Changed Thermostat - Now Random Temp Drop?

[Redmanss]

'97 Sport, 4.0 OHV, 133k miles, pretty much all stock other than normal tune-up maintenance done.

I had a thermostat stuck open so with the recent sub freezing temps I finally got around to swapping it out two days ago. I drained out about 3/4 a gallon before starting, used a 192°F Napa/Balkamp and gasket, made sure to put the outlet tube facing the top, added coolant and let it run a good while on a slight incline with the cap off and heater on to get the air back out of the system, upper hose warm, no more bubbles, called it good. I checked fluid levels again yesterday morning before driving to the city, all good. The fluid in the radiator is nice and clean, could have reused it even if it wasn't for leaves blowing into the pan during the process, last flush was three years/15k miles ago.

On my drive (single digit temps out) at mostly highway speeds, the temp gauge on the dash would be reading middle normal range and holding steady, and then make a sharp drop inside 20 seconds, all the way down to or even below "C" line. It would hang there for a few minutes and I would hear a squeaking from the engine that kept time with RPMs, so my thoughts are it was my water pump running dry. The temp gauge would then creep back up and the squeak would disappear, only to come back later in the drive. This repeated itself at least ten times to town and back (160 miles), on highway and in town.

Today I repeated the burping process, left it running for 30 minutes with the cap off, lots of pulsing in the coolant fluid level and minor bubbles here and there that may have been coming more from the pulses than the radiator itself, not fully sure, but again all hoses warm and flexible, temp stable at just lower than normal, albeit with the hood open and snow falling lightly so figured that was normal enough. Took it to the corner grocery store and temp gauge bottomed out two blocks down the road.

Do you think I still have air in the system, bad gauge/connection, bad thermostat, or something else? All suggestions and advice are of course appreciated.

 

[Mr. Alligator]

Did you have the heater on during the “burping” process?

Just a thought.

 

[poker4me007]

Did you have the heater on during the “burping” process?

Just a thought.

Sounds like air in the system. Do this First ^^^^^
You could have gotten a bad thermostat, If it doesn't straighten out I'd replace the thermostat.
I use a stant #45369 195F with no problems 5yrs now

 

[974X4BLACKSPORT]

Whenever I have drained and refilled a cooling system of an automobile, on the test drive, and after the engine is up to temperature and the thermostat has opened, I do a couple WOT runs to help push any air in the engine block to the radiator where it can escape and allow room for more coolant to be filled.

 

[Redmanss]

I did have the heater both times, blowing nice and warm in the cab, and the second checked the hoses to the heater and they were warm. While I gave it some revs while trying to get the air out, I probably didn't hit past 4k RPMs, I did rev it up a few times for about a ten second count.

It doesn't make sense in my mind why or how the gauge would bottom out if the thermostat was faulty, I would think stopping flow would only cause the gauge to spike instead of bottom, and flowing too much wouldn't drop it all the way down that fast (10 seconds or so), would it?

I guess I'll throw a Motorcraft or Stant t-stat in the RockAuto cart, I'm replacing the head and intake gaskets soon so I'll just do this whole process again next month when I get time to do that one.

 

[boominXplorer]

See if you can find one of the special radiator funnels that attach to the cap area with a big funnel. They let you fill up the funnel 6" above the radiator and you an see bubbles way better. Napa carries them I think.

BTW I'm leaning towards air in the system.

 

[Redmanss]

See if you can find one of the special radiator funnels that attach to the cap area with a big funnel. They let you fill up the funnel 6" above the radiator and you an see bubbles way better. Napa carries them I think.

BTW I'm leaning towards air in the system.

I'll see if I can find one of those, would certainly make it easier to squeeze on the upper hose without dumping fluid out too. I think there's air still in the system too, just stumping me how it's not getting pushed out...

Thanks for the suggestions guys, I'll try to drop another update tomorrow after I try to purge it again.

 

[roscoe 0202]

does the heater still work when the guage bottoms out or is it cooler
roscoe

 

[CDW6212R]

How old is the radiator cap? Have you seen any change in the level in the overflow bottle? The cap is the main thing getting air out of the system. There should be no trouble removing air from the system if the cap works properly.

So stop taking the radiator cap off, check the upper hose to see if it has air in it. It should only take a short while to get most of the air out, and maybe 2-3 engine cool down cycles to remove all of it. These do not need any elaborate methods to remove the air, just drive it(if the cap works properly).

I had a cap go bad about two years after I bought it brand new from Ford. I was swapping it onto my new 98 truck because I knew it was hardly used and should be good. I didn't trust the one on my newly bought 98 truck, so I grabbed one close at hand that I knew was good. It wasn't, the overflow bottle level didn't drop, and opening the cap showed the level down about where it was when I topped it off before the cap went on. A new cap fixed that.

How much heat did you get when the gauge showed full cold?

I have a Scangauge that shows coolant temps constantly. I have a new thermostat in my 98 truck from May, new radiator and hoses too. This 180* stat is running colder than the rating, in Summer it ran around 170-175 most of the time, temps in the 90's outside. Now in 45* weather it shows 140 or so going to work, and the gauge displays full cold, near the bottom of the scale. I have warm heat but it's not hot. I'll know in a couple of days if that will work for me as a work truck, window down and 30-40* outside. I know this stat is functioning lower than its rating(it's a Robert Shaw 180), but I'd want to keep it if the heat will be enough. I much prefer colder coolant temps, it's better for reliability and avoiding over heating. The Scangauge shows it going into closed loop within a minute of starting every time. The PCM has no trouble operating with 140 degree coolant temps, I was worried about that before now.

 

[roscoe 0202]

higher temp thermostats are used by the factory because the coolant can't take heat from the engine as fast and theoretically you will get a little better fuel economy because the heat goes to push the piston down
roscoe

 

[CDW6212R]

higher temp thermostats are used by the factory because the coolant can't take heat from the engine as fast and theoretically you will get a little better fuel economy because the heat goes to push the piston down
roscoe

Manufacturers want the heat as high as reasonable to produce the most heat for the cabin. Emissions is a small part of it, but the fuel economy change is negligible for any reasonable coolant temps. If the computer does jump out of closed loop, that drastically increases fuel usage, because the PCM then uses richer predetermined data maps for A/F. So you should see no change for any typical 160-195 T'stat, if the computer goes into closed loop properly.