The Diesel Stop banner

Sudden Drop in Coolant Temperature

15K views 10 replies 5 participants last post by  Torched 
#1 ·
Hi Folks,
On a recent trip I was driving west up Tioga Pass when I noticed my coolant temperature reading on my ScanGauge suddenly went from about 210 to 170, where it remained for the rest of the drive around Yosemite that day.. The oil temp readings stayed in the normal range. The next morning I started the truck and the temperature was normal-it warmed up from about ambient air temperature to 190ish. I went up the same pass again (we were camping on the east side of the Sierra) and the exact same thing happened and the gauge read low all the way home to the SF bay area. I have no codes or pending codes. I changed the thermostat back in the fall with the factory part so I don;t think this is the issue. If the coolant temp sender was bad, would this throw a code? Is it a problem with the Scangauge? Also, if the pcm thinks the temp is low, would this affect the way the truck runs (mpg, etc)? Thanks
 
#2 ·
It's still doing the same thing driving around town. It gets up to 190 then drops to between 170-180, Does the PCM or the FICM get data from this sensor? If so, I will replace it, if not I will ignore it. The oil temp reading is still normal, and I believe that is what controls the cooling fan.
 
#3 ·
The PCM gets and uses data from the sensor. If the sensor gets too far out of spec on the cold side you may end up with a P0118 code. Also the PCM controls the fan engagement and speed using several different parameters including coolant temperature, oil temp, AC condenser temp, trans temp, and so on. It will normally start fan engagement around 228F, which means your truck could possibly overheat. The best way to check if it is the sensor is to check the coolant temperature and compare it with the ambient temperature after the truck cools overnight. They should be the same.
 
#4 ·
Thanks for the reply-the temperature reading is right on the oil temp (and the ambient air temp) when it's started cold. It only drops once the truck gets to normal operating temperature, and then it won't go back up to normal. This is what is so weird! I'm going to check the wiring and the connection to the sensor right now. Is there an ohm test on the sensor that can be done when it's unplugged?
 
#5 ·
Yes there is I just don't have the specs handy. Even though it is a fairly new thermostat it may still may have gone bad.
 
#6 ·
Agreed. Check your thermostat. Sounds like more water then normal is being pushed into the engine: it just means your new thermostat cant handle the heat and is opening more then it should. Your thermostat is located right next to your coolent temp sensor. You'll just have to move your turbo inlet pipe, from your intercooler, out of the way to gain access.
 
#7 ·
I appreciate the ideas, but I doubt the thermostat would cause the temperature to suddenly drop 20+ degrees. It's got to be electrical or sensor related. The good news is it stopped doing it, so hopefully that will be the end of that-I did check the wiring and maybe the connector was a little loose or needed a "jiggle"??? If it happens again I'm just going to change the sensor.
 
#8 ·
#9 ·
trans temp going down not up

hey there i was just wondering if anyone figured out for sure whats happening with my truck. it runs like a champ and we pull our skidster with it every day when i first bought the truck i had 3 s10 blazers to get rid of and towing was great and the trans temp went down not up and i figured it was the new inter-cooler and cooling system that was put in before i bought it. this was the dead of summer and pretty hot and dry, my 2004 ford f250 turbo diesel with the 6.0l in it, most the time it sets the temp where it should be. with that said its snowing here in colorado and everything moves slower so it was alot of coasting and slow speed damn near in idle and the trans temp was way down and my heater wasnt pumping alot of hot heat out into the cab nor was it blowing as much as it should. is this the thermostat? or the trans temp sensor ? any help would be greatly appreciated
 
#10 ·
These thermostats fail open so It could be that. They are fairly easy to change and pretty cheap.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top