little higher than normal or seems to indicate erratically, (except for red zone spikes to max that are accompanied by fuel gauge spikes) suspect an air bubble has trapped itself (again) under the sender. Especially suspect it if air bubbles can be heard gurgling around when the heater valve is opened (air still in the system).
Once all the air has been fully purged from the cooling system, the sender won't trap air again... until the next time you change the coolant.
Removing the sender (when the engine is cold) to make sure that there isn't air trapped under it is an easy first step to take when trying to troubleshoot apparent hot running engine issues....and it should be the first thing to check too since it's so easy.
Normally if the sender isn't burped, after enough driving and hot and cold cycles (which can sometimes take weeks or longer) the gauge needle will eventually settle down to where it normally runs. My guess to what happens is that the air bubble in the sender fitting eventually gets reabsorbed (somehow) or burped out through expansion via heating. At any rate, 2Fs will e v e n t u a l l y clear the air pocket under the sender on their own, but for those of us who don't want to wait around forever for that to happen, you now know what to do.
Next thread:
Gauge Calibration. Stay tuned.
When the coolant is drained and replaced on a 2F, an air pocket gets created inside the sender's adapter fitting which prevents the sender from being fully immersed in the coolant.
When there's trapped air, only the very tip of the sender touches the coolant. That bubble interferes with the correct display of the coolant temperature on the gauge which gives the impression that the engine coolant is running hotter than it actually is.
This photoshop/composited image illustrates where the coolant touches (green) and where the air pocket exists after draining and replacing the coolant.
As long as that air pocket remains, the temperature sender gets heated more by conduction of the hot cast iron head through the sender fitting threads at the top of the sender than by the much cooler coolant flowing by at the very bottom.
When the coolant gets replaced and the temperature gauge then reads high (for possibly weeks or longer), the engine isn't running hotter than normal, only the sender is.
The Remedy—
After a coolant change, burp the cooling system using the regular tricks of a burping funnel, squeezing the radiator inlet hose, elevating the front of the car and most importantly driving the car for at least an hour. Ignore the temperature gauge. It's not valid and the engine isn't running hot.
Then the next day when the engine is stone cold (and most of the air in the cooling system has been purged) remove the radiator cap to release any vacuum and top off the radiator if needed. Then replace the cap. It's important to release any small vacuum that forms in the cooling system at the cap when the engine cools down so that when you remove the sender in the next step, you don't suck air back into the cooling system at the head.
Next remove the temperature sender and clean up the internal threads of the fitting that the sender screws into and the threads of the sender. You'll notice that the sender fitting "cup" is dry and so is the sender. Possibly even the tip of the sender is dry too.
Then fill the fitting "cup" (adapter/union) up to the brim with water.
Then wrap the sender threads with Teflon thread sealing tape, leaving the first thread bare so that it will make a good ground connection, and screw the sender back into the fitting. Don't use liquid thread sealer on the threads, you're screwing the fitting into wet coolant/water.
Start up the engine and check out the temperature gauge after it warms up and check to make sure the sender isn't leaking.
Your temperature gauge needle will now be back to where it normally was.
At any time in the next few weeks/month, if the temperature gauge needle seems to ride a little higher than normal or seems to indicate erratically, (except for red zone spikes to max that are accompanied by fuel gauge spikes) suspect an air bubble has trapped itself (again) under the sender. Especially suspect it if air bubbles can be heard gurgling around when the heater valve is opened (air still in the system).
Once all the air has been fully purged from the cooling system, the sender won't trap air again... until the next time you change the coolant.
Removing the sender (when the engine is cold) to make sure that there isn't air trapped under it is an easy first step to take when trying to troubleshoot apparent hot running engine issues....and it should be the first thing to check too since it's so easy.
Normally if the sender isn't burped, after enough driving and hot and cold cycles (which can sometimes take weeks or longer) the gauge needle will eventually settle down to where it normally runs. My guess to what happens is that the air bubble in the sender fitting eventually gets reabsorbed (somehow) or burped out through expansion via heating. At any rate, 2Fs will e v e n t u a l l y clear the air pocket under the sender on their own, but for those of us who don't want to wait around forever for that to happen, you now know what to do.
Next thread:
Gauge Calibration. Stay tuned.
Last edited:
