Low Coolant Temp at Highway Speeds

[FranceJohnson]

I have an 80 with the 3-FE, but I assume the coolant system setup is pretty similar to the FZJ80s. A few months back I flushed out the coolant system and have had no problems here in town, but I just left this week to go back out west for the holidays. Temp was about 9 to 20 degrees for a few days while I was back. The truck would warm up normally driving around town, but as soon as you go over about 40 mph it drops down to cold. I thought it might be a problem with the gage, but the hose and coolant was luke warm to the touch. My initial thoughts were bad thermostat, so I installed a new one (180 deg) with gasket, maybe a slight improvment but did not fix the issue. I ended up throwing some cardboard in front of the radiator and now runs about 1/3 up the gage. Is this normal? My next thought went to poor circulation, every couple minutes the needle will hop and return to where ever it was. Anybody know any shade tree methods to make sure my water pump is working? Truck has not overheated or ran hot since I have owned it.

 

[Bomar]

Is your heat working properly?

 

[FranceJohnson]

If the gage is about 1/4 or higher it works fine and there is heat, but there is no heat when the gage is low. It was probably about 45 degrees in that truck for the 13 hour drive home. Doesn't seem to be any change between ON/OFF or any of the climate selections.

 

[FranceJohnson]

After digging through the forums, everyone with this similar problems seems to stem from the O-Ring. I installed put the new one in there with the new stat, and I am sure it was the correct one, since before I realized it came with the stat the clerk brought one (compared them to make sure they matched). So, I will go with the simplest answer first, which is checking to make sure the 0-Ring is getting a good seal between the housing and the thermostat so it cannot be by-passed. If that doesn't fix the issue, I am at a loss again.

 

[FranceJohnson]

Sure enough, after removing the housing again the O-Ring must have shifted and cocked when I put the top back on. Cleaned the whole thing and put some RTV on there, good as new. The consequence of a rushed job.