Coolant temperature after engine shut down? (1 Viewer)

[Tighe]

Howdy,

My '84 FJ60 went to the shop the other day because it was running hot, on an otherwise normal drive to work. The shop said there was a small coolant leak and the culprit was likely low coolant. Sounds plausible to me. They replaced several hoses, the thermostat (which they said was stuck open) and flushed the coolant.

I got it back today and naturally kept a close eye on the (stock) temp gauge. It ran nice and cool (gauge about halfway) despite the varying traffic conditions. I had no trouble or temp fluctuation at all while driving. But, I stopped for gas and when I turned the truck back on after refilling, the temp was very high - nearly into the red. Once it was running again it cooled back down quite quickly, and I had no trouble on the ride from the gas station to my house. Once home I turned the engine off but kept the key turned to ignition, and watched the temp again rise from halfway up to just shy of the red. No signs of steam or leaks.

It makes sense to me that the temp would rise a bit when the engine is turned off due to heatsoak and no electric fan, but should it really go all the way up the gauge, nearly to the red? This is in LA, at night, and it's only about 55F outside. What's going to happen when it's 100F outside and I turn it off after some heavy uphill traffic driving with the AC on?

In order to confirm the temp gauge was working, I took these infrared photos a few minutes after the engine was turned off. These were taken when the gauge was reading the highest - just shy of the red.

Any thoughts on whether this is normal heatsoak, or if something looks awry?

Thanks,

Tighe



When I turned the engine off, temp read halfway. A few minutes later it was almost into the red.

While the gauge was reading high, the thermostat housing read 201F

While the gauge was reading high, the rad inlet hose read 191F

While the gauge was reading high, the radiator read 199F

While the gauge was reading high, the radiator outlet hose read 162F

Any thought on these temperatures? Gauge working properly based on these observed temps? How much should the temp increase after turning the engine off? I can't seem to find this info anywhere.

The FSM says over 230 is considered "overheating". What happens when it's 30 warmer outside, or I was driving harder and the engine was hotter before shut down...

 

[chichiflys]

Normal for that. I believe all of these motors do it (mine does) - it has to do with where the temp sensor is located (top of the block) - so when you turn off the engine, the water by the sensor gets real hot...until you start moving water around after you start the engine again. That it hits the top of the gauge is not a huge concern.

One thing you might want to do is ensure the system was burped properly. Search the forum or You Tube it for that. Also I hear that a bubble of air can get trapped under the temp sensor after cooling system maintenance....not a huge deal, but again something you might want to clear out.

i'm no expert, but I don't think your numbers are concerning...I think it's a 190 or 195 deg thermostat in there.

 

[Mace]

Yeppers, the motor is being constantly cooled till you turn it off. The heat just builds up at that point. Then it cools off. I would not be too worried at those temps.

 

[OSS]

Normal (as stated above). What you're seeing a few minutes after the engine shuts down is the actual temperature of the head. The head is always hotter (a lot hotter) than the coolant when the engine is running and at operating temp. Since the temperature sender is screwed into a fitting that's screwed into the back of the head, when the coolant stops flowing, the sender soaks up the heat from the cast iron head.

Many modern engines have the temperature sender screwed into the thermostat housing, which is a bit removed from the head, so you don't see the big needle spike a few minutes after shutdown like you do on the 2F.

For all its annoyances located back there, it does have one redeeming quality. If the engine were to lose coolant while driving, the sender would still detect the increase in temperature since its screwed into the head. Temperature senders that are installed in the thermostat housing, or worse, the radiator inlet hose, are more dependent on being in contact with coolant to tell you what's going on.

 

[Tighe]

Great to hear it, thanks for the replies! It's very possible that it's always done this, but I'm only paying attention to it now that I'm paranoid. This morning I burped it a bit (nose up on blocks), making sure to run the heat to get any bubbles trapped in there. Got a few small ones but nothing major.

For the sake of comparison I took some more IR photos with the engine on. The gauge was reading a bit under halfway when these were taken.



While the temp gauge was reading in the middle, the thermostat housing read 184F

While the temp gauge was reading in the middle, the rad inlet hose read 183F

While the temp gauge was reading in the middle, the radiator read 186F

While the temp gauge was reading in the middle, the radiator outlet hose read 172F (I assume the rad does a lot more cooling at speed)

I believe this is in line with everybody's comments above. These temps are only marginally lower than the temps noted in the original post. Meaning there's only a little bit of heatsoak - it's not like the entire cooling system is overheating at turnoff.

I guess the next thing to look at would be the temperature of the temp sender when the engine is on, and then a few minutes after it was turned off.

Thanks to all for the input! If there's a takeaway from all this I guess it's to get a real mechanical temp gauge so I don't have to rely on the stock system.

 

[OSS]

Or another takeaway is... if the whole cooling system is replaced with new parts, you don't even need a temperature gauge because the engine will never overheat.

 

[workingdog]

And 200 isn't the end of the world. Even 210 isn't anything to worry too much about.

 

[RodrigzCrzr]

Make sure when you burp it!
To turn on the heater front and rear so that water is cirulated completely through the system.

 

[Tighe]

Hi Jorge! (@89fjlandcruiser )

Thanks for the reply! I was only doing the front heater. To be honest, I don't even know if the rear heater works...I've never touched it. I'll revisit tonight and try to get the rear involved too.

 

[Tighe]

And 200 isn't the end of the world. Even 210 isn't anything to worry too much about.

I guess my concern is what happens when the engine is turned off when it's hot. Will it continue to rise over 230? This concern has been alleviated though, because as @OSS said (and the IR photos seem to show) it's only heating up around the temp sender, not the whole cooling system.

I really appreciate all the replies. This website can certainly make or break my day, and has saved me so much time and effort and money. What an amazing resource.

 

[hj 60]

I think it is normal and find out if the gauge or sensor is faulty , but my diesel is the same temp and diesel is higher than gasoline, I test with a cheap temp pen, funny it can also see tire temp difference when having a leaking flat tire before it is visible.
Your infra red pictures are very informative but to expensive for me.
191 degrees Fahrenheit =88 degrees Celsius

hj60 radiator temp 82 degrees celcius, idle after testdrive
rad 74.8 celcius

 

[Tighe]

Thanks for the followup. Temps look comparable.

FYI the infrared is a relatively inexpensive attachment to an iPhone. Think it was about $300 when I bought it a few years ago, from FLIR. Looks like there are lessexpensive versions available now too.